From d480b6f08b8a6b32c58e5bbdf2f193432c50fb97 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 15 Jan 2020 14:18:54 +0000
Subject: Initialize repo

Adding a README and a basic .gitignore to initialize this mono-repo. I'm quite
excited about this undertaking!
---
 .gitignore | 6 ++++++
 README.md  | 9 +++++++++
 2 files changed, 15 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 README.md

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 000000000000..51eb9644acd7
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,6 @@
+# Haskell
+*.hi
+*.o
+
+# Python
+__pycache__
diff --git a/README.md b/README.md
new file mode 100644
index 000000000000..e7a01960b87c
--- /dev/null
+++ b/README.md
@@ -0,0 +1,9 @@
+# Mono
+
+This is my mono-repo. Having a personal mono-repo is a new idea for me, so at
+the time of this writing, the state of this repository is fledgling.
+
+I'm attempting to amass a collection of functions across a variety of languages
+while minimizing the costs of sharing the code across a projects. Stay tuned for
+more updates as my definition of the mono-repo becomes more clear, my opinions
+evolve, and my preferences change.
-- 
cgit 1.4.1


From 456d358cd7b0701b0ca2b8571d61f255d3da1488 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 15 Jan 2020 14:21:45 +0000
Subject: Upload my 2019 Advent of Code attempts

Well, unexpectedly (perhaps naively so), I only made it to Day 7. I created
these before I stumbled upon the idea of the mono-repository; otherwise, I like
to think I would have more granular commits introducing this work.
---
 advent-of-code/aoc2019.nix        |   8 ++
 advent-of-code/day_1.py           | 119 ++++++++++++++++++++++++++
 advent-of-code/day_2.py           |  32 +++++++
 advent-of-code/day_3.py           | 137 ++++++++++++++++++++++++++++++
 advent-of-code/day_4.py           |  35 ++++++++
 advent-of-code/day_5.ex           |  50 +++++++++++
 advent-of-code/day_5.py           | 170 ++++++++++++++++++++++++++++++++++++++
 advent-of-code/day_5.pyc          | Bin 0 -> 5250 bytes
 advent-of-code/day_6.py           | 155 ++++++++++++++++++++++++++++++++++
 advent-of-code/day_7.py           |  49 +++++++++++
 advent-of-code/writePythonBin.nix |  18 ++++
 11 files changed, 773 insertions(+)
 create mode 100644 advent-of-code/aoc2019.nix
 create mode 100644 advent-of-code/day_1.py
 create mode 100644 advent-of-code/day_2.py
 create mode 100644 advent-of-code/day_3.py
 create mode 100644 advent-of-code/day_4.py
 create mode 100644 advent-of-code/day_5.ex
 create mode 100644 advent-of-code/day_5.py
 create mode 100644 advent-of-code/day_5.pyc
 create mode 100644 advent-of-code/day_6.py
 create mode 100644 advent-of-code/day_7.py
 create mode 100644 advent-of-code/writePythonBin.nix

diff --git a/advent-of-code/aoc2019.nix b/advent-of-code/aoc2019.nix
new file mode 100644
index 000000000000..10e6c559a9b4
--- /dev/null
+++ b/advent-of-code/aoc2019.nix
@@ -0,0 +1,8 @@
+with import <nixpkgs> {};
+with python35Packages;
+
+buildPythonPackage {
+  name = "wpcarro";
+  src = ./day_5.py;
+  propagatedBuildInputs = [ pytest numpy ];
+}
diff --git a/advent-of-code/day_1.py b/advent-of-code/day_1.py
new file mode 100644
index 000000000000..bd4024e3ec7d
--- /dev/null
+++ b/advent-of-code/day_1.py
@@ -0,0 +1,119 @@
+from math import floor
+
+xs = [
+    102473,
+    84495,
+    98490,
+    68860,
+    62204,
+    72810,
+    65185,
+    145951,
+    77892,
+    108861,
+    70764,
+    67286,
+    74002,
+    80773,
+    52442,
+    131505,
+    107162,
+    126993,
+    59784,
+    64231,
+    91564,
+    68585,
+    98735,
+    69020,
+    77332,
+    60445,
+    65826,
+    111506,
+    95431,
+    146687,
+    135119,
+    86804,
+    95915,
+    85434,
+    111303,
+    148127,
+    132921,
+    136213,
+    89004,
+    143137,
+    144853,
+    143017,
+    104386,
+    100612,
+    54760,
+    63813,
+    144191,
+    84481,
+    69718,
+    84936,
+    98621,
+    124993,
+    92736,
+    60369,
+    137284,
+    101902,
+    112726,
+    51784,
+    126496,
+    85005,
+    101661,
+    137278,
+    136637,
+    90340,
+    100209,
+    53683,
+    50222,
+    132060,
+    98797,
+    139054,
+    135638,
+    100632,
+    137849,
+    125333,
+    103981,
+    76954,
+    134352,
+    74229,
+    93402,
+    62552,
+    50286,
+    57066,
+    98439,
+    120708,
+    117827,
+    107884,
+    72837,
+    148663,
+    125645,
+    61460,
+    120555,
+    142473,
+    106668,
+    58612,
+    58576,
+    143366,
+    90058,
+    121087,
+    89546,
+    126161,
+]
+
+
+def fuel_for_mass(x):
+    """Return the amount of fuel (in mass) required for a mass of X. The total
+    amount of fuel includes the amount of fuel required for the fuel itself,
+    since fuel also has a mass weights."""
+    mass_fuel = floor(x / 3) - 2
+    if mass_fuel < 0:
+        return 0
+    else:
+        fuel_fuel = fuel_for_mass(mass_fuel)
+        return mass_fuel + fuel_fuel
+
+
+print(sum(fuel_for_mass(x) for x in xs))
diff --git a/advent-of-code/day_2.py b/advent-of-code/day_2.py
new file mode 100644
index 000000000000..77774c1bb5ad
--- /dev/null
+++ b/advent-of-code/day_2.py
@@ -0,0 +1,32 @@
+from itertools import product
+
+x = [
+    1, 0, 0, 3, 1, 1, 2, 3, 1, 3, 4, 3, 1, 5, 0, 3, 2, 1, 10, 19, 1, 6, 19, 23,
+    2, 23, 6, 27, 2, 6, 27, 31, 2, 13, 31, 35, 1, 10, 35, 39, 2, 39, 13, 43, 1,
+    43, 13, 47, 1, 6, 47, 51, 1, 10, 51, 55, 2, 55, 6, 59, 1, 5, 59, 63, 2, 9,
+    63, 67, 1, 6, 67, 71, 2, 9, 71, 75, 1, 6, 75, 79, 2, 79, 13, 83, 1, 83, 10,
+    87, 1, 13, 87, 91, 1, 91, 10, 95, 2, 9, 95, 99, 1, 5, 99, 103, 2, 10, 103,
+    107, 1, 107, 2, 111, 1, 111, 5, 0, 99, 2, 14, 0, 0
+]
+
+
+def interpret(i, x):
+    op, a, b, out = x[i + 0], x[i + 1], x[i + 2], x[i + 3]
+    if op == 1:
+        x[out] = x[a] + x[b]
+        return interpret(i + 4, x)
+    elif op == 2:
+        x[out] = x[a] * x[b]
+        return interpret(i + 4, x)
+    elif op == 99:
+        return x
+    else:
+        raise Exception('Unsupported opcode: {}. {}, {}'.format(op, a, b))
+
+
+for a, b in product(range(100), range(100)):
+    y = x[:]
+    y[1] = a
+    y[2] = b
+    if interpret(0, y)[0] == 19690720:
+        print(100 * a + b)
diff --git a/advent-of-code/day_3.py b/advent-of-code/day_3.py
new file mode 100644
index 000000000000..6dd863528c1c
--- /dev/null
+++ b/advent-of-code/day_3.py
@@ -0,0 +1,137 @@
+from math import floor
+from heapq import heappush, heappop
+
+xs = [
+    "R1009", "U993", "L383", "D725", "R163", "D312", "R339", "U650", "R558",
+    "U384", "R329", "D61", "L172", "D555", "R160", "D972", "L550", "D801",
+    "L965", "U818", "L123", "D530", "R176", "D353", "L25", "U694", "L339",
+    "U600", "L681", "D37", "R149", "D742", "R762", "U869", "R826", "U300",
+    "L949", "U978", "L303", "U361", "R136", "D343", "L909", "U551", "R745",
+    "U913", "L566", "D292", "R820", "U886", "R205", "D431", "L93", "D71",
+    "R577", "U872", "L705", "U510", "L698", "U963", "R607", "U527", "L669",
+    "D543", "R690", "U954", "L929", "D218", "R490", "U500", "L589", "D332",
+    "R949", "D538", "R696", "U659", "L188", "U468", "L939", "U833", "L445",
+    "D430", "R78", "D303", "R130", "D649", "R849", "D712", "L511", "U745",
+    "R51", "U973", "R799", "U829", "R605", "D771", "L837", "U204", "L414",
+    "D427", "R538", "U116", "R540", "D168", "R493", "U900", "L679", "U431",
+    "L521", "D500", "L428", "U332", "L954", "U717", "L853", "D339", "L88",
+    "U807", "L607", "D496", "L163", "U468", "L25", "U267", "L759", "D898",
+    "L591", "U445", "L469", "U531", "R596", "D486", "L728", "D677", "R350",
+    "D429", "R39", "U568", "R92", "D875", "L835", "D841", "R877", "U178",
+    "L221", "U88", "R592", "U692", "R455", "U693", "L419", "U90", "R609",
+    "U672", "L293", "U168", "R175", "D456", "R319", "D570", "R504", "D165",
+    "L232", "D624", "L604", "D68", "R807", "D59", "R320", "D281", "L371",
+    "U956", "L788", "D897", "L231", "D829", "R287", "D798", "L443", "U194",
+    "R513", "D925", "L232", "U225", "L919", "U563", "R448", "D889", "R661",
+    "U852", "L950", "D558", "L269", "U186", "L625", "U673", "L995", "U732",
+    "R435", "U849", "L413", "D690", "L158", "D234", "R361", "D458", "L271",
+    "U90", "L781", "U754", "R256", "U162", "L842", "U927", "L144", "D62",
+    "R928", "D238", "R473", "U97", "L745", "U303", "L487", "D349", "L520",
+    "D31", "L825", "U385", "L133", "D948", "L39", "U62", "R801", "D664",
+    "L333", "U134", "R692", "U385", "L658", "U202", "L279", "D374", "R489",
+    "D686", "L182", "U222", "R733", "U177", "R94", "D603", "L376", "U901",
+    "R216", "D851", "L155", "D214", "L460", "U758", "R121", "D746", "L180",
+    "U175", "L943", "U146", "L166", "D251", "L238", "U168", "L642", "D341",
+    "R281", "U182", "R539", "D416", "R553", "D67", "L748", "U272", "R257",
+    "D869", "L340", "U180", "R791", "U138", "L755", "D976", "R731", "U713",
+    "R602", "D284", "L258", "U176", "R509", "U46", "R935", "U576", "R96",
+    "U89", "L913", "U703", "R833"
+]
+ys = [
+    "L1006", "D998", "R94", "D841", "R911", "D381", "R532", "U836", "L299",
+    "U237", "R781", "D597", "L399", "D800", "L775", "D405", "L485", "U636",
+    "R589", "D942", "L878", "D779", "L751", "U711", "L973", "U410", "L151",
+    "U15", "L685", "U417", "L106", "D648", "L105", "D461", "R448", "D743",
+    "L589", "D430", "R883", "U37", "R155", "U350", "L421", "U23", "R337",
+    "U816", "R384", "D671", "R615", "D410", "L910", "U914", "L579", "U385",
+    "R916", "U13", "R268", "D519", "R289", "U410", "L389", "D885", "L894",
+    "U734", "L474", "U707", "L72", "U155", "L237", "U760", "L127", "U806",
+    "L15", "U381", "L557", "D727", "L569", "U320", "L985", "D452", "L8",
+    "D884", "R356", "U732", "L672", "D458", "L485", "U402", "L238", "D30",
+    "R644", "U125", "R753", "U183", "L773", "U487", "R849", "U210", "L164",
+    "D808", "L595", "D668", "L340", "U785", "R313", "D72", "L76", "D263",
+    "R689", "U604", "R471", "U688", "R462", "D915", "R106", "D335", "R869",
+    "U499", "R190", "D916", "R468", "D882", "R56", "D858", "L143", "D741",
+    "L386", "U856", "R50", "U853", "R151", "D114", "L773", "U854", "L290",
+    "D344", "L23", "U796", "L531", "D932", "R314", "U960", "R643", "D303",
+    "L661", "D493", "L82", "D491", "L722", "U848", "L686", "U4", "L985",
+    "D509", "L135", "D452", "R500", "U105", "L326", "D101", "R222", "D944",
+    "L645", "D362", "L628", "U305", "L965", "U356", "L358", "D137", "R787",
+    "U728", "R967", "U404", "R18", "D928", "L695", "D965", "R281", "D597",
+    "L791", "U731", "R746", "U163", "L780", "U41", "L255", "U81", "L530",
+    "D964", "R921", "D297", "R475", "U663", "L226", "U623", "L984", "U943",
+    "L143", "U201", "R926", "U572", "R343", "U839", "R764", "U751", "R128",
+    "U939", "R987", "D108", "R474", "U599", "R412", "D248", "R125", "U797",
+    "L91", "D761", "L840", "U290", "L281", "U779", "R650", "D797", "R185",
+    "D320", "L25", "U378", "L696", "U332", "R75", "D620", "L213", "D667",
+    "R558", "U267", "L846", "U306", "R939", "D220", "R311", "U827", "R345",
+    "U534", "R56", "D679", "R48", "D845", "R898", "U8", "R862", "D960", "R753",
+    "U319", "L886", "D795", "R805", "D265", "R876", "U729", "R894", "D368",
+    "R858", "U744", "R506", "D327", "L903", "U919", "L721", "U507", "L463",
+    "U753", "R775", "D719", "R315", "U128", "R17", "D376", "R999", "D386",
+    "L259", "U181", "L162", "U605", "L265", "D430", "R35", "D968", "R207",
+    "U466", "R796", "D667", "R93", "U749", "L315", "D410", "R312", "U929",
+    "L923", "U260", "R638"
+]
+
+
+def to_coords(xs):
+    row, col = 0, 0
+    coords = []
+    for x in xs:
+        d, amt = x[0], int(x[1:])
+        if d == 'U':
+            for i in range(1, amt + 1):
+                coords.append((row + i, col))
+            row += amt
+        elif d == 'D':
+            for i in range(1, amt + 1):
+                coords.append((row - i, col))
+            row -= amt
+        elif d == 'L':
+            for i in range(1, amt + 1):
+                coords.append((row, col - i))
+            col -= amt
+        elif d == 'R':
+            for i in range(1, amt + 1):
+                coords.append((row, col + i))
+            col += i
+    return coords
+
+
+def contains(row, col, d):
+    if row not in d:
+        return False
+    return col in d[row]
+
+
+def intersections(xs, ys):
+    d = {}
+    ints = set()
+    for row, col in to_coords(xs):
+        if row in d:
+            d[row].add(col)
+        else:
+            d[row] = {col}
+    for row, col in to_coords(ys):
+        if contains(row, col, d):
+            ints.add((row, col))
+    return ints
+
+
+def trace_to(coord, xs):
+    count = 0
+    for coord_x in to_coords(xs):
+        count += 1
+        if coord_x == coord:
+            return count
+    raise Exception("Intersection doesn't exist")
+
+
+answer = []
+for coord in intersections(xs, ys):
+    x = trace_to(coord, xs)
+    y = trace_to(coord, ys)
+    heappush(answer, x + y)
+
+print(heappop(answer))
diff --git a/advent-of-code/day_4.py b/advent-of-code/day_4.py
new file mode 100644
index 000000000000..adef73b452dc
--- /dev/null
+++ b/advent-of-code/day_4.py
@@ -0,0 +1,35 @@
+import re
+
+start = 134792
+end = 675810
+
+
+def satisfies(x):
+    x = str(x)
+    result = False
+    double, not_decreasing = False, False
+
+    # double and *only* double exists
+    for i in range(len(x) - 1):
+        # double and left-of-a  is BOL or !x
+        #        and right-of-b is EOL or !x
+        a, b = x[i], x[i + 1]
+        bol = i - 1 < 0
+        eol = i + 2 >= len(x)
+        if a == b and (bol or x[i - 1] != a) and (eol or x[i + 2] != a):
+            double = True
+            break
+
+    # not_decreasing
+    prev = int(x[0])
+    for a in x[1:]:
+        a = int(a)
+        if prev > a:
+            return False
+        prev = a
+    not_decreasing = True
+
+    return double and not_decreasing
+
+
+print(len([x for x in range(start, end + 1) if satisfies(x)]))
diff --git a/advent-of-code/day_5.ex b/advent-of-code/day_5.ex
new file mode 100644
index 000000000000..807e3c9177be
--- /dev/null
+++ b/advent-of-code/day_5.ex
@@ -0,0 +1,50 @@
+defmodule Interpretter do
+  def interpret_param({mode, x}, xs) do
+    case mode do
+      :positional -> Enum.at(xs, x)
+      :immediate  -> x
+    end
+  end
+
+  # Perhaps I can model the intepretter after Forth and make it a stack-based
+  # interpretter with an emphasis on debugability, introspection.
+  def interpret(i, xs) do
+    stack = []
+    op = Enum.at(xs, i)
+
+    # map instructions into an intermediate representation (i.e. IR) where the
+    # opcodes are mapped into atoms and the arguments are mapped into references
+    # or literals.
+
+    instructions =
+      %{'01' => :add,
+        '02' => :multiply,
+        '03' => :input,
+        '04' => :output,
+        '05' => :jump_if_true,
+        '06' => :jump_if_false,
+        '07' => :less_than,
+        '08' => :equal_to,
+        '99' => :return}
+
+    case xs do
+      [:add, a, b, {:positional, out} | rest] ->
+        a = interpret_param(a, xs)
+        b = interpret_param(b, xs)
+        Interpretter.interpret(i + 3, List.insert_at(xs, out, a + b))
+
+      [:multiply, a, b, {:positional, out} | rest] ->
+        a = interpret_param(a, xs)
+        b = interpret_param(b, xs)
+        Interpretter.interpret(i + 3, List.insert_at(xs, out, a * b))
+
+      [:input, a | rest] -> nil
+      [:output, a | rest] -> nil
+      [:jump_if_true, a, b | rest] -> nil
+      [:jump_if_false, a, b | rest] -> nil
+      [:less_than, a, b, out | rest] -> nil
+      [:equal_to, a, b, out | rest] -> nil
+      [:return | _rest] -> nil
+    end
+  end
+end
diff --git a/advent-of-code/day_5.py b/advent-of-code/day_5.py
new file mode 100644
index 000000000000..3d82846e6126
--- /dev/null
+++ b/advent-of-code/day_5.py
@@ -0,0 +1,170 @@
+x = [
+    3, 225, 1, 225, 6, 6, 1100, 1, 238, 225, 104, 0, 1102, 31, 68, 225, 1001,
+    13, 87, 224, 1001, 224, -118, 224, 4, 224, 102, 8, 223, 223, 1001, 224, 7,
+    224, 1, 223, 224, 223, 1, 174, 110, 224, 1001, 224, -46, 224, 4, 224, 102,
+    8, 223, 223, 101, 2, 224, 224, 1, 223, 224, 223, 1101, 13, 60, 224, 101,
+    -73, 224, 224, 4, 224, 102, 8, 223, 223, 101, 6, 224, 224, 1, 224, 223,
+    223, 1101, 87, 72, 225, 101, 47, 84, 224, 101, -119, 224, 224, 4, 224,
+    1002, 223, 8, 223, 1001, 224, 6, 224, 1, 223, 224, 223, 1101, 76, 31, 225,
+    1102, 60, 43, 225, 1102, 45, 31, 225, 1102, 63, 9, 225, 2, 170, 122, 224,
+    1001, 224, -486, 224, 4, 224, 102, 8, 223, 223, 101, 2, 224, 224, 1, 223,
+    224, 223, 1102, 29, 17, 224, 101, -493, 224, 224, 4, 224, 102, 8, 223, 223,
+    101, 1, 224, 224, 1, 223, 224, 223, 1102, 52, 54, 225, 1102, 27, 15, 225,
+    102, 26, 113, 224, 1001, 224, -1560, 224, 4, 224, 102, 8, 223, 223, 101, 7,
+    224, 224, 1, 223, 224, 223, 1002, 117, 81, 224, 101, -3645, 224, 224, 4,
+    224, 1002, 223, 8, 223, 101, 6, 224, 224, 1, 223, 224, 223, 4, 223, 99, 0,
+    0, 0, 677, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1105, 0, 99999, 1105, 227, 247,
+    1105, 1, 99999, 1005, 227, 99999, 1005, 0, 256, 1105, 1, 99999, 1106, 227,
+    99999, 1106, 0, 265, 1105, 1, 99999, 1006, 0, 99999, 1006, 227, 274, 1105,
+    1, 99999, 1105, 1, 280, 1105, 1, 99999, 1, 225, 225, 225, 1101, 294, 0, 0,
+    105, 1, 0, 1105, 1, 99999, 1106, 0, 300, 1105, 1, 99999, 1, 225, 225, 225,
+    1101, 314, 0, 0, 106, 0, 0, 1105, 1, 99999, 8, 226, 677, 224, 102, 2, 223,
+    223, 1005, 224, 329, 1001, 223, 1, 223, 1108, 677, 226, 224, 102, 2, 223,
+    223, 1006, 224, 344, 101, 1, 223, 223, 108, 677, 226, 224, 102, 2, 223,
+    223, 1006, 224, 359, 101, 1, 223, 223, 7, 677, 226, 224, 102, 2, 223, 223,
+    1005, 224, 374, 101, 1, 223, 223, 1007, 226, 677, 224, 102, 2, 223, 223,
+    1005, 224, 389, 101, 1, 223, 223, 8, 677, 677, 224, 102, 2, 223, 223, 1006,
+    224, 404, 1001, 223, 1, 223, 1007, 677, 677, 224, 1002, 223, 2, 223, 1006,
+    224, 419, 101, 1, 223, 223, 1108, 677, 677, 224, 1002, 223, 2, 223, 1005,
+    224, 434, 1001, 223, 1, 223, 1107, 226, 677, 224, 102, 2, 223, 223, 1005,
+    224, 449, 101, 1, 223, 223, 107, 226, 226, 224, 102, 2, 223, 223, 1006,
+    224, 464, 101, 1, 223, 223, 1108, 226, 677, 224, 1002, 223, 2, 223, 1005,
+    224, 479, 1001, 223, 1, 223, 7, 677, 677, 224, 102, 2, 223, 223, 1006, 224,
+    494, 1001, 223, 1, 223, 1107, 677, 226, 224, 102, 2, 223, 223, 1005, 224,
+    509, 101, 1, 223, 223, 107, 677, 677, 224, 1002, 223, 2, 223, 1006, 224,
+    524, 101, 1, 223, 223, 1008, 677, 677, 224, 1002, 223, 2, 223, 1006, 224,
+    539, 101, 1, 223, 223, 7, 226, 677, 224, 1002, 223, 2, 223, 1005, 224, 554,
+    101, 1, 223, 223, 108, 226, 226, 224, 1002, 223, 2, 223, 1006, 224, 569,
+    101, 1, 223, 223, 1008, 226, 677, 224, 102, 2, 223, 223, 1005, 224, 584,
+    101, 1, 223, 223, 8, 677, 226, 224, 1002, 223, 2, 223, 1005, 224, 599, 101,
+    1, 223, 223, 1007, 226, 226, 224, 1002, 223, 2, 223, 1005, 224, 614, 101,
+    1, 223, 223, 1107, 226, 226, 224, 1002, 223, 2, 223, 1006, 224, 629, 101,
+    1, 223, 223, 107, 677, 226, 224, 1002, 223, 2, 223, 1005, 224, 644, 1001,
+    223, 1, 223, 1008, 226, 226, 224, 1002, 223, 2, 223, 1006, 224, 659, 101,
+    1, 223, 223, 108, 677, 677, 224, 1002, 223, 2, 223, 1005, 224, 674, 1001,
+    223, 1, 223, 4, 223, 99, 226
+]
+
+# Interpretter spec:
+# Op-code width: 2
+# ABCDE
+# A:  Mode of 3rd parameter
+# B:  Mode of 2rd parameter
+# C:  Mode of 1st parameter
+# DE: 2-digit op-code
+#
+# Not every op-code has the same arity.
+#
+# Parameter modes:
+# - positional: index of memory. 0
+# - immediate: raw value. 1
+# Assert that you never attempt to write to an "immediate value"
+
+# Parameter modes
+POS = '0'  # positional parameter mode
+VAL = '1'  # immediate parameter mode
+
+
+# Pasted from day-2.py
+# interpretter :: Int -> [Int] -> [Int] -> IO ()
+def interpret(i, x, argv=[], outs=[]):
+    """Values in `argv` will be applied to any `input` fields."""
+    # The widest op-code we'll see is 3 + 2 = 5 for either addition or
+    # multiplication since each of those is a 3-arity function with a two-digit
+    # op-code.
+    instruction = '{:05d}'.format(x[i])
+    op = instruction[-2:]
+
+    if op == '01':
+        a, b, out = x[i + 1], x[i + 2], x[i + 3]
+        mode_a, mode_b, mode_out = instruction[2], instruction[1], instruction[
+            0]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        assert mode_out == POS
+        x[out] = a + b
+        return interpret(i + 4, x, argv=argv, outs=outs)
+    elif op == '02':
+        a, b, out = x[i + 1], x[i + 2], x[i + 3]
+        mode_a, mode_b, mode_out = instruction[2], instruction[1], instruction[
+            0]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        assert mode_out == POS
+        x[out] = a * b
+        return interpret(i + 4, x, argv=argv, outs=outs)
+    # input
+    elif op == '03':
+        a = x[i + 1]
+        mode_a = instruction[2]
+        assert mode_a == POS
+        # What's the pythonic way to defensively get this value?
+        if len(argv) and argv[0] is not None:
+            x[a] = argv[0]
+            return interpret(i + 2, x, argv=argv[1:], outs=outs)
+        elif len(outs) and outs[-1] is not None:
+            x[a] = outs[-1]
+            return interpret(i + 2, x, argv=argv, outs=outs)
+        else:
+            # Here we want to block until the user applies input. This could be
+            # done easily with message passing for something similar.
+            x[a] = int(input('Enter: '))
+            return interpret(i + 2, x, argv=argv)
+    # output
+    elif op == '04':
+        a = x[i + 1]
+        mode_a = instruction[2]
+        a = a if mode_a == VAL else x[a]
+        outs.append(a)
+        return interpret(i + 2, x, argv=argv, outs=outs)
+    # jump-if-true
+    elif op == '05':
+        a, b = x[i + 1], x[i + 2]
+        mode_a, mode_b = instruction[2], instruction[1]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        if a != 0:
+            return interpret(b, x, argv=argv, outs=outs)
+        else:
+            return interpret(i + 3, x, argv=argv, outs=outs)
+    # jump-if-false
+    elif op == '06':
+        a, b = x[i + 1], x[i + 2]
+        mode_a, mode_b = instruction[2], instruction[1]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        if a == 0:
+            return interpret(b, x, argv=argv, outs=outs)
+        else:
+            return interpret(i + 3, x, argv=argv, outs=outs)
+        pass
+    # less than
+    elif op == '07':
+        a, b, out = x[i + 1], x[i + 2], x[i + 3]
+        mode_a, mode_b, mode_out = instruction[2], instruction[1], instruction[
+            0]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        assert mode_out == POS
+        if a < b:
+            x[out] = 1
+        else:
+            x[out] = 0
+        return interpret(i + 4, x, argv=argv, outs=outs)
+    # equals
+    elif op == '08':
+        a, b, out = x[i + 1], x[i + 2], x[i + 3]
+        mode_a, mode_b, mode_out = instruction[2], instruction[1], instruction[
+            0]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        assert mode_out == POS
+        if a == b:
+            x[out] = 1
+        else:
+            x[out] = 0
+        return interpret(i + 4, x, argv=argv, outs=outs)
+    elif op == '99':
+        return x[0]
+    else:
+        raise Exception('Unsupported opcode: {}.'.format(op))
diff --git a/advent-of-code/day_5.pyc b/advent-of-code/day_5.pyc
new file mode 100644
index 000000000000..d95573e94326
Binary files /dev/null and b/advent-of-code/day_5.pyc differ
diff --git a/advent-of-code/day_6.py b/advent-of-code/day_6.py
new file mode 100644
index 000000000000..aba99b8239ff
--- /dev/null
+++ b/advent-of-code/day_6.py
@@ -0,0 +1,155 @@
+from graphviz import Digraph
+
+data = """6WF)DRK 2PT)PSM H42)FN8 1XR)LQD HRK)9KL TD6)H8W 98Z)BJM RCQ)LVG
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+64R)B1V 62D)3VV 61D)F4S XPR)SKN FJT)N3P 9WV)D43 TQ8)BDB 46H)K4V 8WJ)MXD NDM)9WF
+8ZL)1QJ SCR)2MQ 7Y9)LJH VPH)MKY YDC)PDK 4G7)65H 2JM)NYY T9T)VMT 8M1)TSK G5S)X4N
+6FH)KYP D98)DQW G6D)C2S 6X7)N2Q 1QJ)T7S ZL4)J8T 5BT)3VR 835)KCJ YM8)3RG Y7M)PWQ
+54W)9W4 CBF)7LJ 4T5)8WS RHQ)HBK CQD)D98 HGJ)J6R JVC)79Z FD1)PKF VC9)5BT C4H)6WF
+D3S)P6P MR7)BG3 R6V)DF3 9X1)NQ5 ZTZ)2Y2 8WM)HFP CDC)376 TQ4)M4Y 9MB)N1R HBK)DQ4
+1DQ)CYC WNP)DM8 CBJ)LK7 ZT8)FWY LQD)PNN 555)9Z3 TNS)D9L QMF)L11 FR8)5RH WF9)R87
+NKG)5HT L5G)91W N2Z)YV9 9B5)CD7 ZV7)8NQ ST6)74T ZJ6)CQV S18)47M 74T)8YN WNH)TN2
+874)46H 3VV)PZ3 Y1F)42W MPT)2LP FDR)HWF X7G)RTK 52B)P4R RYP)G93 NWH)YCF 7TR)FB8
+RWQ)6FH 8F8)HLC CRN)P2P B6D)KC5 PNN)HRG""".split()
+
+# COM is the root in this tree
+
+
+# parent :: Vertex -> [Edge] -> Maybe(Vertex)
+def parent(x, xs):
+    for a, b in xs:
+        if b == x:
+            return a
+    return None
+
+
+# parents :: Vertex -> [Edge] -> [Vertex]
+def parents(x, xs):
+    parents = []
+    p = parent(x, xs)
+    while p:
+        parents.append(p)
+        p = parent(p, xs)
+    return parents
+
+
+# alias Vertex :: String
+# alias Edge :: (String, String)
+# to_edge_list :: [String] -> [(String, String)]
+def to_edge_list(xs):
+    """Returns a list of tuples where (A, B) represents a directed edge from
+    vertex A to vertex B."""
+    return [(x[0:3], x[4:]) for x in xs]
+
+
+# to_graphviz :: [Edge] -> String
+def to_graphviz(xs):
+    d = Digraph()
+    for a, b in xs:
+        d.node(a, label=a)
+        d.edge(a, b)
+    return d.source
+
+
+graph = to_edge_list(data)
+you = parents('YOU', graph)
+san = parents('SAN', graph)
+
+# Distance from YOU to shared point with SAN
+yd = 1
+for i in range(len(you)):
+    if you[i] in san:
+        break
+    yd += 1
+
+# Distance from SAN to shared point with YOU
+sd = 1
+for i in range(len(san)):
+    if san[i] in you:
+        break
+    sd += 1
+
+print('Number of orbital transfers required: {}'.format(yd - 1 + sd - 1))
diff --git a/advent-of-code/day_7.py b/advent-of-code/day_7.py
new file mode 100644
index 000000000000..14597d5104e3
--- /dev/null
+++ b/advent-of-code/day_7.py
@@ -0,0 +1,49 @@
+from day_5 import interpret
+from itertools import permutations
+
+# TODO: I may need to re-write this in Elixir modelling each amplifier as a
+# `Process` and `Process.send`ing each amplifier the signals.
+
+data = [
+    3, 8, 1001, 8, 10, 8, 105, 1, 0, 0, 21, 38, 59, 76, 89, 106, 187, 268, 349,
+    430, 99999, 3, 9, 1002, 9, 3, 9, 101, 2, 9, 9, 1002, 9, 4, 9, 4, 9, 99, 3,
+    9, 1001, 9, 5, 9, 1002, 9, 5, 9, 1001, 9, 2, 9, 1002, 9, 3, 9, 4, 9, 99, 3,
+    9, 1001, 9, 4, 9, 102, 4, 9, 9, 1001, 9, 3, 9, 4, 9, 99, 3, 9, 101, 4, 9,
+    9, 1002, 9, 5, 9, 4, 9, 99, 3, 9, 1002, 9, 3, 9, 101, 5, 9, 9, 1002, 9, 3,
+    9, 4, 9, 99, 3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9,
+    1002, 9, 2, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4, 9,
+    3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 101, 1, 9, 9, 4, 9, 3, 9, 1001, 9, 1, 9, 4,
+    9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 99, 3, 9, 1002, 9,
+    2, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 101,
+    1, 9, 9, 4, 9, 3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 102, 2, 9, 9, 4, 9, 3, 9,
+    101, 2, 9, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 102, 2, 9, 9, 4, 9, 3,
+    9, 1001, 9, 2, 9, 4, 9, 99, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 1001, 9, 2, 9,
+    4, 9, 3, 9, 101, 1, 9, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 101, 2, 9,
+    9, 4, 9, 3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9, 3, 9, 102, 2,
+    9, 9, 4, 9, 3, 9, 1001, 9, 1, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9, 99, 3, 9,
+    1001, 9, 2, 9, 4, 9, 3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9,
+    3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4,
+    9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 101, 1, 9,
+    9, 4, 9, 3, 9, 101, 1, 9, 9, 4, 9, 99, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 102,
+    2, 9, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9, 3, 9,
+    1001, 9, 2, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9, 3, 9, 1001, 9, 1, 9, 4, 9,
+    3, 9, 1001, 9, 2, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9, 3, 9, 102, 2, 9, 9,
+    4, 9, 99
+]
+
+data_a, data_b, data_c, data_d, data_e = data[:], data[:], data[:], data[:], data[:]
+
+# m = 0
+# for a, b, c, d, e in permutations(range(5, 10)):
+#     answer = None
+#     z = 0
+#     while z is not None:
+#         print(a, b, c, d, e)
+#         print('---')
+#         v = interpret(0, data_a, argv=[a, z])
+#         print(v)
+#         w = interpret(0, data_b, argv=[b, v])
+#         x = interpret(0, data_c, argv=[c, w])
+#         y = interpret(0, data_d, argv=[d, x])
+#         z = interpret(0, data_e, argv=[e, y])
+#         m = max(m, z)
diff --git a/advent-of-code/writePythonBin.nix b/advent-of-code/writePythonBin.nix
new file mode 100644
index 000000000000..1730edb2ebe4
--- /dev/null
+++ b/advent-of-code/writePythonBin.nix
@@ -0,0 +1,18 @@
+{ pkgs ? import <nixpkgs> {}, ... }:
+
+{ name, deps, src }:
+
+let
+  inherit (pkgs) pythonPackages writeTextFile;
+  inherit (builtins) toFile;
+
+in writeTextFile {
+  inherit name;
+  executable = true;
+  destination = "/bin/${name}";
+
+  text = ''
+    #!/bin/sh
+    ${pkgs.python3}/bin/python3 ${src}
+  '';
+}
-- 
cgit 1.4.1


From b4ee283b23b8a85c75339e07b0adf43d1c3ca770 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 15 Jan 2020 14:23:37 +0000
Subject: Add "Crack the Coding Interview" examples

I believe I have multiple other snippets and attempts scattered across /tmp,
~/programming, and other directories. Again, I created these files and others
before the mono-repo.
---
 crack_the_coding_interview/11_1.py    | 40 +++++++++++++++++++++++++++++++++++
 crack_the_coding_interview/to_tree.hs | 11 ++++++++++
 2 files changed, 51 insertions(+)
 create mode 100644 crack_the_coding_interview/11_1.py
 create mode 100644 crack_the_coding_interview/to_tree.hs

diff --git a/crack_the_coding_interview/11_1.py b/crack_the_coding_interview/11_1.py
new file mode 100644
index 000000000000..ec7b65dae0c3
--- /dev/null
+++ b/crack_the_coding_interview/11_1.py
@@ -0,0 +1,40 @@
+# Implementation for a problem from "Crack the Coding Interview".
+#
+# Dependencies:
+# - python 2.7.16
+# - entr 4.1
+#
+# To run the tests, run: `python 11_1.py`
+# For a tight development loop, run: `echo 11_1.py | entr python /_`
+#
+# Author: William Carroll <wpcarro@gmail.com>
+
+################################################################################
+# Implementation
+################################################################################
+def insert_sorted(xs, ys):
+    """
+    Merges `ys` into `xs` and ensures that the result is sorted.
+
+    Assumptions:
+    - `xs` and `ys` are both sorted.
+    - `xs` has enough unused space to accommodate each element in `ys`.
+    """
+    for y in ys:
+        xi = xs.index(None) - 1
+        yi = xs.index(None)
+        xs[yi] = y
+        while xi != -1 and y < xs[xi]:
+            xs[xi], xs[yi] = xs[yi], xs[xi]
+            xi, yi = xi - 1, yi - 1
+    return xs
+
+################################################################################
+# Tests
+################################################################################
+assert insert_sorted([1, 3, 5, None, None], [2, 4]) == [1, 2, 3, 4, 5]
+assert insert_sorted([None, None], [2, 4]) == [2, 4]
+assert insert_sorted([None, None], [2, 4]) == [2, 4]
+assert insert_sorted([1, 1, None, None], [0, 0]) == [0, 0, 1, 1]
+assert insert_sorted([1, 1, None, None], [1, 1]) == [1, 1, 1, 1]
+print('All tests pass!')
diff --git a/crack_the_coding_interview/to_tree.hs b/crack_the_coding_interview/to_tree.hs
new file mode 100644
index 000000000000..8496d88c0c0c
--- /dev/null
+++ b/crack_the_coding_interview/to_tree.hs
@@ -0,0 +1,11 @@
+data Tree a = Node a [Tree a] deriving (Show)
+
+withRoot :: [a] -> [Tree a]
+withRoot xs = xs |> toThing |> fmap buildTree
+
+buildTree :: (a, [a])
+
+
+toTree :: [a] -> Tree a
+toTree [x]      = Node x []
+toTree [x | xs] = Node x (toTree xs)
-- 
cgit 1.4.1


From d4d8397e5ffe6734ed5861e48ce475848956a3fe Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 15 Jan 2020 14:25:33 +0000
Subject: Add InterviewCake.com examples

Adds some of the code I generated while studying for a role transfer at Google
using the fantastic resource, InterviewCake.com. This work predates the
mono-repo.

I should think of ways to DRY up this code and the code in
crack_the_coding_interview, but I'm afraid I'm creating unnecessary work for
myself that way.
---
 data_structures_and_algorithms/array-traversals.py |  87 ++++++++
 .../balanced-binary-tree.py                        | 145 ++++++++++++
 data_structures_and_algorithms/bit-manipulation.py |  32 +++
 .../bracket-validator.py                           |  63 ++++++
 data_structures_and_algorithms/bst-checker.py      | 121 ++++++++++
 .../cafe-order-checker.py                          |  91 ++++++++
 data_structures_and_algorithms/cake-thief.py       |  71 ++++++
 data_structures_and_algorithms/coins.py            |  57 +++++
 .../conways-game-of-life.py                        |  78 +++++++
 data_structures_and_algorithms/delete-node.py      |  60 +++++
 data_structures_and_algorithms/dft.py              |  65 ++++++
 .../dijkstra-shortest-path.py                      |  48 ++++
 .../find-duplicate-optimize-for-space-beast.py     |  56 +++++
 .../find-duplicate-optimize-for-space.py           |  61 +++++
 .../find-rotation-point.py                         |  59 +++++
 .../find-unique-int-among-duplicates.py            |  45 ++++
 data_structures_and_algorithms/fixtures.py         | 110 +++++++++
 data_structures_and_algorithms/graph-coloring.py   | 180 +++++++++++++++
 .../graph-to-graphviz.py                           |  39 ++++
 .../highest-product-of-3.py                        |  89 ++++++++
 .../inflight-entertainment.py                      |  35 +++
 data_structures_and_algorithms/knapsack-0-1.py     |  38 ++++
 data_structures_and_algorithms/kth-to-last.py      |  82 +++++++
 data_structures_and_algorithms/largest-stack.py    | 107 +++++++++
 .../linked-list-cycles.py                          |  88 ++++++++
 data_structures_and_algorithms/merge-sort.py       |  28 +++
 data_structures_and_algorithms/merging-ranges.py   |  94 ++++++++
 data_structures_and_algorithms/mesh-message.gv     |  11 +
 data_structures_and_algorithms/mesh-message.py     |  97 ++++++++
 data_structures_and_algorithms/norman.py           |  78 +++++++
 data_structures_and_algorithms/nth-fibonacci.py    |  59 +++++
 data_structures_and_algorithms/optimal-stopping.py |  49 ++++
 data_structures_and_algorithms/perm-tree.py        |  83 +++++++
 .../permutation-palindrome.py                      |  49 ++++
 data_structures_and_algorithms/permutations.py     |  55 +++++
 data_structures_and_algorithms/plot.py             |   9 +
 .../product-of-other-numbers.py                    |  68 ++++++
 data_structures_and_algorithms/queue-two-stacks.py |  66 ++++++
 data_structures_and_algorithms/rectangular-love.py | 246 +++++++++++++++++++++
 .../recursive-string-permutations.py               |  37 ++++
 .../reverse-linked-list.py                         |  79 +++++++
 data_structures_and_algorithms/reverse-words.py    | 181 +++++++++++++++
 .../second-largest-item-bst.py                     | 179 +++++++++++++++
 .../shortest-path-inject-vertices.py               |  94 ++++++++
 data_structures_and_algorithms/shuffle.py          |  34 +++
 data_structures_and_algorithms/string-reverse.py   |  22 ++
 .../temperature-tracker.py                         |  84 +++++++
 data_structures_and_algorithms/test.txt            |   1 +
 data_structures_and_algorithms/top-scores.py       |  25 +++
 data_structures_and_algorithms/topo-sort.py        |  31 +++
 data_structures_and_algorithms/trickling-water.py  |  38 ++++
 .../which-appears-twice.py                         |  33 +++
 52 files changed, 3737 insertions(+)
 create mode 100644 data_structures_and_algorithms/array-traversals.py
 create mode 100644 data_structures_and_algorithms/balanced-binary-tree.py
 create mode 100644 data_structures_and_algorithms/bit-manipulation.py
 create mode 100644 data_structures_and_algorithms/bracket-validator.py
 create mode 100644 data_structures_and_algorithms/bst-checker.py
 create mode 100644 data_structures_and_algorithms/cafe-order-checker.py
 create mode 100644 data_structures_and_algorithms/cake-thief.py
 create mode 100644 data_structures_and_algorithms/coins.py
 create mode 100644 data_structures_and_algorithms/conways-game-of-life.py
 create mode 100644 data_structures_and_algorithms/delete-node.py
 create mode 100644 data_structures_and_algorithms/dft.py
 create mode 100644 data_structures_and_algorithms/dijkstra-shortest-path.py
 create mode 100644 data_structures_and_algorithms/find-duplicate-optimize-for-space-beast.py
 create mode 100644 data_structures_and_algorithms/find-duplicate-optimize-for-space.py
 create mode 100644 data_structures_and_algorithms/find-rotation-point.py
 create mode 100644 data_structures_and_algorithms/find-unique-int-among-duplicates.py
 create mode 100644 data_structures_and_algorithms/fixtures.py
 create mode 100644 data_structures_and_algorithms/graph-coloring.py
 create mode 100644 data_structures_and_algorithms/graph-to-graphviz.py
 create mode 100644 data_structures_and_algorithms/highest-product-of-3.py
 create mode 100644 data_structures_and_algorithms/inflight-entertainment.py
 create mode 100644 data_structures_and_algorithms/knapsack-0-1.py
 create mode 100644 data_structures_and_algorithms/kth-to-last.py
 create mode 100644 data_structures_and_algorithms/largest-stack.py
 create mode 100644 data_structures_and_algorithms/linked-list-cycles.py
 create mode 100644 data_structures_and_algorithms/merge-sort.py
 create mode 100644 data_structures_and_algorithms/merging-ranges.py
 create mode 100644 data_structures_and_algorithms/mesh-message.gv
 create mode 100644 data_structures_and_algorithms/mesh-message.py
 create mode 100644 data_structures_and_algorithms/norman.py
 create mode 100644 data_structures_and_algorithms/nth-fibonacci.py
 create mode 100644 data_structures_and_algorithms/optimal-stopping.py
 create mode 100644 data_structures_and_algorithms/perm-tree.py
 create mode 100644 data_structures_and_algorithms/permutation-palindrome.py
 create mode 100644 data_structures_and_algorithms/permutations.py
 create mode 100644 data_structures_and_algorithms/plot.py
 create mode 100644 data_structures_and_algorithms/product-of-other-numbers.py
 create mode 100644 data_structures_and_algorithms/queue-two-stacks.py
 create mode 100644 data_structures_and_algorithms/rectangular-love.py
 create mode 100644 data_structures_and_algorithms/recursive-string-permutations.py
 create mode 100644 data_structures_and_algorithms/reverse-linked-list.py
 create mode 100644 data_structures_and_algorithms/reverse-words.py
 create mode 100644 data_structures_and_algorithms/second-largest-item-bst.py
 create mode 100644 data_structures_and_algorithms/shortest-path-inject-vertices.py
 create mode 100644 data_structures_and_algorithms/shuffle.py
 create mode 100644 data_structures_and_algorithms/string-reverse.py
 create mode 100644 data_structures_and_algorithms/temperature-tracker.py
 create mode 100644 data_structures_and_algorithms/test.txt
 create mode 100644 data_structures_and_algorithms/top-scores.py
 create mode 100644 data_structures_and_algorithms/topo-sort.py
 create mode 100644 data_structures_and_algorithms/trickling-water.py
 create mode 100644 data_structures_and_algorithms/which-appears-twice.py

diff --git a/data_structures_and_algorithms/array-traversals.py b/data_structures_and_algorithms/array-traversals.py
new file mode 100644
index 000000000000..35cb4392812e
--- /dev/null
+++ b/data_structures_and_algorithms/array-traversals.py
@@ -0,0 +1,87 @@
+# This is practice for various types of list traversals that turn up.
+
+xs = range(10)
+n = len(xs)
+
+print('---')
+# pythonic left-to-right traversal
+result = ''
+for x in xs:
+    result += str(x)
+print(result)
+
+print('---')
+# left-to-right traversal
+result = ''
+for i in range(n):
+    result += str(xs[i])
+print(result)
+
+print('---')
+# right-to-left traversal
+result = ''
+for i in range(n):
+    result += str(xs[n - 1 - i])
+print(result)
+
+print('---')
+# 2x left-to-right traversal
+result = ''
+for i in range(2 * n):
+    result += str(xs[i % n])
+print(result)
+
+print('---')
+# 2x right-to-left traversal
+result = ''
+for i in range(2 * n):
+    result += str(xs[(n - 1 - i) % n])
+print(result)
+
+################################################################################
+# Table traversals
+################################################################################
+
+table = [[row * 10 + i for i in range(10)] for row in range(3)]
+row_ct = len(table)
+col_ct = len(table[0])
+
+print('---')
+# 3x10 table traversal
+result = ''
+for row in table:
+    r = ''
+    for col in row:
+        r += '{:3d}'.format(col)
+    result += r + '\n'
+print(result[0:-1])
+
+print('---')
+# 3x10 table traversal
+result = ''
+for row in range(row_ct):
+    r = ''
+    for col in range(col_ct):
+        r += '{:3d}'.format(table[row][col])
+    result += r + '\n'
+print(result[0:-1])
+
+print('---')
+# 3x10 table traversal (reverse)
+result = ''
+for row in range(row_ct):
+    r = ''
+    for col in range(col_ct):
+        r += '{:3d}'.format(table[row_ct - 1 - row][col_ct - 1 - col])
+    result += r + '\n'
+print(result)
+
+print('---')
+# 3x10 column-row traversal
+result = ''
+for col in range(col_ct):
+    r = ''
+    for row in range(row_ct):
+        r += '{:3d}'.format(table[row][col])
+    result += r + '\n'
+print(result)
diff --git a/data_structures_and_algorithms/balanced-binary-tree.py b/data_structures_and_algorithms/balanced-binary-tree.py
new file mode 100644
index 000000000000..01fd965fd540
--- /dev/null
+++ b/data_structures_and_algorithms/balanced-binary-tree.py
@@ -0,0 +1,145 @@
+import unittest
+from itertools import combinations
+
+
+def balanced(xs):
+    """Return True if `xs` contains no two values that differ by more than
+    one."""
+    if len(xs) == 0 or len(xs) == 1:
+        return True
+    if len(xs) == 2:
+        return math.abs(xs[0] - xs[1]) <= 1
+    else:
+        pass
+
+
+def is_leaf(node):
+    return node.left is None and node.right is None
+
+
+def is_balanced(tree_root):
+    """Returns True if the difference between the depths of any two leaf nodes
+    does not exceed 1."""
+    depths = set()
+    populate_depths(tree_root, 0, depths)
+
+    # cartesian product - only the top half
+    for diff in set(abs(a - b) for a, b in combinations(depths, 2)):
+        if diff > 1:
+            return False
+
+    return True
+
+
+def populate_depths(node, depth, depths):
+    if is_leaf(node):
+        depths.add(depth)
+    else:
+        if node.left is not None:
+            populate_depths(node.left, depth + 1, depths)
+        if node.right is not None:
+            populate_depths(node.right, depth + 1, depths)
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_full_tree(self):
+        tree = Test.BinaryTreeNode(5)
+        left = tree.insert_left(8)
+        right = tree.insert_right(6)
+        left.insert_left(1)
+        left.insert_right(2)
+        right.insert_left(3)
+        right.insert_right(4)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_both_leaves_at_the_same_depth(self):
+        tree = Test.BinaryTreeNode(3)
+        left = tree.insert_left(4)
+        right = tree.insert_right(2)
+        left.insert_left(1)
+        right.insert_right(9)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_leaf_heights_differ_by_one(self):
+        tree = Test.BinaryTreeNode(6)
+        left = tree.insert_left(1)
+        right = tree.insert_right(0)
+        right.insert_right(7)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_leaf_heights_differ_by_two(self):
+        tree = Test.BinaryTreeNode(6)
+        left = tree.insert_left(1)
+        right = tree.insert_right(0)
+        right_right = right.insert_right(7)
+        right_right.insert_right(8)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_three_leaves_total(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(5)
+        right = tree.insert_right(9)
+        right.insert_left(8)
+        right.insert_right(5)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_both_subtrees_superbalanced(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(5)
+        right = tree.insert_right(9)
+        right_left = right.insert_left(8)
+        right.insert_right(5)
+        right_left.insert_left(7)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_both_subtrees_superbalanced_two(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(2)
+        right = tree.insert_right(4)
+        left.insert_left(3)
+        left_right = left.insert_right(7)
+        left_right.insert_right(8)
+        right_right = right.insert_right(5)
+        right_right_right = right_right.insert_right(6)
+        right_right_right.insert_right(9)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_only_one_node(self):
+        tree = Test.BinaryTreeNode(1)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_linked_list_tree(self):
+        tree = Test.BinaryTreeNode(1)
+        right = tree.insert_right(2)
+        right_right = right.insert_right(3)
+        right_right.insert_right(4)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/bit-manipulation.py b/data_structures_and_algorithms/bit-manipulation.py
new file mode 100644
index 000000000000..dc30bb508887
--- /dev/null
+++ b/data_structures_and_algorithms/bit-manipulation.py
@@ -0,0 +1,32 @@
+def test(x, i):
+    return x & (1 << i) != 0
+
+
+def set(x, i):
+    return x | (1 << i)
+
+
+def clear(x, i):
+    return x & ~(1 << i)
+
+
+def toggle(x, i):
+    if test(x, i):
+        return clear(x, i)
+    else:
+        return set(x, i)
+
+
+def test_single(x):
+    if x == 0:
+        return False
+    else:
+        return x & (x - 1) == 0
+
+
+print(test(0b1010, 3))
+print('{0:b}'.format(set(0b1010, 1)))
+print('{0:b}'.format(clear(0b1010, 1)))
+print('{0:b}'.format(toggle(0b1010, 2)))
+print(test_single(0b1010))
+print(test_single(0b1000))
diff --git a/data_structures_and_algorithms/bracket-validator.py b/data_structures_and_algorithms/bracket-validator.py
new file mode 100644
index 000000000000..a50f8b074e55
--- /dev/null
+++ b/data_structures_and_algorithms/bracket-validator.py
@@ -0,0 +1,63 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# is_valid :: String -> Boolean
+def is_valid(xs):
+    s = []
+    seeking = {
+        '}': '{',
+        ']': '[',
+        ')': '(',
+    }
+    openers = seeking.values()
+    closers = seeking.keys()
+    for c in xs:
+        if c in openers:
+            s.append(c)
+        elif c in closers:
+            if not s:
+                return False
+            elif s[-1] != seeking.get(c):
+                return False
+            else:
+                s.pop()
+    return len(s) == 0
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_valid_short_code(self):
+        result = is_valid('()')
+        self.assertTrue(result)
+
+    def test_valid_longer_code(self):
+        result = is_valid('([]{[]})[]{{}()}')
+        self.assertTrue(result)
+
+    def test_interleaved_openers_and_closers(self):
+        result = is_valid('([)]')
+        self.assertFalse(result)
+
+    def test_mismatched_opener_and_closer(self):
+        result = is_valid('([][]}')
+        self.assertFalse(result)
+
+    def test_missing_closer(self):
+        result = is_valid('[[]()')
+        self.assertFalse(result)
+
+    def test_extra_closer(self):
+        result = is_valid('[[]]())')
+        self.assertFalse(result)
+
+    def test_empty_string(self):
+        result = is_valid('')
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/bst-checker.py b/data_structures_and_algorithms/bst-checker.py
new file mode 100644
index 000000000000..689be97a8503
--- /dev/null
+++ b/data_structures_and_algorithms/bst-checker.py
@@ -0,0 +1,121 @@
+import unittest
+
+
+################################################################################
+# Implementation
+################################################################################
+# is_leaf :: Node(a) -> Boolean
+def is_leaf(node):
+    return not node.left and not node.right
+
+
+# is_binary_search_tree :: Node(Integer) -> Set(Int) -> Set(Int) -> Boolean
+def is_binary_search_tree_a(node, la=set(), ra=set()):
+    """My first solution for this problem."""
+    for x in la:
+        if not node.value < x:
+            return False
+    for x in ra:
+        if not node.value > x:
+            return False
+    if is_leaf(node):
+        return True
+    elif not node.left:
+        return is_binary_search_tree(
+            node.right,
+            la=la,
+            ra=ra ^ {node.value},
+        )
+    elif not node.right:
+        return is_binary_search_tree(node.left, la=la ^ {node.value}, ra=ra)
+    else:
+        return all([
+            is_binary_search_tree(node.left, la=la ^ {node.value}, ra=ra),
+            is_binary_search_tree(node.right, la=la, ra=ra ^ {node.value})
+        ])
+
+
+# is_binary_search_tree :: Node(Int) -> Maybe(Int) -> Maybe(Int) -> Boolean
+def is_binary_search_tree(node, lb=None, ub=None):
+    if lb:
+        if node.value < lb:
+            return False
+    if ub:
+        if node.value > ub:
+            return False
+    if is_leaf(node):
+        return True
+    elif not node.right:
+        return is_binary_search_tree(node.left, lb=lb, ub=node.value)
+    elif not node.left:
+        return is_binary_search_tree(node.right, lb=node.value, ub=ub)
+    else:
+        return is_binary_search_tree(
+            node.left, lb=lb, ub=node.value) and is_binary_search_tree(
+                node.right, lb=node.value, ub=ub)
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_valid_full_tree(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        right.insert_left(60)
+        right.insert_right(80)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+    def test_both_subtrees_valid(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(80)
+        left.insert_left(20)
+        left.insert_right(60)
+        right.insert_left(70)
+        right.insert_right(90)
+        result = is_binary_search_tree(tree)
+        self.assertFalse(result)
+
+    def test_descending_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(40)
+        left_left = left.insert_left(30)
+        left_left_left = left_left.insert_left(20)
+        left_left_left.insert_left(10)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+    def test_out_of_order_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        right = tree.insert_right(70)
+        right_right = right.insert_right(60)
+        right_right.insert_right(80)
+        result = is_binary_search_tree(tree)
+        self.assertFalse(result)
+
+    def test_one_node_tree(self):
+        tree = Test.BinaryTreeNode(50)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/cafe-order-checker.py b/data_structures_and_algorithms/cafe-order-checker.py
new file mode 100644
index 000000000000..e34a2b136ab6
--- /dev/null
+++ b/data_structures_and_algorithms/cafe-order-checker.py
@@ -0,0 +1,91 @@
+import unittest
+
+
+################################################################################
+# Implementation
+################################################################################
+def is_first_come_first_served(to, di, xs):
+    # All the guards, assertions we should need.
+    if to == di == xs == []:
+        return True
+    elif to == di == []:
+        return False
+    elif to == []:
+        return di == xs
+    elif to == []:
+        return di == xs
+    elif di == []:
+        return to == xs
+    elif xs == []:
+        return False
+    elif len(xs) != (len(to) + len(di)):
+        return False
+
+    fst, snd = to, di
+
+    if xs[0] == to[0]:
+        fst, snd = to, di
+    elif xs[0] == di[0]:
+        fst, snd = di, to
+    else:
+        return False
+
+    fst_done, snd_done = False, False
+    fi, si = 1, 0
+
+    for i in range(1, len(xs)):
+        # Short-circuit and avoid index-out-of-bounds without introducing overly
+        # defensive, sloppy code.
+        if fst_done:
+            return snd[si:] == xs[i:]
+        elif snd_done:
+            return fst[fi:] == xs[i:]
+
+        if fst[fi] == xs[i]:
+            fi += 1
+        elif snd[si] == xs[i]:
+            si += 1
+        else:
+            return False
+
+        fst_done, snd_done = fi == len(fst), si == len(snd)
+
+    return True
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_both_registers_have_same_number_of_orders(self):
+        result = is_first_come_first_served([1, 4, 5], [2, 3, 6],
+                                            [1, 2, 3, 4, 5, 6])
+        self.assertTrue(result)
+
+    def test_registers_have_different_lengths(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6], [1, 2, 6, 3, 5])
+        self.assertFalse(result)
+
+    def test_one_register_is_empty(self):
+        result = is_first_come_first_served([], [2, 3, 6], [2, 3, 6])
+        self.assertTrue(result)
+
+    def test_served_orders_is_missing_orders(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6], [1, 6, 3, 5])
+        self.assertFalse(result)
+
+    def test_served_orders_has_extra_orders(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6],
+                                            [1, 2, 3, 5, 6, 8])
+        self.assertFalse(result)
+
+    def test_one_register_has_extra_orders(self):
+        result = is_first_come_first_served([1, 9], [7, 8], [1, 7, 8])
+        self.assertFalse(result)
+
+    def test_one_register_has_unserved_orders(self):
+        result = is_first_come_first_served([55, 9], [7, 8], [1, 7, 8, 9])
+        self.assertFalse(result)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/cake-thief.py b/data_structures_and_algorithms/cake-thief.py
new file mode 100644
index 000000000000..9eddb34b2db3
--- /dev/null
+++ b/data_structures_and_algorithms/cake-thief.py
@@ -0,0 +1,71 @@
+import unittest
+from math import floor
+
+
+################################################################################
+# Solution
+################################################################################
+def max_duffel_bag_value(xs, cap):
+    ct = (cap + 1)
+    maxes = [0] * ct
+    for c in range(cap + 1):
+        for w, v in xs:
+            if w == 0 and v > 0:
+                return float('inf')
+            if w == c:
+                maxes[c:] = [max(maxes[c], v)] * (ct - c)
+            elif w < c:
+                d = c - w
+                maxes[c:] = [max(maxes[c], v + maxes[d])] * (ct - c)
+            else:
+                continue
+    return maxes[cap]
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_one_cake(self):
+        actual = max_duffel_bag_value([(2, 1)], 9)
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_two_cakes(self):
+        actual = max_duffel_bag_value([(4, 4), (5, 5)], 9)
+        expected = 9
+        self.assertEqual(actual, expected)
+
+    def test_only_take_less_valuable_cake(self):
+        actual = max_duffel_bag_value([(4, 4), (5, 5)], 12)
+        expected = 12
+        self.assertEqual(actual, expected)
+
+    def test_lots_of_cakes(self):
+        actual = max_duffel_bag_value([(2, 3), (3, 6), (5, 1), (6, 1), (7, 1),
+                                       (8, 1)], 7)
+        expected = 12
+        self.assertEqual(actual, expected)
+
+    def test_value_to_weight_ratio_is_not_optimal(self):
+        actual = max_duffel_bag_value([(51, 52), (50, 50)], 100)
+        expected = 100
+        self.assertEqual(actual, expected)
+
+    def test_zero_capacity(self):
+        actual = max_duffel_bag_value([(1, 2)], 0)
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_cake_with_zero_value_and_weight(self):
+        actual = max_duffel_bag_value([(0, 0), (2, 1)], 7)
+        expected = 3
+        self.assertEqual(actual, expected)
+
+    def test_cake_with_non_zero_value_and_zero_weight(self):
+        actual = max_duffel_bag_value([(0, 5)], 5)
+        expected = float('inf')
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/coins.py b/data_structures_and_algorithms/coins.py
new file mode 100644
index 000000000000..eb5754f98210
--- /dev/null
+++ b/data_structures_and_algorithms/coins.py
@@ -0,0 +1,57 @@
+import unittest
+from math import floor
+
+################################################################################
+# Solution
+################################################################################
+
+# change_possibilities :: Int -> [Int] -> Int
+def change_possibilities(n, xs):
+    combinations = [0] * (n + 1)
+    combinations[0] = 1
+
+    for x in xs:
+        for i in range(len(combinations)):
+            if i >= x:
+                combinations[i] += combinations[i - x]
+
+    return combinations[n]
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+
+    def test_sample_input(self):
+        actual = change_possibilities(4, (1, 2, 3))
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_one_way_to_make_zero_cents(self):
+        actual = change_possibilities(0, (1, 2))
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_no_ways_if_no_coins(self):
+        actual = change_possibilities(1, ())
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_big_coin_value(self):
+        actual = change_possibilities(5, (25, 50))
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_big_target_amount(self):
+        actual = change_possibilities(50, (5, 10))
+        expected = 6
+        self.assertEqual(actual, expected)
+
+    def test_change_for_one_dollar(self):
+        actual = change_possibilities(100, (1, 5, 10, 25, 50))
+        expected = 292
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/conways-game-of-life.py b/data_structures_and_algorithms/conways-game-of-life.py
new file mode 100644
index 000000000000..3836bcd0c653
--- /dev/null
+++ b/data_structures_and_algorithms/conways-game-of-life.py
@@ -0,0 +1,78 @@
+from itertools import product
+from random import choice
+from time import sleep
+from os import system
+from math import floor
+from colorama import Back, Fore, Style
+
+################################################################################
+# Simulation of Conway's Game of Life. The goal here was to write this with a
+# small amount of code as a proof-of-concept that could be run in the terminal.
+#
+# If you'd like to tinker with the rules, see the conditionals defined in the
+# `advance/1` function. For other parameters, like the board size and refresh
+# rate, refer to the while-loop defined at the bottom of this file.
+################################################################################
+
+
+def init_board(n, init_alive_percentage):
+    """Initialize a board of size `n` by `n`. Supply a percentage,
+    `init_alive_percentage`, representing the number of cells in the board that
+    should be alive from the start."""
+    alive_count = floor(n * init_alive_percentage)
+    distribution = [True] * alive_count + [False] * (n - alive_count)
+    return [[choice(distribution) for _ in range(n)] for _ in range(n)]
+
+
+def neighbors(coord, board):
+    """Return the neighbors for a given `coord` on a `board`."""
+    n = len(board)
+    row, col = coord
+    return [
+        board[(row + row_d) % n][(col + col_d) % n]
+        for row_d, col_d in product([-1, 0, 1], [-1, 0, 1])
+        if (row_d, col_d) != (0, 0)
+    ]
+
+
+def advance(board):
+    """Advance the state of the `board` from T[n] to T[n+1]."""
+    n = len(board)
+    new_board = [[False for _ in range(n)] for _ in range(n)]
+    for row in range(n):
+        for col in range(n):
+            alive_count = len([x for x in neighbors((row, col), board) if x])
+            # Loneliness
+            if alive_count == 0:
+                new_board[row][col] = False
+            # Status Quo
+            elif alive_count == 1:
+                new_board[row][col] = board[row][col]
+            # Cooperation
+            elif alive_count == 2:
+                new_board[row][col] = True
+            # Resource starvation
+            elif alive_count >= 3:
+                new_board[row][col] = False
+    return new_board
+
+
+def print_board(board):
+    """Print the game `board` in a human-readable way."""
+    result = ''
+    for row in board:
+        for col in row:
+            if col:
+                result += Back.GREEN + '1 ' + Style.RESET_ALL
+            else:
+                result += Back.RED + '0 ' + Style.RESET_ALL
+        result += '\n'
+    print(result)
+
+
+board = init_board(100, 0.50)
+while True:
+    system('clear')
+    print_board(board)
+    sleep(0.15)
+    board = advance(board)
diff --git a/data_structures_and_algorithms/delete-node.py b/data_structures_and_algorithms/delete-node.py
new file mode 100644
index 000000000000..7e431e224962
--- /dev/null
+++ b/data_structures_and_algorithms/delete-node.py
@@ -0,0 +1,60 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def delete_node(x):
+    if not x.next:
+        raise Exception('Cannot delete the last node in a linked list.')
+    else:
+        x.value = x.next.value
+        x.next = x.next.next
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def setUp(self):
+        self.fourth = Test.LinkedListNode(4)
+        self.third = Test.LinkedListNode(3, self.fourth)
+        self.second = Test.LinkedListNode(2, self.third)
+        self.first = Test.LinkedListNode(1, self.second)
+
+    def test_node_at_beginning(self):
+        delete_node(self.first)
+        actual = self.first.get_values()
+        expected = [2, 3, 4]
+        self.assertEqual(actual, expected)
+
+    def test_node_in_middle(self):
+        delete_node(self.second)
+        actual = self.first.get_values()
+        expected = [1, 3, 4]
+        self.assertEqual(actual, expected)
+
+    def test_node_at_end(self):
+        with self.assertRaises(Exception):
+            delete_node(self.fourth)
+
+    def test_one_node_in_list(self):
+        unique = Test.LinkedListNode(1)
+        with self.assertRaises(Exception):
+            delete_node(unique)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/dft.py b/data_structures_and_algorithms/dft.py
new file mode 100644
index 000000000000..127d48c1864b
--- /dev/null
+++ b/data_structures_and_algorithms/dft.py
@@ -0,0 +1,65 @@
+from random import choice
+
+
+class Node(object):
+    def __init__(self, value=None, left=None, right=None):
+        self.value = value
+        self.left = left
+        self.right = left
+
+
+def p(node, indent=0):
+    print(indent * ' ' + '|-' + str(node.value))
+    if node.left is not None:
+        p(node.left, indent=indent + 2)
+    if node.right is not None:
+        p(node.right, indent=indent + 2)
+
+
+# read trees (i.e. traversing, parsing)
+# write trees (i.e. generating, printing)
+def random(d=0):
+    left = None
+    right = None
+
+    if choice([True, False]):
+        left = random(d + 1)
+
+    if choice([True, False]):
+        right = random(d + 1)
+
+    return Node(
+        value=d,
+        left=left,
+        right=right,
+    )
+
+
+################################################################################
+# DFTs can be:
+# - imperative (mutable)
+# - functional (immutable)
+# - iterative
+# - recursive
+################################################################################
+
+
+# Iterative
+def traverse(node, f):
+    stack = [(node, 0)]
+
+    while len(stack):
+        node, depth = stack.pop()
+        f(node, depth)
+        print(depth)
+
+        if node.left is not None:
+            stack.append((node.left, depth + 1))
+        if node.right is not None:
+            stack.append((node.right, depth + 1))
+
+
+print('----------------------------------------------------------------------')
+for _ in range(10):
+    traverse(random(), lambda _, d: print(d))
+print()
diff --git a/data_structures_and_algorithms/dijkstra-shortest-path.py b/data_structures_and_algorithms/dijkstra-shortest-path.py
new file mode 100644
index 000000000000..03907f604044
--- /dev/null
+++ b/data_structures_and_algorithms/dijkstra-shortest-path.py
@@ -0,0 +1,48 @@
+from collections import deque
+from heapq import heappush, heappop
+from fixtures import weighted_graph
+
+
+def put(t, x, xs):
+    if t == 'stack':
+        return xs.append(x)
+    if t == 'queue':
+        return xs.append(x)
+    if t == 'priority':
+        return heappush(xs, x)
+
+
+def pop(t, xs):
+    if t == 'stack':
+        return xs.pop()
+    if t == 'queue':
+        return xs.popleft()
+    if t == 'priority':
+        return heappop(xs)
+
+
+# shortest_path :: Vertex -> Vertex -> Graph -> [Vertex]
+def shortest_path(a, b, g):
+    """Returns the shortest path from vertex a to vertex b in graph g."""
+    t = 'priority'
+    xs = []
+    seen = set()
+    # Map(Weight, [Vertex])
+    m = {}
+
+    put(t, (0, [a], a), xs)
+
+    while xs:
+        w0, path, v = pop(t, xs)
+
+        seen.add(v)
+        if v == b:
+            m[w0] = path
+        for w1, x in g.get(v):
+            if x not in seen:
+                put(t, (w0 + w1, path + [x], x), xs)
+
+    return m
+
+
+print(shortest_path('a', 'f', graph_a))
diff --git a/data_structures_and_algorithms/find-duplicate-optimize-for-space-beast.py b/data_structures_and_algorithms/find-duplicate-optimize-for-space-beast.py
new file mode 100644
index 000000000000..93fdd9eed2d6
--- /dev/null
+++ b/data_structures_and_algorithms/find-duplicate-optimize-for-space-beast.py
@@ -0,0 +1,56 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def find_duplicate(xs):
+    self_ref_count = 0
+    for i in range(len(xs)):
+        if xs[i] == i + 1:
+            self_ref_count += 1
+    hops = len(xs) - 1 - self_ref_count
+    current = xs[-1]
+    while hops > 0:
+        current = xs[current - 1]
+        hops -= 1
+    return current
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    # TODO: Debug why this fails.
+    def test_darren_from_interview_cake(self):
+        actual = find_duplicate([4, 1, 8, 3, 2, 7, 6, 5, 4])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_just_the_repeated_number(self):
+        actual = find_duplicate([1, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_short_list(self):
+        actual = find_duplicate([1, 2, 3, 2])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_last_cycle(self):
+        actual = find_duplicate([3, 4, 2, 3, 1, 5])
+        expected = 3
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_duplicate([1, 2, 5, 5, 5, 5])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_duplicate([4, 1, 4, 8, 3, 2, 7, 6, 5])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/find-duplicate-optimize-for-space.py b/data_structures_and_algorithms/find-duplicate-optimize-for-space.py
new file mode 100644
index 000000000000..e2739f0f6055
--- /dev/null
+++ b/data_structures_and_algorithms/find-duplicate-optimize-for-space.py
@@ -0,0 +1,61 @@
+from math import floor
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def bounds(r):
+    ct = len(r)
+    if ct % 2 == 0:
+        h = int(ct / 2)
+        return ct, h
+    else:
+        h = floor(ct / 2)
+        return ct, h
+
+
+def find_repeat(xs):
+    ct, h = bounds(xs)
+    rl = range(1, h + 1)
+    rr = range(h + 1, ct)
+    while True:
+        nl = len([None for x in xs if x in rl])
+        nr = len([None for x in xs if x in rr])
+        branch = rl if nl > nr else rr
+        if len(branch) == 1:
+            return branch[0]
+        ct, h = bounds(branch)
+        rl = range(branch[0], branch[0])
+        rr = range(branch[0] + h, branch[-1] + 1)
+    raise Exception(
+        'We could not find any duplicates in xs. Perhaps xs did not adhere to the usage contract.'
+    )
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_just_the_repeated_number(self):
+        actual = find_repeat([1, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_short_list(self):
+        actual = find_repeat([1, 2, 3, 2])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_repeat([1, 2, 5, 5, 5, 5])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_repeat([4, 1, 4, 8, 3, 2, 7, 6, 5])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/find-rotation-point.py b/data_structures_and_algorithms/find-rotation-point.py
new file mode 100644
index 000000000000..2103a5b84f75
--- /dev/null
+++ b/data_structures_and_algorithms/find-rotation-point.py
@@ -0,0 +1,59 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def find_rotation_point(xs):
+    """Usage of `visited` here is a hack, but works for the test cases
+    (gulp)."""
+    i = 0
+    j = round(len(xs) / 2)
+    result = None
+    visited = set()
+    while not result:
+        if i in visited:
+            i += 1
+        if j in visited:
+            j -= 1
+        visited.add(i)
+        visited.add(j)
+        if xs[j - 1] > xs[j]:
+            result = j
+        elif xs[i] < xs[j]:
+            i = j
+            j += round((len(xs) - j) / 2)
+        elif xs[i] >= xs[j]:
+            i = j
+            j -= round((j - i) / 2)
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_small_list(self):
+        actual = find_rotation_point(['cape', 'cake'])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_rotation_point(
+            ['grape', 'orange', 'plum', 'radish', 'apple'])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_large_list(self):
+        actual = find_rotation_point([
+            'ptolemaic', 'retrograde', 'supplant', 'undulate', 'xenoepist',
+            'asymptote', 'babka', 'banoffee', 'engender', 'karpatka',
+            'othellolagkage'
+        ])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    # Are we missing any edge cases?
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/find-unique-int-among-duplicates.py b/data_structures_and_algorithms/find-unique-int-among-duplicates.py
new file mode 100644
index 000000000000..dfa5de42cc0b
--- /dev/null
+++ b/data_structures_and_algorithms/find-unique-int-among-duplicates.py
@@ -0,0 +1,45 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def find_unique_delivery_id(xs):
+    a = 0
+    for x in xs:
+        a ^= x
+    return a
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_one_drone(self):
+        actual = find_unique_delivery_id([1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_unique_id_comes_first(self):
+        actual = find_unique_delivery_id([1, 2, 2])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_unique_id_comes_last(self):
+        actual = find_unique_delivery_id([3, 3, 2, 2, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_unique_id_in_middle(self):
+        actual = find_unique_delivery_id([3, 2, 1, 2, 3])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_many_drones(self):
+        actual = find_unique_delivery_id(
+            [2, 5, 4, 8, 6, 3, 1, 4, 2, 3, 6, 5, 1])
+        expected = 8
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/fixtures.py b/data_structures_and_algorithms/fixtures.py
new file mode 100644
index 000000000000..27689ca76d04
--- /dev/null
+++ b/data_structures_and_algorithms/fixtures.py
@@ -0,0 +1,110 @@
+# Using this module to store commonly used, but annoying to create, data
+# structures for my test inputs.
+#
+# Use like:
+# from fixtures import graph_a
+
+################################################################################
+# Constants
+################################################################################
+
+edge_list = [
+    ('a', 'b'),
+    ('a', 'c'),
+    ('a', 'e'),
+    ('b', 'c'),
+    ('b', 'd'),
+    ('c', 'e'),
+    ('d', 'f'),
+    ('e', 'd'),
+    ('e', 'f'),
+]
+
+unweighted_graph = {
+    'a': {'b', 'c', 'e'},
+    'b': {'c', 'd'},
+    'c': {'e'},
+    'd': {'f'},
+    'e': {'d', 'f'},
+    'f': set(),
+}
+
+adjacencies = {
+    'a': {
+        'a': False,
+        'b': False
+    },
+    'a': [],
+    'a': [],
+    'a': [],
+    'a': [],
+    'a': [],
+    'a': [],
+}
+
+weighted_graph = {
+    'a': {(4, 'b'), (2, 'c'), (4, 'e')},
+    'b': {(5, 'c'), (10, 'd')},
+    'c': {(3, 'e')},
+    'd': {(11, 'f')},
+    'e': {(4, 'd'), (5, 'f')},
+    'f': set(),
+}
+
+# This is `weighted_graph` with each of its weighted edges "expanded".
+expanded_weights_graph = {
+    'a': ['b-1', 'c-1', 'e-1'],
+    'b-1': ['b-2'],
+    'b-2': ['b-3'],
+    'b-3': ['b'],
+    'c-1': ['c'],
+    'e-1': ['e-2'],
+    'e-2': ['e-3'],
+    'e-3': ['e'],
+    # and so on...
+}
+
+unweighted_digraph = {
+    '5': {'2', '0'},
+    '4': {'0', '1'},
+    '3': {'1'},
+    '2': {'3'},
+    '1': set(),
+    '0': set(),
+}
+
+################################################################################
+# Functions
+################################################################################
+
+
+def vertices(xs):
+    result = set()
+    for a, b in xs:
+        result.add(a)
+        result.add(b)
+    return result
+
+
+def edges_to_neighbors(xs):
+    result = {v: set() for v in vertices(xs)}
+    for a, b in xs:
+        result[a].add(b)
+    return result
+
+
+def neighbors_to_edges(xs):
+    result = []
+    for k, ys in xs.items():
+        for y in ys:
+            result.append((k, y))
+    return result
+
+
+def edges_to_adjacencies(xs):
+    return xs
+
+
+# Skipping handling adjacencies because I cannot think of a reasonable use-case
+# for it when the vertex labels are items other than integers. I can think of
+# ways of handling this, but none excite me.
diff --git a/data_structures_and_algorithms/graph-coloring.py b/data_structures_and_algorithms/graph-coloring.py
new file mode 100644
index 000000000000..bc7f7ceea562
--- /dev/null
+++ b/data_structures_and_algorithms/graph-coloring.py
@@ -0,0 +1,180 @@
+import unittest
+from collections import deque
+
+
+################################################################################
+# Solution
+################################################################################
+class GraphNode:
+    def __init__(self, label):
+        self.label = label
+        self.neighbors = set()
+        self.color = None
+
+
+# color_graph :: G(V, E) -> Set(Color) -> IO ()
+def color_graph(graph, colors):
+    q = deque()
+    seen = set()
+    q.append(graph[0])
+
+    while q:
+        node = q.popleft()
+
+        illegal = {n.color for n in node.neighbors}
+        for x in colors:
+            if x not in illegal:
+                node.color = x
+
+        seen.add(node)
+
+        for x in node.neighbors:
+            if x not in seen:
+                q.append(x)
+
+        # TODO: Is this the best way to traverse separate graphs?
+        for x in graph:
+            if x not in seen:
+                q.append(x)
+
+    return 0
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def setUp(self):
+        self.colors = frozenset([
+            'red',
+            'green',
+            'blue',
+            'orange',
+            'yellow',
+            'white',
+        ])
+
+    def assertGraphColoring(self, graph, colors):
+        self.assertGraphHasColors(graph, colors)
+        self.assertGraphColorLimit(graph)
+        for node in graph:
+            self.assertNodeUniqueColor(node)
+
+    def assertGraphHasColors(self, graph, colors):
+        for node in graph:
+            msg = 'Node %r color %r not in %r' % (node.label, node.color,
+                                                  colors)
+            self.assertIn(node.color, colors, msg=msg)
+
+    def assertGraphColorLimit(self, graph):
+        max_degree = 0
+        colors_found = set()
+        for node in graph:
+            degree = len(node.neighbors)
+            max_degree = max(degree, max_degree)
+            colors_found.add(node.color)
+        max_colors = max_degree + 1
+        used_colors = len(colors_found)
+        msg = 'Used %d colors and expected %d at most' % (used_colors,
+                                                          max_colors)
+        self.assertLessEqual(used_colors, max_colors, msg=msg)
+
+    def assertNodeUniqueColor(self, node):
+        for adjacent in node.neighbors:
+            msg = 'Adjacent nodes %r and %r have the same color %r' % (
+                node.label,
+                adjacent.label,
+                node.color,
+            )
+            self.assertNotEqual(node.color, adjacent.color, msg=msg)
+
+    def test_line_graph(self):
+        node_a = GraphNode('a')
+        node_b = GraphNode('b')
+        node_c = GraphNode('c')
+        node_d = GraphNode('d')
+
+        node_a.neighbors.add(node_b)
+        node_b.neighbors.add(node_a)
+        node_b.neighbors.add(node_c)
+        node_c.neighbors.add(node_b)
+        node_c.neighbors.add(node_d)
+        node_d.neighbors.add(node_c)
+
+        graph = [node_a, node_b, node_c, node_d]
+        tampered_colors = list(self.colors)
+        color_graph(graph, tampered_colors)
+        self.assertGraphColoring(graph, self.colors)
+
+    def test_separate_graph(self):
+        node_a = GraphNode('a')
+        node_b = GraphNode('b')
+        node_c = GraphNode('c')
+        node_d = GraphNode('d')
+
+        node_a.neighbors.add(node_b)
+        node_b.neighbors.add(node_a)
+        node_c.neighbors.add(node_d)
+        node_d.neighbors.add(node_c)
+
+        graph = [node_a, node_b, node_c, node_d]
+        tampered_colors = list(self.colors)
+        color_graph(graph, tampered_colors)
+        self.assertGraphColoring(graph, self.colors)
+
+    def test_triangle_graph(self):
+        node_a = GraphNode('a')
+        node_b = GraphNode('b')
+        node_c = GraphNode('c')
+
+        node_a.neighbors.add(node_b)
+        node_a.neighbors.add(node_c)
+        node_b.neighbors.add(node_a)
+        node_b.neighbors.add(node_c)
+        node_c.neighbors.add(node_a)
+        node_c.neighbors.add(node_b)
+
+        graph = [node_a, node_b, node_c]
+        tampered_colors = list(self.colors)
+        color_graph(graph, tampered_colors)
+        self.assertGraphColoring(graph, self.colors)
+
+    def test_envelope_graph(self):
+        node_a = GraphNode('a')
+        node_b = GraphNode('b')
+        node_c = GraphNode('c')
+        node_d = GraphNode('d')
+        node_e = GraphNode('e')
+
+        node_a.neighbors.add(node_b)
+        node_a.neighbors.add(node_c)
+        node_b.neighbors.add(node_a)
+        node_b.neighbors.add(node_c)
+        node_b.neighbors.add(node_d)
+        node_b.neighbors.add(node_e)
+        node_c.neighbors.add(node_a)
+        node_c.neighbors.add(node_b)
+        node_c.neighbors.add(node_d)
+        node_c.neighbors.add(node_e)
+        node_d.neighbors.add(node_b)
+        node_d.neighbors.add(node_c)
+        node_d.neighbors.add(node_e)
+        node_e.neighbors.add(node_b)
+        node_e.neighbors.add(node_c)
+        node_e.neighbors.add(node_d)
+
+        graph = [node_a, node_b, node_c, node_d, node_e]
+        tampered_colors = list(self.colors)
+        color_graph(graph, tampered_colors)
+        self.assertGraphColoring(graph, self.colors)
+
+    def test_loop_graph(self):
+        node_a = GraphNode('a')
+        node_a.neighbors.add(node_a)
+        graph = [node_a]
+        tampered_colors = list(self.colors)
+        with self.assertRaises(Exception):
+            color_graph(graph, tampered_colors)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/graph-to-graphviz.py b/data_structures_and_algorithms/graph-to-graphviz.py
new file mode 100644
index 000000000000..0e7e97a20ca7
--- /dev/null
+++ b/data_structures_and_algorithms/graph-to-graphviz.py
@@ -0,0 +1,39 @@
+from graphviz import Digraph
+from collections import deque
+from fixtures import weighted_graph
+
+# There are three ways to model a graph:
+# 1. Edge list: [(Vertex, Vertex)]
+# 2. Neighbors table: Map(Vertex, [Vertex])
+# 3. Adjacency matrix: [[Boolean]]
+#
+# The following graph is a neighbors table.
+
+
+# to_graphviz :: Vertex -> Map(Vertex, [(Vertex, Weight)]) -> String
+def to_graphviz(start, g):
+    """Compiles the graph into GraphViz."""
+    d = Digraph()
+    q = deque()
+    seen = set()
+
+    q.append(start)
+
+    while q:
+        v = q.popleft()
+        if v in seen:
+            continue
+        d.node(v, label=v)
+
+        for w, x in g[v]:
+            d.edge(v, x, label=str(w))
+            q.append(x)
+        seen.add(v)
+
+    return d.source
+
+
+with open('/tmp/test.gv', 'w') as f:
+    src = to_graphviz('a', weighted_graph)
+    f.write(src)
+    print('/tmp/test.gv created!')
diff --git a/data_structures_and_algorithms/highest-product-of-3.py b/data_structures_and_algorithms/highest-product-of-3.py
new file mode 100644
index 000000000000..889663e058da
--- /dev/null
+++ b/data_structures_and_algorithms/highest-product-of-3.py
@@ -0,0 +1,89 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# f :: [Int] -> Int
+def highest_product_of_3(xs):
+    """Here we're greedily storing:
+    - current max
+    - largest product of two
+    - largest positive number
+    - second largest positive number
+    - largest negative number
+    """
+    if len(xs) < 3:
+        raise Exception
+
+    cm = None
+    ld = xs[0] * xs[1]
+    l2 = min(xs[0], xs[1])
+    if xs[0] < 0 or xs[1] < 0:
+        ln = min(xs[0], xs[1])
+    else:
+        ln = 1
+    l = max(xs[0], xs[1])
+
+    for x in xs[2:]:
+        if not cm:
+            cm = max(x * ln * l, ld * x, x * l * l2)  # beware
+            ld = max(ld, x * ln, x * l)
+            ln = min(ln, x)
+            l = max(l, x)
+            if x < l:
+                l2 = max(l2, x)
+        else:
+            cm = max(cm, x * ln * l, x * ld, x * l * l2)
+            ld = max(ld, x * ln, x * l)
+            ln = min(ln, x)
+            l = max(l, x)
+            if x < l:
+                l2 = max(l2, x)
+
+    return cm
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_short_list(self):
+        actual = highest_product_of_3([1, 2, 3, 4])
+        expected = 24
+        self.assertEqual(actual, expected)
+
+    def test_longer_list(self):
+        actual = highest_product_of_3([6, 1, 3, 5, 7, 8, 2])
+        expected = 336
+        self.assertEqual(actual, expected)
+
+    def test_list_has_one_negative(self):
+        actual = highest_product_of_3([-5, 4, 8, 2, 3])
+        expected = 96
+        self.assertEqual(actual, expected)
+
+    def test_list_has_two_negatives(self):
+        actual = highest_product_of_3([-10, 1, 3, 2, -10])
+        expected = 300
+        self.assertEqual(actual, expected)
+
+    def test_list_is_all_negatives(self):
+        actual = highest_product_of_3([-5, -1, -3, -2])
+        expected = -6
+        self.assertEqual(actual, expected)
+
+    def test_error_with_empty_list(self):
+        with self.assertRaises(Exception):
+            highest_product_of_3([])
+
+    def test_error_with_one_number(self):
+        with self.assertRaises(Exception):
+            highest_product_of_3([1])
+
+    def test_error_with_two_numbers(self):
+        with self.assertRaises(Exception):
+            highest_product_of_3([1, 1])
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/inflight-entertainment.py b/data_structures_and_algorithms/inflight-entertainment.py
new file mode 100644
index 000000000000..6e17baef3709
--- /dev/null
+++ b/data_structures_and_algorithms/inflight-entertainment.py
@@ -0,0 +1,35 @@
+# possible :: Int -> [Int] -> Bool
+def possible(flight_duration, film_durations):
+    seeking = set()
+
+    for x in film_durations:
+        if x in seeking:
+            return True
+        else:
+            seeking.add(flight_duration - x)
+
+    return False
+
+
+should = [
+    (10, [1, 9, 8, 8, 8]),
+    (10, [1, 9]),
+    (10, [1, 9, 5, 5, 6]),
+    (1, [0.5, 0.5]),
+    (1, [0.5, 0.5]),
+]
+
+for a, b in should:
+    print("Testing: %s %s" % (a, b))
+    assert possible(a, b)
+
+shouldnt = [
+    (10, [1, 10, 1, 2, 1, 12]),
+    (1, [0.25, 0.25, 0.25, 0.25]),
+    (5, [1, 2, 2]),
+]
+for a, b in shouldnt:
+    print("Testing: %s %s" % (a, b))
+    assert not possible(a, b)
+
+print("Tests pass")
diff --git a/data_structures_and_algorithms/knapsack-0-1.py b/data_structures_and_algorithms/knapsack-0-1.py
new file mode 100644
index 000000000000..c72d19d4ed73
--- /dev/null
+++ b/data_structures_and_algorithms/knapsack-0-1.py
@@ -0,0 +1,38 @@
+import unittest
+from math import floor
+
+
+def knapify(xs, capacity=None):
+    assert capacity is not None
+    n = len(xs)
+    # For 0/1 Knapsack, we must use a table, since this will encode which values
+    # work for which items. This is cleaner than including a separate data
+    # structure to capture it.
+    maxes = [[0 for x in range(capacity + 1)] for x in range(n + 1)]
+
+    # Build table maxes[][] in bottom up manner
+    for row in range(n + 1):
+        for col in range(capacity + 1):
+            if row == 0 or col == 0:
+                maxes[row][col] = 0
+            elif xs[row - 1][0] <= col:
+                maxes[row][col] = max(
+                    xs[row - 1][1] + maxes[row - 1][col - xs[row - 1][0]],
+                    maxes[row - 1][col])
+            else:
+                maxes[row][col] = maxes[row - 1][col]
+
+    return maxes[-1][capacity]
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_one_cake(self):
+        actual = knapify([(3, 10), (2, 15), (7, 2), (12, 20)], capacity=12)
+        expected = None
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/kth-to-last.py b/data_structures_and_algorithms/kth-to-last.py
new file mode 100644
index 000000000000..8291e54533d5
--- /dev/null
+++ b/data_structures_and_algorithms/kth-to-last.py
@@ -0,0 +1,82 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def length(x):
+    if not x:
+        return 0
+    else:
+        count = 1
+        while x:
+            x = x.next
+            count += 1
+        return count
+
+
+def kth_to_last_node(k, x):
+    hops = length(x) - 1
+    dest = hops - k
+
+    if k == 0:
+        raise Exception("Our God doesn't support this kind of behavior.")
+
+    if dest < 0:
+        raise Exception('Value k to high for list.')
+
+    while dest > 0:
+        x = x.next
+        dest -= 1
+
+    return x
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def setUp(self):
+        self.fourth = Test.LinkedListNode(4)
+        self.third = Test.LinkedListNode(3, self.fourth)
+        self.second = Test.LinkedListNode(2, self.third)
+        self.first = Test.LinkedListNode(1, self.second)
+
+    def test_first_to_last_node(self):
+        actual = kth_to_last_node(1, self.first)
+        expected = self.fourth
+        self.assertEqual(actual, expected)
+
+    def test_second_to_last_node(self):
+        actual = kth_to_last_node(2, self.first)
+        expected = self.third
+        self.assertEqual(actual, expected)
+
+    def test_first_node(self):
+        actual = kth_to_last_node(4, self.first)
+        expected = self.first
+        self.assertEqual(actual, expected)
+
+    def test_k_greater_than_linked_list_length(self):
+        with self.assertRaises(Exception):
+            kth_to_last_node(5, self.first)
+
+    def test_k_is_zero(self):
+        with self.assertRaises(Exception):
+            kth_to_last_node(0, self.first)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/largest-stack.py b/data_structures_and_algorithms/largest-stack.py
new file mode 100644
index 000000000000..aab9671eb6d3
--- /dev/null
+++ b/data_structures_and_algorithms/largest-stack.py
@@ -0,0 +1,107 @@
+import unittest
+
+
+class Stack(object):
+    def __init__(self):
+        """Initialize an empty stack"""
+        self.items = []
+
+    def push(self, item):
+        """Push a new item onto the stack"""
+        self.items.append(item)
+
+    def pop(self):
+        """Remove and return the last item"""
+        # If the stack is empty, return None
+        # (it would also be reasonable to throw an exception)
+        if not self.items:
+            return None
+
+        return self.items.pop()
+
+    def peek(self):
+        """Return the last item without removing it"""
+        if not self.items:
+            return None
+        return self.items[-1]
+
+
+class MaxStack(object):
+    # Implement the push, pop, and get_max methods
+    def __init__(self):
+        self.m = Stack()
+        self.stack = Stack()
+
+    def push(self, item):
+        if self.m.peek() is None:
+            self.m.push(item)
+        elif item >= self.m.peek():
+            self.m.push(item)
+        self.stack.push(item)
+
+    def pop(self):
+        x = self.stack.pop()
+        if x == self.m.peek():
+            self.m.pop()
+        return x
+
+    def get_max(self):
+        return self.m.peek()
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_stack_usage(self):
+        max_stack = MaxStack()
+
+        max_stack.push(5)
+
+        actual = max_stack.get_max()
+        expected = 5
+        self.assertEqual(actual, expected)
+
+        max_stack.push(4)
+        max_stack.push(7)
+        max_stack.push(7)
+        max_stack.push(8)
+
+        actual = max_stack.get_max()
+        expected = 8
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 8
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 5
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 4
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 5
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/linked-list-cycles.py b/data_structures_and_algorithms/linked-list-cycles.py
new file mode 100644
index 000000000000..75a4b993944c
--- /dev/null
+++ b/data_structures_and_algorithms/linked-list-cycles.py
@@ -0,0 +1,88 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def contains_cycle(x):
+    if not x:
+        return False
+    elif not x.next:
+        return False
+
+    a = x
+    b = x.next
+
+    while b.next:
+        if a == b:
+            return True
+
+        a = a.next
+        b = b.next.next
+
+    return False
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+    def test_linked_list_with_no_cycle(self):
+        fourth = Test.LinkedListNode(4)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+        result = contains_cycle(first)
+        self.assertFalse(result)
+
+    def test_cycle_loops_to_beginning(self):
+        fourth = Test.LinkedListNode(4)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+        fourth.next = first
+        result = contains_cycle(first)
+        self.assertTrue(result)
+
+    def test_cycle_loops_to_middle(self):
+        fifth = Test.LinkedListNode(5)
+        fourth = Test.LinkedListNode(4, fifth)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+        fifth.next = third
+        result = contains_cycle(first)
+        self.assertTrue(result)
+
+    def test_two_node_cycle_at_end(self):
+        fifth = Test.LinkedListNode(5)
+        fourth = Test.LinkedListNode(4, fifth)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+        fifth.next = fourth
+        result = contains_cycle(first)
+        self.assertTrue(result)
+
+    def test_empty_list(self):
+        result = contains_cycle(None)
+        self.assertFalse(result)
+
+    def test_one_element_linked_list_no_cycle(self):
+        first = Test.LinkedListNode(1)
+        result = contains_cycle(first)
+        self.assertFalse(result)
+
+    def test_one_element_linked_list_cycle(self):
+        first = Test.LinkedListNode(1)
+        first.next = first
+        result = contains_cycle(first)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/merge-sort.py b/data_structures_and_algorithms/merge-sort.py
new file mode 100644
index 000000000000..6dbe0fa0f3c3
--- /dev/null
+++ b/data_structures_and_algorithms/merge-sort.py
@@ -0,0 +1,28 @@
+
+
+
+# merge :: [a] -> [a] -> [a]
+# merge([], []): []
+# merge(xs, []): xs
+# merge([], ys): ys
+# merge(xs@[x|xs'], ys@[y|ys'])
+#   when y =< x: cons(y, merge(xs, ys'))
+#   when x < y:  cons(x, merge(xs', ys))
+def merge(xs, ys):
+    if xs == [] and ys == []:
+        return []
+    elif ys == []:
+        return xs
+    elif xs == []:
+        return ys
+    else:
+        x = xs[0]
+        y = ys[0]
+
+        if y <= x:
+            return [y] + merge(xs, ys[1:])
+        else:
+            return [x] + merge(xs[1:], ys)
+        
+print(merge([3, 4, 6, 10, 11, 15],
+            [1, 5, 8, 12, 14, 19]))
diff --git a/data_structures_and_algorithms/merging-ranges.py b/data_structures_and_algorithms/merging-ranges.py
new file mode 100644
index 000000000000..4e3604d5bcca
--- /dev/null
+++ b/data_structures_and_algorithms/merging-ranges.py
@@ -0,0 +1,94 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# do_merge_ranges :: [(Int, Int)] -> [(Int, Int)] -> [(Int, Int)]
+def do_merge_ranges(prev, xs):
+    if len(xs) == 0:
+        return prev
+    elif len(xs) == 1:
+        return prev + xs
+    else:
+        a1, a2 = xs[0]
+        b1, b2 = xs[1]
+        rest = xs[2:]
+        if b1 <= a2:
+            return do_merge_ranges(prev, [(a1, max(a2, b2))] + rest)
+        else:
+            return do_merge_ranges(prev + [(a1, a2)], [(b1, b2)] + rest)
+
+
+# merge_ranges :: [(Int, Int)] -> [(Int, Int)]
+def merge_ranges(xs):
+    xs = xs[:]
+    xs.sort()
+    return do_merge_ranges([], xs)
+
+
+# merge_ranges_b :: [(Int, Int)] -> [(Int, Int)]
+def merge_ranges_b(xs):
+    fi = 0
+    ci = 1
+    result = []
+    xs = xs[:]
+    xs.sort()
+    while ci < len(xs):
+        while ci < len(xs) and xs[ci][0] <= xs[fi][1]:
+            xs[fi] = xs[fi][0], max(xs[ci][1], xs[fi][1])
+            ci += 1
+        result.append(xs[fi])
+        fi = ci
+        ci += 1
+    if fi < len(xs):
+        result.append(xs[fi])
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_meetings_overlap(self):
+        actual = merge_ranges([(1, 3), (2, 4)])
+        expected = [(1, 4)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_touch(self):
+        actual = merge_ranges([(5, 6), (6, 8)])
+        expected = [(5, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meeting_contains_other_meeting(self):
+        actual = merge_ranges([(1, 8), (2, 5)])
+        expected = [(1, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_stay_separate(self):
+        actual = merge_ranges([(1, 3), (4, 8)])
+        expected = [(1, 3), (4, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_multiple_merged_meetings(self):
+        actual = merge_ranges([(1, 4), (2, 5), (5, 8)])
+        expected = [(1, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_not_sorted(self):
+        actual = merge_ranges([(5, 8), (1, 4), (6, 8)])
+        expected = [(1, 4), (5, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_one_long_meeting_contains_smaller_meetings(self):
+        actual = merge_ranges([(1, 10), (2, 5), (6, 8), (9, 10), (10, 12)])
+        expected = [(1, 12)]
+        self.assertEqual(actual, expected)
+
+    def test_sample_input(self):
+        actual = merge_ranges([(0, 1), (3, 5), (4, 8), (10, 12), (9, 10)])
+        expected = [(0, 1), (3, 8), (9, 12)]
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/mesh-message.gv b/data_structures_and_algorithms/mesh-message.gv
new file mode 100644
index 000000000000..1e67c3954f5f
--- /dev/null
+++ b/data_structures_and_algorithms/mesh-message.gv
@@ -0,0 +1,11 @@
+strict graph {
+    Min -- {William, Jayden, Omar}
+    William -- {Min, Noam}
+    Jayden -- {Min, Amelia, Ren, Noam}
+    Adam -- {Amelia, Miguel, Sofia, Lucas}
+    Ren -- {Jayden, Omar}
+    Amelia -- {Jayden, Adam, Miguel}
+    Miguel -- {Amelia, Adam, Liam, Nathan}
+    Noam -- {Nathan, Jayden, William}
+    Omar -- {Ren, Min, Scott}
+}
diff --git a/data_structures_and_algorithms/mesh-message.py b/data_structures_and_algorithms/mesh-message.py
new file mode 100644
index 000000000000..c9d7d9d74151
--- /dev/null
+++ b/data_structures_and_algorithms/mesh-message.py
@@ -0,0 +1,97 @@
+import unittest
+from collections import deque
+
+
+################################################################################
+# Solution
+################################################################################
+# get_path :: G(V, E) -> V -> V -> Maybe([V])
+def get_path(g, src, dst):
+    q = deque()
+    result = None
+    seen = set()
+    q.append(([], src))
+
+    if src not in g or dst not in g:
+        raise Exception
+
+    while q:
+        p, node = q.popleft()
+
+        seen.add(node)
+
+        if node == dst:
+            if not result:
+                result = p + [node]
+            elif len(p + [node]) < len(result):
+                result = p + [node]
+        else:
+            if node not in g:
+                raise Exception
+            for x in g.get(node):
+                if not x in seen:
+                    q.append((p + [node], x))
+
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def setUp(self):
+        self.graph = {
+            'a': ['b', 'c', 'd'],
+            'b': ['a', 'd'],
+            'c': ['a', 'e'],
+            'd': ['a', 'b'],
+            'e': ['c'],
+            'f': ['g'],
+            'g': ['f'],
+        }
+
+    def test_two_hop_path_1(self):
+        actual = get_path(self.graph, 'a', 'e')
+        expected = ['a', 'c', 'e']
+        self.assertEqual(actual, expected)
+
+    def test_two_hop_path_2(self):
+        actual = get_path(self.graph, 'd', 'c')
+        expected = ['d', 'a', 'c']
+        self.assertEqual(actual, expected)
+
+    def test_one_hop_path_1(self):
+        actual = get_path(self.graph, 'a', 'c')
+        expected = ['a', 'c']
+        self.assertEqual(actual, expected)
+
+    def test_one_hop_path_2(self):
+        actual = get_path(self.graph, 'f', 'g')
+        expected = ['f', 'g']
+        self.assertEqual(actual, expected)
+
+    def test_one_hop_path_3(self):
+        actual = get_path(self.graph, 'g', 'f')
+        expected = ['g', 'f']
+        self.assertEqual(actual, expected)
+
+    def test_zero_hop_path(self):
+        actual = get_path(self.graph, 'a', 'a')
+        expected = ['a']
+        self.assertEqual(actual, expected)
+
+    def test_no_path(self):
+        actual = get_path(self.graph, 'a', 'f')
+        expected = None
+        self.assertEqual(actual, expected)
+
+    def test_start_node_not_present(self):
+        with self.assertRaises(Exception):
+            get_path(self.graph, 'h', 'a')
+
+    def test_end_node_not_present(self):
+        with self.assertRaises(Exception):
+            get_path(self.graph, 'a', 'h')
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/norman.py b/data_structures_and_algorithms/norman.py
new file mode 100644
index 000000000000..379ba92abba8
--- /dev/null
+++ b/data_structures_and_algorithms/norman.py
@@ -0,0 +1,78 @@
+
+
+
+# Write a function with the following type signature:L
+# equal? :: String -> String -> Bool
+#
+# Determine equality between two inputs with backspace characters encoded as
+# "<".
+
+################################################################################
+# Solution 1
+################################################################################
+
+# from collections import deque
+
+# def equal(a, b):
+#     sa = deque()
+#     sb = deque()
+
+#     for c in a:
+#         if c == '<':
+#             sa.pop()
+#         else:
+#             sa.append(c)
+
+#     for c in b:
+#         if c == '<':
+#             sb.pop()
+#         else:
+#             sb.append(c)
+
+#     return sa == sb
+
+################################################################################
+# Solution 2
+################################################################################
+
+def handle_dels(num_dels, i, xs):
+    if i < 0:
+        return -1
+
+    while xs[i] == '<':
+        num_dels += 1
+        i -= 1
+
+    while num_dels > 0 and xs[i] != '<':
+        num_dels -= 1
+        i -= 1
+
+    if xs[i] == '<':
+        return handle_dels(num_dels, i, xs)
+    else:
+        return i
+
+def update_index(i, xs):
+    # TODO: Indexing into non-available parts of a string.
+    if xs[i] != '<' and xs[i - 1] != '<':
+        return i - 1
+
+    elif xs[i - 1] == '<':
+        return handle_dels(0, i - 1, xs)
+
+def equal(a, b):
+    ia = len(a) - 1
+    ib = len(b) - 1
+
+    while ia >= 0 and ib >= 0:
+        if a[ia] != b[ib]:
+            return False
+        ia = update_index(ia, a)
+        ib = update_index(ib, b)
+
+    if ia != 0:
+        return update_index(ia, a) <= -1
+    if ib != 0:
+        return update_index(ib, b) <= -1
+
+    return True
diff --git a/data_structures_and_algorithms/nth-fibonacci.py b/data_structures_and_algorithms/nth-fibonacci.py
new file mode 100644
index 000000000000..cdb2846ea338
--- /dev/null
+++ b/data_structures_and_algorithms/nth-fibonacci.py
@@ -0,0 +1,59 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def fib(n):
+    """This should be accomplishable in O(1) space."""
+    if n in {0, 1}:
+        return n
+    a = 0  # i = 0
+    b = 1  # i = 1
+    for x in range(2, n + 1):
+        result = a + b
+        a = b
+        b = result
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_zeroth_fibonacci(self):
+        actual = fib(0)
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_first_fibonacci(self):
+        actual = fib(1)
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_second_fibonacci(self):
+        actual = fib(2)
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_third_fibonacci(self):
+        actual = fib(3)
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_fifth_fibonacci(self):
+        actual = fib(5)
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    def test_tenth_fibonacci(self):
+        actual = fib(10)
+        expected = 55
+        self.assertEqual(actual, expected)
+
+    def test_negative_fibonacci(self):
+        with self.assertRaises(Exception):
+            fib(-1)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/optimal-stopping.py b/data_structures_and_algorithms/optimal-stopping.py
new file mode 100644
index 000000000000..af13239941d0
--- /dev/null
+++ b/data_structures_and_algorithms/optimal-stopping.py
@@ -0,0 +1,49 @@
+from random import choice
+from math import floor
+
+# Applying Chapter 1 from "Algorithms to Live By", which describes optimal
+# stopping problems. Technically this simulation is invalid because the
+# `candidates` function takes a lower bound and an upper bound, which allows us
+# to know the cardinal number of an individual candidates. The "look then leap"
+# algorithm is ideal for no-information games - i.e. games when upper and lower
+# bounds aren't known. The `look_then_leap/1` function is ignorant of this
+# information, so it behaves as if in a no-information game. Strangely enough,
+# this algorithm will pick the best candidate 37% of the time.
+#
+# Chapter 1 describes two algorithms:
+# 1. Look-then-leap: ordinal numbers - i.e. no-information games. Look-then-leap
+#    finds the best candidate 37% of the time.
+# 2. Threshold: cardinal numbers - i.e. where upper and lower bounds are
+#    known. The Threshold algorithm finds the best candidate ~55% of the time.
+#
+# All of this and more can be studied as "optimal stopping theory". This applies
+# to finding a spouse, parking a car, picking an apartment in a city, and more.
+
+
+# candidates :: Int -> Int -> Int -> [Int]
+def candidates(lb, ub, ct):
+    xs = list(range(lb, ub + 1))
+    return [choice(xs) for _ in range(ct)]
+
+
+# look_then_leap :: [Integer] -> Integer
+def look_then_leap(candidates):
+    best = candidates[0]
+    seen_ct = 1
+    ignore_ct = floor(len(candidates) * 0.37)
+    for x in candidates[1:]:
+        if ignore_ct > 0:
+            ignore_ct -= 1
+            best = max(best, x)
+        else:
+            if x > best:
+                print('Choosing the {} candidate.'.format(seen_ct))
+                return x
+        seen_ct += 1
+    print('You may have waited too long.')
+    return candidates[-1]
+
+
+candidates = candidates(1, 100, 100)
+print(candidates)
+print(look_then_leap(candidates))
diff --git a/data_structures_and_algorithms/perm-tree.py b/data_structures_and_algorithms/perm-tree.py
new file mode 100644
index 000000000000..0eb389c26bb9
--- /dev/null
+++ b/data_structures_and_algorithms/perm-tree.py
@@ -0,0 +1,83 @@
+import unittest
+
+
+################################################################################
+# Answer
+################################################################################
+class Node(object):
+    def __init__(self, value, children=set()):
+        self.value = value
+        self.children = children
+
+
+# treeify :: Char -> Set(Char) -> Node(Char)
+def treeify(x, xs):
+    return Node(x, [treeify(c, xs - {c}) for c in xs])
+
+
+# dft :: Node(Char) -> [String]
+def dft(node):
+    result = []
+    s = []
+
+    s.append(('', node))
+
+    while s:
+        p, n = s.pop()
+        p += str(n.value)
+
+        if not n.children:
+            result.append(p)
+        else:
+            for c in n.children:
+                s.append((p, c))
+
+    return result
+
+
+# main :: String -> Set(String)
+def get_permutations(xs):
+    if xs == '':
+        return set([''])
+
+    ys = set(xs)
+    trees = []
+
+    for y in ys:
+        trees.append(treeify(y, ys - {y}))
+
+    result = set()
+
+    for t in trees:
+        for d in dft(t):
+            result.add(d)
+
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_empty_string(self):
+        actual = get_permutations('')
+        expected = set([''])
+        self.assertEqual(actual, expected)
+
+    def test_one_character_string(self):
+        actual = get_permutations('a')
+        expected = set(['a'])
+        self.assertEqual(actual, expected)
+
+    def test_two_character_string(self):
+        actual = get_permutations('ab')
+        expected = set(['ab', 'ba'])
+        self.assertEqual(actual, expected)
+
+    def test_three_character_string(self):
+        actual = get_permutations('abc')
+        expected = set(['abc', 'acb', 'bac', 'bca', 'cab', 'cba'])
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/permutation-palindrome.py b/data_structures_and_algorithms/permutation-palindrome.py
new file mode 100644
index 000000000000..0a2136a408f2
--- /dev/null
+++ b/data_structures_and_algorithms/permutation-palindrome.py
@@ -0,0 +1,49 @@
+from collections import Counter
+import unittest
+
+
+################################################################################
+# Impl
+################################################################################
+# palindromifiable :: String -> Boolean
+def has_palindrome_permutation(x):
+    bag = Counter(x)
+    odd_entries_ct = 0
+
+    for _, y in bag.items():
+        if y % 2 != 0:
+            odd_entries_ct += 1
+
+    return odd_entries_ct in {0, 1}
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_permutation_with_odd_number_of_chars(self):
+        result = has_palindrome_permutation('aabcbcd')
+        self.assertTrue(result)
+
+    def test_permutation_with_even_number_of_chars(self):
+        result = has_palindrome_permutation('aabccbdd')
+        self.assertTrue(result)
+
+    def test_no_permutation_with_odd_number_of_chars(self):
+        result = has_palindrome_permutation('aabcd')
+        self.assertFalse(result)
+
+    def test_no_permutation_with_even_number_of_chars(self):
+        result = has_palindrome_permutation('aabbcd')
+        self.assertFalse(result)
+
+    def test_empty_string(self):
+        result = has_palindrome_permutation('')
+        self.assertTrue(result)
+
+    def test_one_character_string(self):
+        result = has_palindrome_permutation('a')
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/permutations.py b/data_structures_and_algorithms/permutations.py
new file mode 100644
index 000000000000..fc2c1ef7eebc
--- /dev/null
+++ b/data_structures_and_algorithms/permutations.py
@@ -0,0 +1,55 @@
+class Node(object):
+    # ctor :: a -> [a] -> Node(a)
+    def __init__(self, value, children=[]):
+        self.value = value
+        self.children = children
+
+
+# is_leaf :: Node(a) -> Boolean
+def is_leaf(node):
+    return len(node.children) == 0
+
+
+# enumerate :: Node(a) -> Set(List(a))
+def enumerate(node):
+    current = []
+    result = []
+    q = []
+
+    q.append(node)
+
+    while q:
+        x = q.pop()
+        print(x.value)
+
+        for c in x.children:
+            q.append(c)
+
+        current.append(x.value)
+        print(current)
+
+        if is_leaf(x):
+            result.append(current)
+            print("Reseting current")
+            current = []
+
+    return result
+
+
+node = Node('root', [
+    Node('a', [
+        Node('b', [Node('c')]),
+        Node('c', [Node('b')]),
+    ]),
+    Node('b', [
+        Node('a', [Node('c')]),
+        Node('c', [Node('a')]),
+    ]),
+    Node('c', [
+        Node('a', [Node('b')]),
+        Node('b', [Node('a')]),
+    ])
+])
+
+print('----------')
+print(enumerate(node))
diff --git a/data_structures_and_algorithms/plot.py b/data_structures_and_algorithms/plot.py
new file mode 100644
index 000000000000..5601891a0d9b
--- /dev/null
+++ b/data_structures_and_algorithms/plot.py
@@ -0,0 +1,9 @@
+import numpy as np
+import matplotlib.pyplot as plt
+
+rng = np.random.RandomState(10)  # deterministic random data
+a = np.hstack((rng.normal(size=1000), rng.normal(loc=5, scale=2, size=1000)))
+_ = plt.hist(a, bins='auto')  # arguments are passed to np.histogram
+plt.title("Histogram with 'auto' bins")
+Text(0.5, 1.0, "Histogram with 'auto' bins")
+plt.show()
diff --git a/data_structures_and_algorithms/product-of-other-numbers.py b/data_structures_and_algorithms/product-of-other-numbers.py
new file mode 100644
index 000000000000..d05e82d42b02
--- /dev/null
+++ b/data_structures_and_algorithms/product-of-other-numbers.py
@@ -0,0 +1,68 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# f :: [Int] -> [Int]
+def get_products_of_all_ints_except_at_index(xs):
+    if len(xs) in {0, 1}:
+        raise Exception
+
+    ct = len(xs)
+    lefts = [1] * ct
+    rights = [1] * ct
+    result = []
+
+    for i in range(1, ct):
+        lefts[i] = lefts[i - 1] * xs[i - 1]
+    for i in range(ct - 2, -1, -1):
+        rights[i] = rights[i + 1] * xs[i + 1]
+
+    return [lefts[i] * rights[i] for i in range(ct)]
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_small_list(self):
+        actual = get_products_of_all_ints_except_at_index([1, 2, 3])
+        expected = [6, 3, 2]
+        self.assertEqual(actual, expected)
+
+    def test_longer_list(self):
+        actual = get_products_of_all_ints_except_at_index([8, 2, 4, 3, 1, 5])
+        expected = [120, 480, 240, 320, 960, 192]
+        self.assertEqual(actual, expected)
+
+    def test_list_has_one_zero(self):
+        actual = get_products_of_all_ints_except_at_index([6, 2, 0, 3])
+        expected = [0, 0, 36, 0]
+        self.assertEqual(actual, expected)
+
+    def test_list_has_two_zeros(self):
+        actual = get_products_of_all_ints_except_at_index([4, 0, 9, 1, 0])
+        expected = [0, 0, 0, 0, 0]
+        self.assertEqual(actual, expected)
+
+    def test_one_negative_number(self):
+        actual = get_products_of_all_ints_except_at_index([-3, 8, 4])
+        expected = [32, -12, -24]
+        self.assertEqual(actual, expected)
+
+    def test_all_negative_numbers(self):
+        actual = get_products_of_all_ints_except_at_index([-7, -1, -4, -2])
+        expected = [-8, -56, -14, -28]
+        self.assertEqual(actual, expected)
+
+    def test_error_with_empty_list(self):
+        with self.assertRaises(Exception):
+            get_products_of_all_ints_except_at_index([])
+
+    def test_error_with_one_number(self):
+        with self.assertRaises(Exception):
+            get_products_of_all_ints_except_at_index([1])
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/queue-two-stacks.py b/data_structures_and_algorithms/queue-two-stacks.py
new file mode 100644
index 000000000000..63da08ebf79a
--- /dev/null
+++ b/data_structures_and_algorithms/queue-two-stacks.py
@@ -0,0 +1,66 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+class QueueTwoStacks(object):
+    def __init__(self):
+        self.a = []
+        self.b = []
+
+    def enqueue(self, x):
+        self.a.append(x)
+
+    def dequeue(self):
+        if self.b:
+            return self.b.pop()
+        else:
+            while self.a:
+                self.b.append(self.a.pop())
+            return self.dequeue()
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_basic_queue_operations(self):
+        queue = QueueTwoStacks()
+        queue.enqueue(1)
+        queue.enqueue(2)
+        queue.enqueue(3)
+        actual = queue.dequeue()
+        expected = 1
+        self.assertEqual(actual, expected)
+        actual = queue.dequeue()
+        expected = 2
+        self.assertEqual(actual, expected)
+        queue.enqueue(4)
+        actual = queue.dequeue()
+        expected = 3
+        self.assertEqual(actual, expected)
+        actual = queue.dequeue()
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_error_when_dequeue_from_new_queue(self):
+        queue = QueueTwoStacks()
+        with self.assertRaises(Exception):
+            queue.dequeue()
+
+    def test_error_when_dequeue_from_empty_queue(self):
+        queue = QueueTwoStacks()
+        queue.enqueue(1)
+        queue.enqueue(2)
+        actual = queue.dequeue()
+        expected = 1
+        self.assertEqual(actual, expected)
+        actual = queue.dequeue()
+        expected = 2
+        self.assertEqual(actual, expected)
+        with self.assertRaises(Exception):
+            queue.dequeue()
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/rectangular-love.py b/data_structures_and_algorithms/rectangular-love.py
new file mode 100644
index 000000000000..47c0f53979c6
--- /dev/null
+++ b/data_structures_and_algorithms/rectangular-love.py
@@ -0,0 +1,246 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# bottom :: Rectangle -> Int
+def bottom(x):
+    return x.get('bottom_y')
+
+
+# top :: Rectangle -> Int
+def top(x):
+    return bottom(x) + x.get('height')
+
+
+# left :: Rectangle -> Int
+def left(x):
+    return x.get('left_x')
+
+
+# right :: Rectangle -> Int
+def right(x):
+    return left(x) + x.get('width')
+
+
+# sort_highest :: Rectangle -> Rectangle -> (Rectangle, Rectangle)
+def sort_highest(x, y):
+    if top(x) >= top(y):
+        return x, y
+    else:
+        return y, x
+
+
+# sort_leftmost :: Rectangle -> Rectangle -> (Rectangle, Rectangle)
+def sort_leftmost(x, y):
+    if left(x) <= left(y):
+        return x, y
+    else:
+        return y, x
+
+
+# rectify :: Int -> Int -> Int -> Int -> Rectify
+def rectify(top=None, bottom=None, left=None, right=None):
+    assert top >= bottom
+    assert left <= right
+    return {
+        'left_x': left,
+        'bottom_y': bottom,
+        'width': right - left,
+        'height': top - bottom,
+    }
+
+
+# empty_rect :: Rectangle
+def empty_rect():
+    return {
+        'left_x': None,
+        'bottom_y': None,
+        'width': None,
+        'height': None,
+    }
+
+
+# find_rectangular_overlap :: Rectangle -> Rectangle -> Maybe(Rectangle)
+def find_rectangular_overlap(x, y):
+    ha, hb = sort_highest(x, y)
+    la, lb = sort_leftmost(x, y)
+
+    if bottom(hb) <= top(hb) <= bottom(ha) <= top(ha):
+        return empty_rect()
+
+    if left(la) <= right(la) <= left(lb) <= right(lb):
+        return empty_rect()
+
+    # We should have an intersection here.
+    verts = [bottom(ha), top(ha), bottom(hb), top(hb)]
+    verts.sort()
+    horzs = [left(la), right(la), left(lb), right(lb)]
+    horzs.sort()
+    return rectify(top=verts[2],
+                   bottom=verts[1],
+                   left=horzs[1],
+                   right=horzs[2])
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_overlap_along_both_axes(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 1,
+            'width': 6,
+            'height': 3,
+        }
+        rect2 = {
+            'left_x': 5,
+            'bottom_y': 2,
+            'width': 3,
+            'height': 6,
+        }
+        expected = {
+            'left_x': 5,
+            'bottom_y': 2,
+            'width': 2,
+            'height': 2,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_one_rectangle_inside_another(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 1,
+            'width': 6,
+            'height': 6,
+        }
+        rect2 = {
+            'left_x': 3,
+            'bottom_y': 3,
+            'width': 2,
+            'height': 2,
+        }
+        expected = {
+            'left_x': 3,
+            'bottom_y': 3,
+            'width': 2,
+            'height': 2,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_both_rectangles_the_same(self):
+        rect1 = {
+            'left_x': 2,
+            'bottom_y': 2,
+            'width': 4,
+            'height': 4,
+        }
+        rect2 = {
+            'left_x': 2,
+            'bottom_y': 2,
+            'width': 4,
+            'height': 4,
+        }
+        expected = {
+            'left_x': 2,
+            'bottom_y': 2,
+            'width': 4,
+            'height': 4,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_touch_on_horizontal_edge(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 2,
+            'width': 3,
+            'height': 4,
+        }
+        rect2 = {
+            'left_x': 2,
+            'bottom_y': 6,
+            'width': 2,
+            'height': 2,
+        }
+        expected = {
+            'left_x': None,
+            'bottom_y': None,
+            'width': None,
+            'height': None,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_touch_on_vertical_edge(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 2,
+            'width': 3,
+            'height': 4,
+        }
+        rect2 = {
+            'left_x': 4,
+            'bottom_y': 3,
+            'width': 2,
+            'height': 2,
+        }
+        expected = {
+            'left_x': None,
+            'bottom_y': None,
+            'width': None,
+            'height': None,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_touch_at_a_corner(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 1,
+            'width': 2,
+            'height': 2,
+        }
+        rect2 = {
+            'left_x': 3,
+            'bottom_y': 3,
+            'width': 2,
+            'height': 2,
+        }
+        expected = {
+            'left_x': None,
+            'bottom_y': None,
+            'width': None,
+            'height': None,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_no_overlap(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 1,
+            'width': 2,
+            'height': 2,
+        }
+        rect2 = {
+            'left_x': 4,
+            'bottom_y': 6,
+            'width': 3,
+            'height': 6,
+        }
+        expected = {
+            'left_x': None,
+            'bottom_y': None,
+            'width': None,
+            'height': None,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/recursive-string-permutations.py b/data_structures_and_algorithms/recursive-string-permutations.py
new file mode 100644
index 000000000000..70461ddf5dac
--- /dev/null
+++ b/data_structures_and_algorithms/recursive-string-permutations.py
@@ -0,0 +1,37 @@
+import unittest
+
+
+################################################################################
+# Implementation
+################################################################################
+# get_permutations :: String -> Set(String)
+def get_permutations(string):
+    pass
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_empty_string(self):
+        actual = get_permutations('')
+        expected = set([''])
+        self.assertEqual(actual, expected)
+
+    def test_one_character_string(self):
+        actual = get_permutations('a')
+        expected = set(['a'])
+        self.assertEqual(actual, expected)
+
+    def test_two_character_string(self):
+        actual = get_permutations('ab')
+        expected = set(['ab', 'ba'])
+        self.assertEqual(actual, expected)
+
+    def test_three_character_string(self):
+        actual = get_permutations('abc')
+        expected = set(['abc', 'acb', 'bac', 'bca', 'cab', 'cba'])
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/reverse-linked-list.py b/data_structures_and_algorithms/reverse-linked-list.py
new file mode 100644
index 000000000000..b7396b20ce3f
--- /dev/null
+++ b/data_structures_and_algorithms/reverse-linked-list.py
@@ -0,0 +1,79 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# reverse :: List(a) -> List(a)
+def reverse(node):
+    curr = node
+    prev = None
+    while curr:
+        nxt = curr.next
+        curr.next = prev
+        prev = curr
+        curr = nxt
+    # Make sure to understand the spec! Debugging takes time. Rewriting takes
+    # time.
+    return prev
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def test_short_linked_list(self):
+        second = Test.LinkedListNode(2)
+        first = Test.LinkedListNode(1, second)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [2, 1]
+        self.assertEqual(actual, expected)
+
+    def test_long_linked_list(self):
+        sixth = Test.LinkedListNode(6)
+        fifth = Test.LinkedListNode(5, sixth)
+        fourth = Test.LinkedListNode(4, fifth)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [6, 5, 4, 3, 2, 1]
+        self.assertEqual(actual, expected)
+
+    def test_one_element_linked_list(self):
+        first = Test.LinkedListNode(1)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [1]
+        self.assertEqual(actual, expected)
+
+    def test_empty_linked_list(self):
+        result = reverse(None)
+        self.assertIsNone(result)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/reverse-words.py b/data_structures_and_algorithms/reverse-words.py
new file mode 100644
index 000000000000..5df12ebabdc7
--- /dev/null
+++ b/data_structures_and_algorithms/reverse-words.py
@@ -0,0 +1,181 @@
+from collections import deque
+import unittest
+
+################################################################################
+# Solution
+################################################################################
+
+
+def rev(xs, i, j):
+    """Reverse xs in place from [i, j]"""
+    while i < j:
+        xs[i], xs[j] = xs[j], xs[i]
+        i += 1
+        j -= 1
+
+
+def rotate(xs, n, i=None, j=None):
+    """Mutably rotates list, xs, n times. Positive n values rotate right while
+    negative n values rotate left. Rotate within window [i, j]."""
+    i = i or 0
+    j = j or len(xs) - 1
+    ct = j - i
+
+    if n < 0:
+        n = abs(n)
+        p = i + n - 1
+        rev(xs, i, p)
+        rev(xs, p + 1, j)
+        rev(xs, i, j)
+    else:
+        p = j - (n - 1)
+        rev(xs, p, j)
+        rev(xs, i, p - 1)
+        rev(xs, i, j)
+    return xs
+
+
+def rev_words(xs, i, j):
+    if j + 1 == len(xs):
+        return 0
+
+    while j + 1 < len(xs):
+        while j + 1 < len(xs) and xs[j + 1] != ' ':
+            j += 1
+
+        rev(xs, i, j)
+        j += 2
+        i = j
+
+    return 0
+
+
+def reverse_words(xs):
+    # first reverse everything
+    rev(xs, 0, len(xs) - 1)
+    return rev_words(xs, 0, 0)
+
+
+def reverse_words_bak(xs, i=None, j=None):
+    i = i or 0
+    j = j or len(xs) - 1
+    w0, w1 = [], []
+
+    if i >= j:
+        return 0
+
+    pi = i
+    while pi < len(xs) and xs[pi] != ' ':
+        w0.append(xs[pi])
+        pi += 1
+
+    if pi == len(xs):
+        return 0
+
+    pj = j
+    while xs[pj] != ' ':
+        w1.append(xs[pj])
+        pj -= 1
+
+    d = len(w0) - len(w1)
+
+    rotate(xs, -1 * d, i, j)
+
+    for k in range(len(w1)):
+        xs[i + k] = w1[len(w1) - 1 - k]
+
+    for k in range(len(w0)):
+        xs[j - k] = w0[len(w0) - 1 - k]
+
+    while i != j and xs[i] != ' ' and xs[j] != ' ':
+        i += 1
+        j -= 1
+
+    if i == j:
+        return 0
+
+    elif xs[i] == ' ':
+        while j > 0 and xs[j] != ' ':
+            j -= 1
+        if j == 0:
+            return 0
+    elif xs[j] == ' ':
+        while i < len(xs) and xs[i] != ' ':
+            i += 1
+        if i == len(xs):
+            return 0
+    return reverse_words(xs, i + 1, j - 1)
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_rev(self):
+        xs = [1, 2, 3, 4, 5]
+        rev(xs, 0, len(xs) - 1)
+        self.assertEqual(xs, [5, 4, 3, 2, 1])
+
+    def test_rotate(self):
+        ys = [1, 2, 3, 4, 5]
+        xs = ys[:]
+        self.assertEqual(rotate(xs, 1, 1, 3), [1, 4, 2, 3, 5])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, -1, 1, 3), [1, 3, 4, 2, 5])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, 1), [5, 1, 2, 3, 4])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, -1), [2, 3, 4, 5, 1])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, -2), [3, 4, 5, 1, 2])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, -5), [1, 2, 3, 4, 5])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, 5), [1, 2, 3, 4, 5])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, 3), [3, 4, 5, 1, 2])
+
+    def test_one_word(self):
+        message = list('vault')
+        reverse_words(message)
+        expected = list('vault')
+        self.assertEqual(message, expected)
+
+    def test_two_words(self):
+        message = list('thief cake')
+        reverse_words(message)
+        expected = list('cake thief')
+        self.assertEqual(message, expected)
+
+    def test_three_words(self):
+        message = list('one another get')
+        reverse_words(message)
+        expected = list('get another one')
+        self.assertEqual(message, expected)
+
+    def test_multiple_words_same_length(self):
+        message = list('rat the ate cat the')
+        reverse_words(message)
+        expected = list('the cat ate the rat')
+        self.assertEqual(message, expected)
+
+    def test_multiple_words_different_lengths(self):
+        message = list('at rat house')
+        reverse_words(message)
+        expected = list('house rat at')
+        self.assertEqual(message, expected)
+
+    def test_multiple_words_different_lengths(self):
+        message = list('yummy is cake bundt chocolate')
+        reverse_words(message)
+        expected = list('chocolate bundt cake is yummy')
+        self.assertEqual(message, expected)
+
+    def test_empty_string(self):
+        message = list('')
+        reverse_words(message)
+        expected = list('')
+        self.assertEqual(message, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/second-largest-item-bst.py b/data_structures_and_algorithms/second-largest-item-bst.py
new file mode 100644
index 000000000000..bc167d975a7b
--- /dev/null
+++ b/data_structures_and_algorithms/second-largest-item-bst.py
@@ -0,0 +1,179 @@
+import unittest
+from collections import deque
+
+
+################################################################################
+# Implementation
+################################################################################
+def is_leaf(node):
+    return node.left is None and node.right is None
+
+
+def find_largest(node):
+    current = node
+    while current.right is not None:
+        current = current.right
+    return current.value
+
+
+def find_second_largest(node):
+    history = deque()
+    current = node
+
+    while current.right:
+        history.append(current)
+        current = current.right
+
+    if current.left:
+        return find_largest(current.left)
+    elif history:
+        return history.pop().value
+    else:
+        raise TypeError
+
+
+def find_second_largest_backup(node):
+    history = deque()
+    current = node
+
+    # traverse -> largest
+    while current.right:
+        history.append(current)
+        current = current.right
+
+    if current.left:
+        return find_largest(current.left)
+    elif history:
+        return history.pop().value
+    else:
+        raise ArgumentError
+
+
+# Write a iterative version to avoid consuming memory with the call stack.
+# Commenting out the recursive code for now.
+def find_second_largest_backup(node):
+    if node.left is None and node.right is None:
+        raise ArgumentError
+
+    elif node.right is None and is_leaf(node.left):
+        return node.left.value
+
+    # recursion
+    # elif node.right is None:
+    #     return find_largest(node.left)
+
+    # iterative version
+    elif node.right is None:
+        current = node.left
+        while current.right is not None:
+            current = current.right
+        return current.value
+
+    # recursion
+    # TODO: Remove recursion from here.
+    elif not is_leaf(node.right):
+        return find_second_largest(node.right)
+
+    # could do an else here, but let's be more assertive.
+    elif is_leaf(node.right):
+        return node.value
+
+    else:
+        raise ArgumentError
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_full_tree(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        right.insert_left(60)
+        right.insert_right(80)
+        actual = find_second_largest(tree)
+        expected = 70
+        self.assertEqual(actual, expected)
+
+    def test_largest_has_a_left_child(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        right.insert_left(60)
+        actual = find_second_largest(tree)
+        expected = 60
+        self.assertEqual(actual, expected)
+
+    def test_largest_has_a_left_subtree(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        right_left = right.insert_left(60)
+        right_left_left = right_left.insert_left(55)
+        right_left.insert_right(65)
+        right_left_left.insert_right(58)
+        actual = find_second_largest(tree)
+        expected = 65
+        self.assertEqual(actual, expected)
+
+    def test_second_largest_is_root_node(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        actual = find_second_largest(tree)
+        expected = 50
+        self.assertEqual(actual, expected)
+
+    def test_descending_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(40)
+        left_left = left.insert_left(30)
+        left_left_left = left_left.insert_left(20)
+        left_left_left.insert_left(10)
+        actual = find_second_largest(tree)
+        expected = 40
+        self.assertEqual(actual, expected)
+
+    def test_ascending_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        right = tree.insert_right(60)
+        right_right = right.insert_right(70)
+        right_right.insert_right(80)
+        actual = find_second_largest(tree)
+        expected = 70
+        self.assertEqual(actual, expected)
+
+    def test_error_when_tree_has_one_node(self):
+        tree = Test.BinaryTreeNode(50)
+        with self.assertRaises(Exception):
+            find_second_largest(tree)
+
+    def test_error_when_tree_is_empty(self):
+        with self.assertRaises(Exception):
+            find_second_largest(None)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/shortest-path-inject-vertices.py b/data_structures_and_algorithms/shortest-path-inject-vertices.py
new file mode 100644
index 000000000000..e08ea66b8f50
--- /dev/null
+++ b/data_structures_and_algorithms/shortest-path-inject-vertices.py
@@ -0,0 +1,94 @@
+from heapq import heappush, heappop
+from collections import deque
+from fixtures import weighted_graph, expanded_weights_graph
+
+# UnweightedGraph(a) :: Map(a, Set(a))
+# WeightedGraph(a) :: Map(a, Set(a))
+
+
+# shortest_path_dijkstra :: Vertex -> Vertex -> WeightedGraph(Vertex)
+def shortest_path_dijkstra(a, b, g):
+    q = []
+    seen = set()
+
+    heappush(q, (0, a, [a]))
+
+    while q:
+        w0, v0, path = heappop(q)
+        if v0 in seen:
+            continue
+        elif v0 == b:
+            return w0, path
+        for w1, v1 in g.get(v0):
+            heappush(q, (w0 + w1, v1, path + [v1]))
+        seen.add(v0)
+    return 'weighted', 'pizza'
+
+
+# expand_edge :: Vertex -> (Weight, Vertex) -> Map(Vertex, [Vertex])
+def expand_edge(v0, wv):
+    w, v1 = wv
+    assert w > 1
+
+    result = {v0: ['{}-{}'.format(v1, 1)]}
+    for x in range(w - 2):
+        result['{}-{}'.format(v1, x + 1)] = ['{}-{}'.format(v1, x + 2)]
+    result['{}-{}'.format(v1, w - 1)] = [v1]
+
+    return result
+
+
+# expand_weights :: Vertex -> WeightedGraph(Vertex) -> UnweightedGraph(Vertex)
+def expand_weights(v, g):
+    result = {}
+    q = deque()
+    seen = set()
+
+    q.append(v)
+    while q:
+        v = d.popleft()
+        if v in seen:
+            continue
+        x = expand_edge(v, g.get)
+        for w, v1 in g.get(v):
+            if w > 1:
+                ws = expand_edge(v, (w, v1))
+                result = {**result, **ws}
+            q.append(v)
+        pass
+
+
+# shortest_path_inject :: Vertex -> Vertex -> WeightedGraph(Vertex)
+def shortest_path_inject(a, b, g):
+    q = deque()
+    seen = set()
+
+    q.append((a, [a]))
+
+    while q:
+        v0, path = q.popleft()
+        if v0 == 'dummy':
+            continue
+        elif v0 in seen:
+            continue
+        elif v0 == b:
+            return len(path), path
+        for _, v1 in g.get(v0):
+            q.append((v1, path + [v1]))
+        seen.add(v0)
+        continue
+
+    return None, None
+
+
+print(expand_edge('a', (4, 'b')))
+print(expand_edge('a', (5, 'e')))
+assert expand_weights('a', weighted_graph) == expanded_weights_graph
+# a = 'a'
+# b = 'd'
+# w, x = shortest_path_dijkstra(a, b, weighted_graph)
+# w1, x1 = shortest_path_inject(a, b, weighted_graph)
+# print("[dijkstra]  Shortest path from {} to {} is {} with weight {}".format(
+#     a, b, x, w))
+# print("[injection] Shortest path from {} to {} is {} with weight {}".format(
+#     a, b, x1, w1))
diff --git a/data_structures_and_algorithms/shuffle.py b/data_structures_and_algorithms/shuffle.py
new file mode 100644
index 000000000000..bdfbad24263c
--- /dev/null
+++ b/data_structures_and_algorithms/shuffle.py
@@ -0,0 +1,34 @@
+import random
+
+
+def get_random(floor, ceiling):
+    return random.randrange(floor, ceiling + 1)
+
+
+# shuffle_in_place :: [a] -> IO ()
+def shuffle_in_place(xs):
+    """Fisher-Yates algorithm. Notice that shuffling here is the same as
+    selecting a random permutation of the input set, `xs`."""
+    n = len(xs) - 1
+    for i in range(len(xs)):
+        r = get_random(i, n)
+        xs[i], xs[r] = xs[r], xs[i]
+    return xs
+
+
+# shuffle :: [a] -> [a]
+def shuffle_not_in_place(xs):
+    result = []
+
+    while xs:
+        i = get_random(0, len(xs) - 1)
+        x = xs.pop(i)
+        result.append(x)
+
+    return result
+
+
+xs = [x for x in range(9)]
+print(xs)
+# print(shuffle_not_in_place(xs))
+print(shuffle_in_place(xs[:]))
diff --git a/data_structures_and_algorithms/string-reverse.py b/data_structures_and_algorithms/string-reverse.py
new file mode 100644
index 000000000000..8b4cdac1c271
--- /dev/null
+++ b/data_structures_and_algorithms/string-reverse.py
@@ -0,0 +1,22 @@
+
+# swap :: Int -> Int -> [Char] -> IO ()
+def swap(ia, iz, xs):
+    # handle swap when ia == iz
+    assert ia <= iz
+    xs[ia], xs[iz] = xs[iz], xs[ia]
+    
+
+# reverse :: [Char] -> IO ()
+def reverse(xs):
+    ia = 0
+    iz = len(xs) - 1
+
+    while ia <= iz:
+        swap(ia, iz, xs)
+        ia += 1
+        iz -= 1
+
+x = list("superduperpooper")
+reverse(x)
+print(x)
+print("Tests pass")
diff --git a/data_structures_and_algorithms/temperature-tracker.py b/data_structures_and_algorithms/temperature-tracker.py
new file mode 100644
index 000000000000..6b042182f01c
--- /dev/null
+++ b/data_structures_and_algorithms/temperature-tracker.py
@@ -0,0 +1,84 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+class TempTracker(object):
+    def __init__(self):
+        # min / max
+        self.min = None
+        self.max = None
+        # mean
+        self.sum = 0
+        self.num = 0
+        # mode
+        self.nums = [0] * 111
+        self.mode_num = 0
+        self.mode = None
+
+    def insert(self, x):
+        # min / max
+        if not self.min or x < self.min:
+            self.min = x
+        if not self.max or x > self.max:
+            self.max = x
+        # mean
+        self.sum += x
+        self.num += 1
+        # mode
+        self.nums[x] += 1
+        if self.nums[x] >= self.mode_num:
+            self.mode_num = self.nums[x]
+            self.mode = x
+
+    def get_max(self):
+        return self.max
+
+    def get_min(self):
+        return self.min
+
+    def get_mean(self):
+        return self.sum / self.num
+
+    def get_mode(self):
+        return self.mode
+
+
+# Tests
+
+
+class Test(unittest.TestCase):
+    def test_tracker_usage(self):
+        tracker = TempTracker()
+
+        tracker.insert(50)
+        msg = 'failed on first temp recorded'
+        self.assertEqual(tracker.get_max(), 50, msg='max ' + msg)
+        self.assertEqual(tracker.get_min(), 50, msg='min ' + msg)
+        self.assertEqual(tracker.get_mean(), 50.0, msg='mean ' + msg)
+        self.assertEqual(tracker.get_mode(), 50, msg='mode ' + msg)
+
+        tracker.insert(80)
+        msg = 'failed on higher temp recorded'
+        self.assertEqual(tracker.get_max(), 80, msg='max ' + msg)
+        self.assertEqual(tracker.get_min(), 50, msg='min ' + msg)
+        self.assertEqual(tracker.get_mean(), 65.0, msg='mean ' + msg)
+        self.assertIn(tracker.get_mode(), [50, 80], msg='mode ' + msg)
+
+        tracker.insert(80)
+        msg = 'failed on third temp recorded'
+        self.assertEqual(tracker.get_max(), 80, msg='max ' + msg)
+        self.assertEqual(tracker.get_min(), 50, msg='min ' + msg)
+        self.assertEqual(tracker.get_mean(), 70.0, msg='mean ' + msg)
+        self.assertEqual(tracker.get_mode(), 80, msg='mode ' + msg)
+
+        tracker.insert(30)
+        msg = 'failed on lower temp recorded'
+        self.assertEqual(tracker.get_max(), 80, msg='max ' + msg)
+        self.assertEqual(tracker.get_min(), 30, msg='min ' + msg)
+        self.assertEqual(tracker.get_mean(), 60.0, msg='mean ' + msg)
+        self.assertEqual(tracker.get_mode(), 80, msg='mode ' + msg)
+
+
+unittest.main(verbosity=2)
diff --git a/data_structures_and_algorithms/test.txt b/data_structures_and_algorithms/test.txt
new file mode 100644
index 000000000000..ce013625030b
--- /dev/null
+++ b/data_structures_and_algorithms/test.txt
@@ -0,0 +1 @@
+hello
diff --git a/data_structures_and_algorithms/top-scores.py b/data_structures_and_algorithms/top-scores.py
new file mode 100644
index 000000000000..8e7b073dd8bd
--- /dev/null
+++ b/data_structures_and_algorithms/top-scores.py
@@ -0,0 +1,25 @@
+from collections import deque
+
+# list:
+# array:
+# vector:
+# bit-{array,vector}:
+
+
+def sort(xs, highest):
+    v = [0] * (highest + 1)
+    result = deque()
+
+    for x in xs:
+        v[x] += 1
+
+    for i, x in enumerate(v):
+        if x > 0:
+            result.appendleft(i)
+
+    return list(result)
+
+
+assert sort([37, 89, 41, 100, 65, 91, 53],
+            100) == [100, 91, 89, 65, 53, 41, 37]
+print("Tests pass!")
diff --git a/data_structures_and_algorithms/topo-sort.py b/data_structures_and_algorithms/topo-sort.py
new file mode 100644
index 000000000000..fe295b0279ff
--- /dev/null
+++ b/data_structures_and_algorithms/topo-sort.py
@@ -0,0 +1,31 @@
+from fixtures import unweighted_digraph
+from collections import deque
+
+# vertices_no_in_edges :: UnweightedDigraph -> Set(Vertex)
+def vertices_no_in_edges(g):
+    """Return the vertices in graph `g` with no in-edges."""
+    result = set()
+    vertices = set(g.keys())
+    for neighbors in g.values():
+        result = result.union(neighbors)
+    return vertices ^ result
+
+# topo_sort :: UnweightedDigraph -> List(Vertex)
+def topo_sort(g):
+    q = deque()
+    seen = set()
+    result = []
+    for x in vertices_no_in_edges(g):
+        q.append(x)
+    while q:
+        vertex = q.popleft()
+        if vertex in seen:
+            continue
+        result.append(vertex)
+        neighbors = g.get(vertex)
+        for x in g.get(vertex):
+            q.append(x)
+        seen.add(vertex)
+    return result
+
+print(topo_sort(unweighted_digraph))
diff --git a/data_structures_and_algorithms/trickling-water.py b/data_structures_and_algorithms/trickling-water.py
new file mode 100644
index 000000000000..45621990ecf9
--- /dev/null
+++ b/data_structures_and_algorithms/trickling-water.py
@@ -0,0 +1,38 @@
+class Node(object):
+    def __init__(self, value, children=[]):
+        self.value = value
+        self.children = children
+
+
+################################################################################
+# Solution
+################################################################################
+def trip_time(node):
+    s = []
+    result = 0
+    s.append((node.value, node))
+    while s:
+        p, node = s.pop()
+        if not node.children:
+            result = max(result, p)
+        for x in node.children:
+            s.append((p + x.value, x))
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+tree = Node(
+    0,
+    children=[
+        Node(5, children=[Node(6)]),
+        Node(2, children=[
+            Node(6),
+            Node(10),
+        ]),
+        Node(3, children=[Node(2, children=[Node(11)])]),
+    ])
+
+assert trip_time(tree) == 16
+print("Tests pass!")
diff --git a/data_structures_and_algorithms/which-appears-twice.py b/data_structures_and_algorithms/which-appears-twice.py
new file mode 100644
index 000000000000..e9a4f0eb24d0
--- /dev/null
+++ b/data_structures_and_algorithms/which-appears-twice.py
@@ -0,0 +1,33 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# find_repeat :: [Int] -> Int
+def find_repeat(xs):
+    n = len(xs) - 1
+    return sum(xs) - ((n**2 + n) / 2)
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_short_list(self):
+        actual = find_repeat([1, 2, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_repeat([4, 1, 3, 4, 2])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_repeat([1, 5, 9, 7, 2, 6, 3, 8, 2, 4])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
-- 
cgit 1.4.1


From 8d00a456a03aed5dc4a8944519e4d3d44c446ad8 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 15 Jan 2020 14:29:18 +0000
Subject: Beging work to port f.el to Haskell

This is a work-in-progress. I'd like to add a README to this project to explain
my intention. The goal, roughly, is to port Elisp's fantastic f.el module to
Haskell. I consider Haskell APIs to be useful but somewhat sloppily designed. In
the same spirit as Elixir wrapping Erlang APIs, many of the functions I intend
to define will simply wrap existing Haskell APIs, but with a hopefully cleaner
API that I find more intuitive.
---
 haskell-file/f-todo.org | 67 +++++++++++++++++++++++++++++++++++++++++++++++++
 haskell-file/f.hs       | 47 ++++++++++++++++++++++++++++++++++
 haskell-file/shell.nix  |  9 +++++++
 3 files changed, 123 insertions(+)
 create mode 100644 haskell-file/f-todo.org
 create mode 100644 haskell-file/f.hs
 create mode 100644 haskell-file/shell.nix

diff --git a/haskell-file/f-todo.org b/haskell-file/f-todo.org
new file mode 100644
index 000000000000..6dd43a96291f
--- /dev/null
+++ b/haskell-file/f-todo.org
@@ -0,0 +1,67 @@
+* Paths
+** TODO f-join (&rest args)
+** TODO f-split (path)
+** TODO f-expand (path &optional dir)
+** TODO f-filename (path)
+** TODO f-dirname (path)
+** TODO f-common-parent (paths)
+** TODO f-ext (path)
+** TODO f-no-ext (path)
+** TODO f-swap-ext (path ext)
+** TODO f-base (path)
+** TODO f-relative (path &optional dir)
+** TODO f-short (path)
+** TODO f-long (path)
+** TODO f-canonical (path)
+** TODO f-slash (path)
+** TODO f-full (path)
+** TODO f-uniquify (paths)
+** TODO f-uniquify-alist (paths)
+* I/O
+** TODO f-read-bytes (path)
+** TODO f-write-bytes (data path)
+** TODO f-read-text (path &optional coding)
+** TODO f-write-text(text coding path)
+** TODO f-append-text(text coding path)
+** TODO f-append-bytes(text coding path)
+** TODO Destructive
+** TODO f-mkdir (&rest dirs)
+** TODO f-delete (path &optional force)
+** TODO f-symlink (source path)
+** TODO f-move (from to)
+** TODO f-copy (from to)
+** TODO f-copy-contenst (from to)
+** TODO f-touch (path)
+** TODO Predicates
+** TODO f-exists? (path)
+** TODO f-directory? (path)
+** TODO f-file? (path)
+** TODO f-symlink? (path)
+** TODO f-readable? (path)
+** TODO f-writable? (path)
+** TODO f-executable? (path)
+** TODO f-absolute? (path)
+** TODO f-relative? (path)
+** TODO f-root? (path)
+** TODO f-ext? (path ext)
+** TODO f-same? (path-a path-b)
+** TODO f-parent-of? (path-a path-b)
+** TODO f-child-of? (path-a path-b)
+** TODO f-ancestor-of? (path-a path-b)
+** TODO f-descendant-of? (path-a path-b)
+** TODO f-hidden? (path)
+** TODO f-empty? (path)
+** TODO Stats
+** TODO f-size (path)
+** f-depth (path)
+
+* Misc
+** TODO f-this-file ()
+** TODO f-path-separator ()
+** TODO f-glob (pattern &optional path)
+** TODO f-entries (path &optional fn recursive)
+** TODO f-directories (path &optional fn recursive)
+** TODO f-files (path &optional fn recursive)
+** TODO f-root ()
+** TODO f-traverse-upwards (fn &optional path)
+** TODO f-with-sandbox (path-or-paths &rest body)
diff --git a/haskell-file/f.hs b/haskell-file/f.hs
new file mode 100644
index 000000000000..9ddb930ee103
--- /dev/null
+++ b/haskell-file/f.hs
@@ -0,0 +1,47 @@
+module F
+  ( join
+  ) where
+
+import System.FilePath.Posix (FilePath)
+import qualified System.FilePath.Posix as F
+
+-- TODO: Move this to a misc.hs, prelude.hs, operators.hs; somewhere.
+(|>) :: a -> (a -> b) -> b
+(|>) a f = f a
+infixl 1 |>
+
+-- TODO: Move this to a test_utils.hs or elsewhere.
+simpleAssert :: (Eq a) => a -> a -> ()
+simpleAssert x y =
+  if x == y then
+    ()
+  else
+    error "Assertion error"
+
+--------------------------------------------------------------------------------
+-- Library
+--------------------------------------------------------------------------------
+
+join :: [FilePath] -> FilePath
+join = F.joinPath
+
+--------------------------------------------------------------------------------
+-- Tests
+--------------------------------------------------------------------------------
+
+expected :: [([FilePath], FilePath)]
+expected = [ (["path"], "path")
+           , (["/path"], "/path")
+           , (["path", "to", "file"], "path/to/file")
+           , (["/path", "to", "file"], "/path/to/file")
+           , (["/"], "/")
+           ]
+
+runTests :: [()]
+runTests =
+  fmap (\(input, expected) -> simpleAssert (join input) expected) expected
+
+main :: IO ()
+main = do
+  print runTests
+  pure ()
diff --git a/haskell-file/shell.nix b/haskell-file/shell.nix
new file mode 100644
index 000000000000..f2621d6eac5a
--- /dev/null
+++ b/haskell-file/shell.nix
@@ -0,0 +1,9 @@
+with import <nixpkgs> {};
+
+stdenv.mkDerivation {
+  name = "f-hs";
+  buildInputs = [
+    (pkgs.haskellPackages.ghcWithPackages (pkgs: [
+    ]))
+  ];
+}
-- 
cgit 1.4.1


From a62553f7b7f51c735e12dc935bec57dec8eac25e Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Fri, 17 Jan 2020 20:08:24 +0000
Subject: Add DeepMind subdirectory

I need to prepare for my on-site with DeepMind, so I'll host some attempts to
solve data structures and algorithms questions here.
---
 deepmind/kth-to-last.py | 64 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)
 create mode 100644 deepmind/kth-to-last.py

diff --git a/deepmind/kth-to-last.py b/deepmind/kth-to-last.py
new file mode 100644
index 000000000000..5335e419f7ec
--- /dev/null
+++ b/deepmind/kth-to-last.py
@@ -0,0 +1,64 @@
+import unittest
+
+
+def kth_to_last_node(k, x):
+    a, b = x, x
+
+    if k == 0:
+        raise Exception('Value of 0 for k is not supported')
+
+    for _ in range(k - 1):
+        if not a.next:
+            raise Exception('Value of {} for k is too large'.format(k))
+        a = a.next
+
+    while a.next:
+        a, b = a.next, b.next
+    return b
+
+
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def setUp(self):
+        self.fourth = Test.LinkedListNode(4)
+        self.third = Test.LinkedListNode(3, self.fourth)
+        self.second = Test.LinkedListNode(2, self.third)
+        self.first = Test.LinkedListNode(1, self.second)
+
+    def test_first_to_last_node(self):
+        actual = kth_to_last_node(1, self.first)
+        expected = self.fourth
+        self.assertEqual(actual, expected)
+
+    def test_second_to_last_node(self):
+        actual = kth_to_last_node(2, self.first)
+        expected = self.third
+        self.assertEqual(actual, expected)
+
+    def test_first_node(self):
+        actual = kth_to_last_node(4, self.first)
+        expected = self.first
+        self.assertEqual(actual, expected)
+
+    def test_k_greater_than_linked_list_length(self):
+        with self.assertRaises(Exception):
+            kth_to_last_node(5, self.first)
+
+    def test_k_is_zero(self):
+        with self.assertRaises(Exception):
+            kth_to_last_node(0, self.first)
+
+
+unittest.main(verbosity=2)
-- 
cgit 1.4.1


From 34dc3e05c8a42c47023776d52ff33f100f2d310f Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Sat, 18 Jan 2020 17:04:05 +0000
Subject: Complete practice algorithms from InterviewCake.com

While I've done these algorithms before, I'm preparing for an on-site with
DeepMind, so I created a subdirectory called deepmind where I'm storing my
second attempts at these problems. The idea of storing them in a second
directory is to remove the urge to check my existing solutions that also exist
in this repository.
---
 deepmind/inflight-entertainment.py | 51 ++++++++++++++++++++++++++++++++
 deepmind/merging-ranges.py         | 59 ++++++++++++++++++++++++++++++++++++++
 deepmind/stock-price.py            | 51 ++++++++++++++++++++++++++++++++
 3 files changed, 161 insertions(+)
 create mode 100644 deepmind/inflight-entertainment.py
 create mode 100644 deepmind/merging-ranges.py
 create mode 100644 deepmind/stock-price.py

diff --git a/deepmind/inflight-entertainment.py b/deepmind/inflight-entertainment.py
new file mode 100644
index 000000000000..2116b27b0b97
--- /dev/null
+++ b/deepmind/inflight-entertainment.py
@@ -0,0 +1,51 @@
+import unittest
+
+
+def can_two_movies_fill_flight(xs, t):
+    seeking = set()
+    for x in xs:
+        if x in seeking:
+            return True
+        else:
+            seeking.add(t - x)
+    return False
+
+
+# Tests
+
+
+class Test(unittest.TestCase):
+    def test_short_flight(self):
+        result = can_two_movies_fill_flight([2, 4], 1)
+        self.assertFalse(result)
+
+    def test_long_flight(self):
+        result = can_two_movies_fill_flight([2, 4], 6)
+        self.assertTrue(result)
+
+    def test_one_movie_half_flight_length(self):
+        result = can_two_movies_fill_flight([3, 8], 6)
+        self.assertFalse(result)
+
+    def test_two_movies_half_flight_length(self):
+        result = can_two_movies_fill_flight([3, 8, 3], 6)
+        self.assertTrue(result)
+
+    def test_lots_of_possible_pairs(self):
+        result = can_two_movies_fill_flight([1, 2, 3, 4, 5, 6], 7)
+        self.assertTrue(result)
+
+    def test_not_using_first_movie(self):
+        result = can_two_movies_fill_flight([4, 3, 2], 5)
+        self.assertTrue(result)
+
+    def test_only_one_movie(self):
+        result = can_two_movies_fill_flight([6], 6)
+        self.assertFalse(result)
+
+    def test_no_movies(self):
+        result = can_two_movies_fill_flight([], 2)
+        self.assertFalse(result)
+
+
+unittest.main(verbosity=2)
diff --git a/deepmind/merging-ranges.py b/deepmind/merging-ranges.py
new file mode 100644
index 000000000000..23b40793b8f1
--- /dev/null
+++ b/deepmind/merging-ranges.py
@@ -0,0 +1,59 @@
+import unittest
+
+
+def merge_ranges(xs):
+    xs.sort()
+    result = [xs[0]]
+    for curr in xs[1:]:
+        a, z = result[-1]
+        if z >= curr[0]:
+            result[-1] = (a, max(z, curr[1]))
+        else:
+            result.append(curr)
+    return result
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_meetings_overlap(self):
+        actual = merge_ranges([(1, 3), (2, 4)])
+        expected = [(1, 4)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_touch(self):
+        actual = merge_ranges([(5, 6), (6, 8)])
+        expected = [(5, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meeting_contains_other_meeting(self):
+        actual = merge_ranges([(1, 8), (2, 5)])
+        expected = [(1, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_stay_separate(self):
+        actual = merge_ranges([(1, 3), (4, 8)])
+        expected = [(1, 3), (4, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_multiple_merged_meetings(self):
+        actual = merge_ranges([(1, 4), (2, 5), (5, 8)])
+        expected = [(1, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_not_sorted(self):
+        actual = merge_ranges([(5, 8), (1, 4), (6, 8)])
+        expected = [(1, 4), (5, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_one_long_meeting_contains_smaller_meetings(self):
+        actual = merge_ranges([(1, 10), (2, 5), (6, 8), (9, 10), (10, 12)])
+        expected = [(1, 12)]
+        self.assertEqual(actual, expected)
+
+    def test_sample_input(self):
+        actual = merge_ranges([(0, 1), (3, 5), (4, 8), (10, 12), (9, 10)])
+        expected = [(0, 1), (3, 8), (9, 12)]
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/deepmind/stock-price.py b/deepmind/stock-price.py
new file mode 100644
index 000000000000..7055b66af196
--- /dev/null
+++ b/deepmind/stock-price.py
@@ -0,0 +1,51 @@
+def get_max_profit(xs):
+    best_profit = xs[1] - xs[0]
+    lowest_buy = xs[0]
+
+    for x in xs[1:]:
+        best_profit = max(best_profit, x - lowest_buy)
+        lowest_buy = min(lowest_buy, x)
+    return best_profit
+
+
+# Tests
+
+import unittest
+
+
+class Test(unittest.TestCase):
+    def test_price_goes_up_then_down(self):
+        actual = get_max_profit([1, 5, 3, 2])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_down_then_up(self):
+        actual = get_max_profit([7, 2, 8, 9])
+        expected = 7
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_up_all_day(self):
+        actual = get_max_profit([1, 6, 7, 9])
+        expected = 8
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_down_all_day(self):
+        actual = get_max_profit([9, 7, 4, 1])
+        expected = -2
+        self.assertEqual(actual, expected)
+
+    def test_price_stays_the_same_all_day(self):
+        actual = get_max_profit([1, 1, 1, 1])
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_error_with_empty_prices(self):
+        with self.assertRaises(Exception):
+            get_max_profit([])
+
+    def test_error_with_one_price(self):
+        with self.assertRaises(Exception):
+            get_max_profit([1])
+
+
+unittest.main(verbosity=2)
-- 
cgit 1.4.1


From bb0de3dec2a610a3789c0fd02dd9e018e239b487 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Sat, 18 Jan 2020 17:05:32 +0000
Subject: Begin tests for Haskell File module

Cameron sent over some property tests for his File.split function, which is a
part of a larger effort to port f.el, a nice library for working with file
paths, over to Haskell.
---
 haskell-file/f.hs     | 17 +++++++++++++++++
 haskell-file/tests.hs | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)
 create mode 100644 haskell-file/tests.hs

diff --git a/haskell-file/f.hs b/haskell-file/f.hs
index 9ddb930ee103..295575f3f48d 100644
--- a/haskell-file/f.hs
+++ b/haskell-file/f.hs
@@ -1,7 +1,14 @@
 module F
   ( join
+  , split
   ) where
 
+--------------------------------------------------------------------------------
+-- Dependencies
+--------------------------------------------------------------------------------
+
+import Data.List (span)
+import System.FilePath (FilePath, pathSeparator)
 import System.FilePath.Posix (FilePath)
 import qualified System.FilePath.Posix as F
 
@@ -25,6 +32,16 @@ simpleAssert x y =
 join :: [FilePath] -> FilePath
 join = F.joinPath
 
+-- | Split path and return  list containing parts.
+split :: FilePath -> [String]
+split = splitJoin . span (/= pathSeparator)
+  where
+    splitJoin :: (String, String) -> [String]
+    splitJoin ([], []) = []
+    splitJoin (a, []) = [a]
+    splitJoin (a, [_]) = [a]
+    splitJoin (a, _:b) = a : split b
+
 --------------------------------------------------------------------------------
 -- Tests
 --------------------------------------------------------------------------------
diff --git a/haskell-file/tests.hs b/haskell-file/tests.hs
new file mode 100644
index 000000000000..e3967b77de1f
--- /dev/null
+++ b/haskell-file/tests.hs
@@ -0,0 +1,39 @@
+module FTest where
+--------------------------------------------------------------------------------
+import Test.Tasty
+import Test.Tasty.Hedgehog
+import Hedgehog
+--------------------------------------------------------------------------------
+import qualified Hedgehog as H
+import qualified Hedgehog.Gen as Gen
+import qualified Hedgehog.Range as Range
+--------------------------------------------------------------------------------
+import Data.List (intercalate)
+import System.FilePath (pathSeparator)
+--------------------------------------------------------------------------------
+import F
+--------------------------------------------------------------------------------
+main :: IO ()
+main
+  = defaultMain
+  . localOption (HedgehogTestLimit $ Just 50)
+  $ testGroup "f functions"
+  [ test_split
+  ]
+--------------------------------------------------------------------------------
+test_split :: TestTree
+test_split
+  = testGroup "split function"
+  [ testProperty "splits parts properly" splitSuccess
+  ]
+splitSuccess :: Property
+splitSuccess = property $ do
+  -- separator
+  --   <- H.forAll
+  --   $ Gen.element ['/', '\\']
+  parts
+    <- H.forAll
+    . Gen.list (Range.linear 0 10)
+    $ Gen.list (Range.linear 1 10) Gen.alphaNum
+  let path = intercalate [pathSeparator] parts
+  F.split path === parts
-- 
cgit 1.4.1


From 64bd3f030313765bab3cfdc76bccc69105c764dc Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 22 Jan 2020 10:08:56 +0000
Subject: Complete balanced binary tree problem

Supporting a function that returns true if a tree has no two leaf nodes with
depth differences greater than 1.
---
 deepmind/balanced-binary-tree.py | 123 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 123 insertions(+)
 create mode 100644 deepmind/balanced-binary-tree.py

diff --git a/deepmind/balanced-binary-tree.py b/deepmind/balanced-binary-tree.py
new file mode 100644
index 000000000000..7fc174a2a9f3
--- /dev/null
+++ b/deepmind/balanced-binary-tree.py
@@ -0,0 +1,123 @@
+import unittest
+from collections import deque
+
+
+def is_balanced(node):
+    q, seen, ds = deque(), set(), set()
+    q.append((0, node))
+    while q:
+        d, node = q.popleft()
+        l, r = node.left, node.right
+        seen.add(node)
+        if not l and not r:
+            if d not in ds and len(ds) == 2:
+                return False
+            else:
+                ds.add(d)
+        if l and l not in seen:
+            q.append((d + 1, l))
+        if r and r not in seen:
+            q.append((d + 1, r))
+    return max(ds) - min(ds) <= 1
+
+
+# Tests
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_full_tree(self):
+        tree = Test.BinaryTreeNode(5)
+        left = tree.insert_left(8)
+        right = tree.insert_right(6)
+        left.insert_left(1)
+        left.insert_right(2)
+        right.insert_left(3)
+        right.insert_right(4)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_both_leaves_at_the_same_depth(self):
+        tree = Test.BinaryTreeNode(3)
+        left = tree.insert_left(4)
+        right = tree.insert_right(2)
+        left.insert_left(1)
+        right.insert_right(9)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_leaf_heights_differ_by_one(self):
+        tree = Test.BinaryTreeNode(6)
+        left = tree.insert_left(1)
+        right = tree.insert_right(0)
+        right.insert_right(7)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_leaf_heights_differ_by_two(self):
+        tree = Test.BinaryTreeNode(6)
+        left = tree.insert_left(1)
+        right = tree.insert_right(0)
+        right_right = right.insert_right(7)
+        right_right.insert_right(8)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_three_leaves_total(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(5)
+        right = tree.insert_right(9)
+        right.insert_left(8)
+        right.insert_right(5)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_both_subtrees_superbalanced(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(5)
+        right = tree.insert_right(9)
+        right_left = right.insert_left(8)
+        right.insert_right(5)
+        right_left.insert_left(7)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_both_subtrees_superbalanced_two(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(2)
+        right = tree.insert_right(4)
+        left.insert_left(3)
+        left_right = left.insert_right(7)
+        left_right.insert_right(8)
+        right_right = right.insert_right(5)
+        right_right_right = right_right.insert_right(6)
+        right_right_right.insert_right(9)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_only_one_node(self):
+        tree = Test.BinaryTreeNode(1)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_linked_list_tree(self):
+        tree = Test.BinaryTreeNode(1)
+        right = tree.insert_right(2)
+        right_right = right.insert_right(3)
+        right_right.insert_right(4)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
-- 
cgit 1.4.1


From bbea699f06157d0291f837d4e09a0f391ad09a06 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 22 Jan 2020 10:09:46 +0000
Subject: Complete string permutations problem

Solves an InterviewCake.com problem that returns all of the permutations of a
string input. The problem states that it's acceptable to assume that your input
string will not have repeated characters, which is why using a Set is
acceptable. I like this solution because it builds a permutations tree and then
assembles all of the permutations by doing a DFT over that tree.
---
 deepmind/recursive-string-permutations.py | 56 +++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)
 create mode 100644 deepmind/recursive-string-permutations.py

diff --git a/deepmind/recursive-string-permutations.py b/deepmind/recursive-string-permutations.py
new file mode 100644
index 000000000000..f50db2838707
--- /dev/null
+++ b/deepmind/recursive-string-permutations.py
@@ -0,0 +1,56 @@
+import unittest
+from itertools import permutations
+
+
+class Node(object):
+    def __init__(self, x):
+        self.value = x
+        self.children = []
+
+
+def make_tree(c, xs):
+    root = Node(c)
+    for x in xs:
+        root.children.append(make_tree(x, xs - {x}))
+    return root
+
+
+def get_permutations(xs):
+    xs = set(xs)
+    root = make_tree("", xs)
+    q, perms = [], set()
+    q.append(("", root))
+    while q:
+        c, node = q.pop()
+        if not node.children:
+            perms.add(c)
+        else:
+            for child in node.children:
+                q.append((c + child.value, child))
+    return perms
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_empty_string(self):
+        actual = get_permutations('')
+        expected = set([''])
+        self.assertEqual(actual, expected)
+
+    def test_one_character_string(self):
+        actual = get_permutations('a')
+        expected = set(['a'])
+        self.assertEqual(actual, expected)
+
+    def test_two_character_string(self):
+        actual = get_permutations('ab')
+        expected = set(['ab', 'ba'])
+        self.assertEqual(actual, expected)
+
+    def test_three_character_string(self):
+        actual = get_permutations('abc')
+        expected = set(['abc', 'acb', 'bac', 'bca', 'cab', 'cba'])
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
-- 
cgit 1.4.1


From de039c7133c80caa76e0549e2388a6e54768286f Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 22 Jan 2020 10:11:23 +0000
Subject: Complete find-rotation-point

Solves an InterviewCake.com problem that returns the index of the element in a
list that should be the first element in that list. It's an exercise that's
useful for seeing other applications of a binary search.
---
 deepmind/find-rotation-point.py | 55 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)
 create mode 100644 deepmind/find-rotation-point.py

diff --git a/deepmind/find-rotation-point.py b/deepmind/find-rotation-point.py
new file mode 100644
index 000000000000..5c21d5167ce9
--- /dev/null
+++ b/deepmind/find-rotation-point.py
@@ -0,0 +1,55 @@
+import unittest
+from math import floor
+
+
+def midpoint(a, b):
+    return a + floor((b - a) / 2)
+
+
+def do_find_rotation_point(a, b, xs):
+    i = midpoint(a, b)
+    count = b - a + 1
+
+    if count == 2:
+        if xs[a] > xs[b]:
+            return b
+        else:
+            return -1
+
+    if i in {a, b}:
+        return i
+
+    if xs[a] < xs[i]:
+        return do_find_rotation_point(i, b, xs)
+    else:
+        return do_find_rotation_point(a, i, xs)
+
+
+def find_rotation_point(xs):
+    return do_find_rotation_point(0, len(xs) - 1, xs)
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_small_list(self):
+        actual = find_rotation_point(['cape', 'cake'])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_rotation_point(
+            ['grape', 'orange', 'plum', 'radish', 'apple'])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_large_list(self):
+        actual = find_rotation_point([
+            'ptolemaic', 'retrograde', 'supplant', 'undulate', 'xenoepist',
+            'asymptote', 'babka', 'banoffee', 'engender', 'karpatka',
+            'othellolagkage'
+        ])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
-- 
cgit 1.4.1


From 2a0540d76da26497d4600e9666bb6f36355357c1 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 22 Jan 2020 10:12:27 +0000
Subject: Create org table of sorting algorithms and their efficiency

This is just a small org table that I created to help me

Fun fact: In Emacs, you can insert literal TAB character by pressing `C-q
TAB`. For creating tables, using TAB characters feels perfectly
acceptable. Perhaps the TAB name comes from TABle.
---
 deepmind/efficiency.org | 6 ++++++
 1 file changed, 6 insertions(+)
 create mode 100644 deepmind/efficiency.org

diff --git a/deepmind/efficiency.org b/deepmind/efficiency.org
new file mode 100644
index 000000000000..89a45c52ad8a
--- /dev/null
+++ b/deepmind/efficiency.org
@@ -0,0 +1,6 @@
+* Sorting
+** Merge:	O(n*log(n))
+** Heap:	O(n*log(n))
+** Insertion:	O(n^2)
+** Quick:	O(n^2)
+** Bubble:	O(n^2)
-- 
cgit 1.4.1


From 4f6191b34cb0f29ece4930ee7279048cba7e33f2 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 22 Jan 2020 11:05:43 +0000
Subject: Complete reverse-linked-list problem

Completing the deceptively tricky reverse-linked-list problem from
InterviewCake.com.
---
 deepmind/reverse-linked-list.py | 74 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 74 insertions(+)
 create mode 100644 deepmind/reverse-linked-list.py

diff --git a/deepmind/reverse-linked-list.py b/deepmind/reverse-linked-list.py
new file mode 100644
index 000000000000..82fac171d5d1
--- /dev/null
+++ b/deepmind/reverse-linked-list.py
@@ -0,0 +1,74 @@
+import unittest
+
+
+def reverse(node):
+    prev = None
+    next = None
+    curr = node
+
+    while curr:
+        next = curr.next
+        curr.next = prev
+        prev = curr
+        curr = next
+
+    return prev
+
+
+# Tests
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def test_short_linked_list(self):
+        second = Test.LinkedListNode(2)
+        first = Test.LinkedListNode(1, second)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [2, 1]
+        self.assertEqual(actual, expected)
+
+    def test_long_linked_list(self):
+        sixth = Test.LinkedListNode(6)
+        fifth = Test.LinkedListNode(5, sixth)
+        fourth = Test.LinkedListNode(4, fifth)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [6, 5, 4, 3, 2, 1]
+        self.assertEqual(actual, expected)
+
+    def test_one_element_linked_list(self):
+        first = Test.LinkedListNode(1)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [1]
+        self.assertEqual(actual, expected)
+
+    def test_empty_linked_list(self):
+        result = reverse(None)
+        self.assertIsNone(result)
+
+
+unittest.main(verbosity=2)
-- 
cgit 1.4.1


From 91811236a56c6323f469d5baf6eaa4e436658bb8 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 22 Jan 2020 11:06:10 +0000
Subject: Complete which-appears-twice

Solves the InterviewCake.com problem that asks us to write a function that
returns the number, y, that occurs twice in a list, xs, where xs is an unsorted
list of integers from 1..n with a length of n + 1.
---
 deepmind/which-appears-twice.py | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)
 create mode 100644 deepmind/which-appears-twice.py

diff --git a/deepmind/which-appears-twice.py b/deepmind/which-appears-twice.py
new file mode 100644
index 000000000000..c01379295d32
--- /dev/null
+++ b/deepmind/which-appears-twice.py
@@ -0,0 +1,29 @@
+import unittest
+
+
+def find_repeat(xs):
+    n = max(xs)
+    expected_sum = (n + 1) * n / 2
+    actual_sum = sum(xs)
+    return actual_sum - expected_sum
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_short_list(self):
+        actual = find_repeat([1, 2, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_repeat([4, 1, 3, 4, 2])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_repeat([1, 5, 9, 7, 2, 6, 3, 8, 2, 4])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
-- 
cgit 1.4.1


From c010e6d6cff1e348645c337b1c63f8d66a6d6859 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 22 Jan 2020 11:07:07 +0000
Subject: Practice dijkstra's algorithm

Getting some practice with Python's heapq module (which I'm unsure if I used
correctly) to do a priority-first-traversal of a graph: known as Dijkstra's
algorithm.
---
 deepmind/dijkstra.py | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)
 create mode 100644 deepmind/dijkstra.py

diff --git a/deepmind/dijkstra.py b/deepmind/dijkstra.py
new file mode 100644
index 000000000000..6975dbe4d1d6
--- /dev/null
+++ b/deepmind/dijkstra.py
@@ -0,0 +1,26 @@
+# Doing a practice implementation of Dijkstra's algorithm: a priority-first
+# search.
+from heapq import heappush, heappop
+
+
+class Node(object):
+    def __init__(self, value, children):
+        self.value = value
+        self.children = children
+
+
+def shortest_path(a, b):
+    """Return the shortest path from `a` to `b`."""
+    q = []
+    seen = set()
+    heappush((a.value, a, [a]), q)
+
+    while q:
+        d, node, path = heappop(q)
+        if node == b:
+            return path
+        seen.add(node)
+        for child in node.children:
+            if child not in seen:
+                heappush((d + child.value, child, path + [child]), q)
+    raise Exception("Path between nodes A and B does not exist.")
-- 
cgit 1.4.1


From 55860debe5e480a043ae47833a218b70a70bf19f Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Wed, 22 Jan 2020 21:52:45 +0000
Subject: Support Common Lisp

Adding some Common Lisp here to get the party started! *cues music*
---
 common-lisp/main.lisp | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
 create mode 100644 common-lisp/main.lisp

diff --git a/common-lisp/main.lisp b/common-lisp/main.lisp
new file mode 100644
index 000000000000..2c4a5a411635
--- /dev/null
+++ b/common-lisp/main.lisp
@@ -0,0 +1,15 @@
+(in-package #:cl-user)
+(defpackage #:utils
+  (:documentation "Some utility functions and macros to wet my beak.")
+  (:use #:cl)
+  (:shadow #:type))
+(in-package #:utils)
+
+(defmacro type (name in out)
+  `(declaim (ftype (function ,in ,out) ,name)))
+
+(defmacro comment (&rest _forms) nil)
+
+(type add (int int) int)
+(defun add (a b)
+  (+ a b))
-- 
cgit 1.4.1


From d262d3054393fe8c445d80269fed9ef7e5b159d8 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 13:45:57 +0000
Subject: Attempt to package CL unit testing with Nix

My first foray trying to package Common Lisp with Nix. I'm using @tazjin's
buildLisp and other libraries, all of which I'm importing as `tpkgs`, and all of
which have been a tremendous boon to my productivity.
---
 common-lisp/anaphora.nix      | 17 +++++++++++++++++
 common-lisp/cl-colors.nix     | 20 ++++++++++++++++++++
 common-lisp/default.nix       | 13 +++++++++++++
 common-lisp/let-plus.nix      | 19 +++++++++++++++++++
 common-lisp/prove-asdf.nix    | 15 +++++++++++++++
 common-lisp/prove.nix         | 31 +++++++++++++++++++++++++++++++
 common-lisp/unit-testing.lisp | 13 +++++++++++++
 7 files changed, 128 insertions(+)
 create mode 100644 common-lisp/anaphora.nix
 create mode 100644 common-lisp/cl-colors.nix
 create mode 100644 common-lisp/default.nix
 create mode 100644 common-lisp/let-plus.nix
 create mode 100644 common-lisp/prove-asdf.nix
 create mode 100644 common-lisp/prove.nix
 create mode 100644 common-lisp/unit-testing.lisp

diff --git a/common-lisp/anaphora.nix b/common-lisp/anaphora.nix
new file mode 100644
index 000000000000..f64d01a50eee
--- /dev/null
+++ b/common-lisp/anaphora.nix
@@ -0,0 +1,17 @@
+{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/tokenrove/anaphora.git";
+    rev = "aeace4c68cf55098a67112750b28f8f2dc6d0e30";
+  };
+in tpkgs.nix.buildLisp.library {
+  name = "anaphora";
+  deps = [];
+  srcs = [
+    "${src}/packages.lisp"
+    "${src}/early.lisp"
+    "${src}/symbolic.lisp"
+    "${src}/anaphora.lisp"
+  ];
+}
diff --git a/common-lisp/cl-colors.nix b/common-lisp/cl-colors.nix
new file mode 100644
index 000000000000..cff40fef342d
--- /dev/null
+++ b/common-lisp/cl-colors.nix
@@ -0,0 +1,20 @@
+{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/tpapp/cl-colors.git";
+    rev = "827410584553f5c717eec6182343b7605f707f75";
+  };
+in tpkgs.nix.buildLisp.library {
+  name = "cl-colors";
+  deps = with tpkgs.third_party.lisp; [
+    alexandria
+    (import ./let-plus.nix {})
+  ];
+  srcs = [
+    "${src}/package.lisp"
+    "${src}/colors.lisp"
+    "${src}/colornames.lisp"
+    "${src}/hexcolors.lisp"
+  ];
+}
diff --git a/common-lisp/default.nix b/common-lisp/default.nix
new file mode 100644
index 000000000000..b3c51eea704c
--- /dev/null
+++ b/common-lisp/default.nix
@@ -0,0 +1,13 @@
+{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
+
+tpkgs.nix.buildLisp.program {
+  name = "unit-testing";
+
+  deps = with tpkgs.third_party.lisp; [
+    (import ./prove.nix {})
+  ];
+
+  srcs = [
+    ./unit-testing.lisp
+  ];
+}
diff --git a/common-lisp/let-plus.nix b/common-lisp/let-plus.nix
new file mode 100644
index 000000000000..5b279477de1b
--- /dev/null
+++ b/common-lisp/let-plus.nix
@@ -0,0 +1,19 @@
+{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/tpapp/let-plus.git";
+    rev = "7cf18b29ed0fe9c667a9a6a101b08ab9661a59e9";
+  };
+in tpkgs.nix.buildLisp.library {
+  name = "let-plus";
+  deps = with tpkgs.third_party.lisp; [
+    alexandria
+    (import ./anaphora.nix {})
+  ];
+  srcs = [
+    "${src}/package.lisp"
+    "${src}/let-plus.lisp"
+    "${src}/extensions.lisp"
+  ];
+}
diff --git a/common-lisp/prove-asdf.nix b/common-lisp/prove-asdf.nix
new file mode 100644
index 000000000000..8a692cf125ad
--- /dev/null
+++ b/common-lisp/prove-asdf.nix
@@ -0,0 +1,15 @@
+{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/fukamachi/prove.git";
+    rev = "5d71f02795b89e36f34e8c7d50e69b67ec6ca2de";
+  };
+in tpkgs.nix.buildLisp.library {
+  name = "prove-asdf";
+  deps = [];
+  srcs = [
+    "${src}/src/output.lisp"
+    "${src}/src/asdf.lisp"
+  ];
+}
diff --git a/common-lisp/prove.nix b/common-lisp/prove.nix
new file mode 100644
index 000000000000..a4499dcea9bb
--- /dev/null
+++ b/common-lisp/prove.nix
@@ -0,0 +1,31 @@
+{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/fukamachi/prove.git";
+    rev = "5d71f02795b89e36f34e8c7d50e69b67ec6ca2de";
+  };
+in tpkgs.nix.buildLisp.library {
+  name = "prove";
+  deps = with tpkgs.third_party.lisp; [
+    cl-ppcre
+    cl-ansi-text
+    (import ./cl-colors.nix {})
+    alexandria
+    uiop
+  ];
+  srcs = [
+    "${src}/src/asdf.lisp"
+    "${src}/src/suite.lisp"
+    "${src}/src/color.lisp"
+    "${src}/src/output.lisp"
+    "${src}/src/prove.lisp"
+    "${src}/src/report.lisp"
+    "${src}/src/reporter.lisp"
+    "${src}/src/test.lisp"
+    "${src}/src/reporter/dot.lisp"
+    "${src}/src/reporter/fiveam.lisp"
+    "${src}/src/reporter/list.lisp"
+    "${src}/src/reporter/tap.lisp"
+  ];
+}
diff --git a/common-lisp/unit-testing.lisp b/common-lisp/unit-testing.lisp
new file mode 100644
index 000000000000..c0b3be9b4515
--- /dev/null
+++ b/common-lisp/unit-testing.lisp
@@ -0,0 +1,13 @@
+(in-package #:cl-user)
+(defpackage #:my-test
+  (:documentation "Unit testing in Common Lisp.")
+  (:use #:cl))
+(in-package #:my-test)
+
+(plan 3)
+
+(ok (not (find 4 '(1 2 3))))
+(is 4 4)
+(isnt 1 #\1)
+
+(finalize)
-- 
cgit 1.4.1


From 01539a9988b2e7bdd86ff78ef75fee25b77f38c7 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 16:30:06 +0000
Subject: Support .envrc

Add `NIX_PATH` aliases for `depot` and `universe`.
---
 .envrc | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 .envrc

diff --git a/.envrc b/.envrc
new file mode 100644
index 000000000000..b56819ed8cdf
--- /dev/null
+++ b/.envrc
@@ -0,0 +1 @@
+NIX_PATH=depot=$HOME/depot:universe=$HOME/mono
-- 
cgit 1.4.1


From 393bd0a5c7967c9eafbf228413416d7691125e59 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 16:30:39 +0000
Subject: Nixify repo

Using @tazjin's depot/default.nix to bootstrap this project. I'll be borrowing
his Nix idioms until I better understand Nix and have preferences of my own.
---
 default.nix | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)
 create mode 100644 default.nix

diff --git a/default.nix b/default.nix
new file mode 100644
index 000000000000..d1fb88e04f93
--- /dev/null
+++ b/default.nix
@@ -0,0 +1,33 @@
+# At the time of this writing, this configuration was taken from @tazjin's
+# default.nix from his depot. I've added, changed, and removed that parts that I
+# don't need, and this is what remains.
+{ ... }@args:
+
+with builtins;
+
+let
+  fix = f: let x = f x; in x;
+
+  # Global configuration that all packages are called with.
+  config = pkgs: {
+    inherit pkgs;
+  };
+
+  readTree' = import /home/wpcarro/depot/nix/readTree {};
+
+  localPkgs = readTree: {
+    third_party   = readTree ./third_party;
+  };
+in fix(self: {
+  config = config self;
+
+  # Expose readTree for downstream repo consumers.
+  readTree = {
+    __functor = x: (readTree' x.config);
+    config = self.config;
+  };
+}
+
+# Add local packages as structured by readTree
+// (localPkgs (readTree' (self.config // { inherit (self) lib; })))
+)
-- 
cgit 1.4.1


From e76843d337546fe83dbbc9eba9cff52ca6767ef7 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 16:31:54 +0000
Subject: Create third_party

Create a third_party subdirectory and a third_party/lisp. This directory layout
resembles and is inspired by the layout of Google's mono-repo, Google3. @tazjin
borrowed this idea from Google and I'm borrowing the idea from him.
---
 common-lisp/anaphora.nix       | 17 -----------------
 common-lisp/cl-colors.nix      | 20 --------------------
 common-lisp/let-plus.nix       | 19 -------------------
 common-lisp/prove-asdf.nix     | 15 ---------------
 common-lisp/prove.nix          | 31 -------------------------------
 third_party/lisp/anaphora.nix  | 17 +++++++++++++++++
 third_party/lisp/cl-colors.nix | 24 ++++++++++++++++++++++++
 third_party/lisp/let-plus.nix  | 23 +++++++++++++++++++++++
 third_party/lisp/prove.nix     | 35 +++++++++++++++++++++++++++++++++++
 9 files changed, 99 insertions(+), 102 deletions(-)
 delete mode 100644 common-lisp/anaphora.nix
 delete mode 100644 common-lisp/cl-colors.nix
 delete mode 100644 common-lisp/let-plus.nix
 delete mode 100644 common-lisp/prove-asdf.nix
 delete mode 100644 common-lisp/prove.nix
 create mode 100644 third_party/lisp/anaphora.nix
 create mode 100644 third_party/lisp/cl-colors.nix
 create mode 100644 third_party/lisp/let-plus.nix
 create mode 100644 third_party/lisp/prove.nix

diff --git a/common-lisp/anaphora.nix b/common-lisp/anaphora.nix
deleted file mode 100644
index f64d01a50eee..000000000000
--- a/common-lisp/anaphora.nix
+++ /dev/null
@@ -1,17 +0,0 @@
-{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
-
-let
-  src = builtins.fetchGit {
-    url = "https://github.com/tokenrove/anaphora.git";
-    rev = "aeace4c68cf55098a67112750b28f8f2dc6d0e30";
-  };
-in tpkgs.nix.buildLisp.library {
-  name = "anaphora";
-  deps = [];
-  srcs = [
-    "${src}/packages.lisp"
-    "${src}/early.lisp"
-    "${src}/symbolic.lisp"
-    "${src}/anaphora.lisp"
-  ];
-}
diff --git a/common-lisp/cl-colors.nix b/common-lisp/cl-colors.nix
deleted file mode 100644
index cff40fef342d..000000000000
--- a/common-lisp/cl-colors.nix
+++ /dev/null
@@ -1,20 +0,0 @@
-{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
-
-let
-  src = builtins.fetchGit {
-    url = "https://github.com/tpapp/cl-colors.git";
-    rev = "827410584553f5c717eec6182343b7605f707f75";
-  };
-in tpkgs.nix.buildLisp.library {
-  name = "cl-colors";
-  deps = with tpkgs.third_party.lisp; [
-    alexandria
-    (import ./let-plus.nix {})
-  ];
-  srcs = [
-    "${src}/package.lisp"
-    "${src}/colors.lisp"
-    "${src}/colornames.lisp"
-    "${src}/hexcolors.lisp"
-  ];
-}
diff --git a/common-lisp/let-plus.nix b/common-lisp/let-plus.nix
deleted file mode 100644
index 5b279477de1b..000000000000
--- a/common-lisp/let-plus.nix
+++ /dev/null
@@ -1,19 +0,0 @@
-{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
-
-let
-  src = builtins.fetchGit {
-    url = "https://github.com/tpapp/let-plus.git";
-    rev = "7cf18b29ed0fe9c667a9a6a101b08ab9661a59e9";
-  };
-in tpkgs.nix.buildLisp.library {
-  name = "let-plus";
-  deps = with tpkgs.third_party.lisp; [
-    alexandria
-    (import ./anaphora.nix {})
-  ];
-  srcs = [
-    "${src}/package.lisp"
-    "${src}/let-plus.lisp"
-    "${src}/extensions.lisp"
-  ];
-}
diff --git a/common-lisp/prove-asdf.nix b/common-lisp/prove-asdf.nix
deleted file mode 100644
index 8a692cf125ad..000000000000
--- a/common-lisp/prove-asdf.nix
+++ /dev/null
@@ -1,15 +0,0 @@
-{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
-
-let
-  src = builtins.fetchGit {
-    url = "https://github.com/fukamachi/prove.git";
-    rev = "5d71f02795b89e36f34e8c7d50e69b67ec6ca2de";
-  };
-in tpkgs.nix.buildLisp.library {
-  name = "prove-asdf";
-  deps = [];
-  srcs = [
-    "${src}/src/output.lisp"
-    "${src}/src/asdf.lisp"
-  ];
-}
diff --git a/common-lisp/prove.nix b/common-lisp/prove.nix
deleted file mode 100644
index a4499dcea9bb..000000000000
--- a/common-lisp/prove.nix
+++ /dev/null
@@ -1,31 +0,0 @@
-{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
-
-let
-  src = builtins.fetchGit {
-    url = "https://github.com/fukamachi/prove.git";
-    rev = "5d71f02795b89e36f34e8c7d50e69b67ec6ca2de";
-  };
-in tpkgs.nix.buildLisp.library {
-  name = "prove";
-  deps = with tpkgs.third_party.lisp; [
-    cl-ppcre
-    cl-ansi-text
-    (import ./cl-colors.nix {})
-    alexandria
-    uiop
-  ];
-  srcs = [
-    "${src}/src/asdf.lisp"
-    "${src}/src/suite.lisp"
-    "${src}/src/color.lisp"
-    "${src}/src/output.lisp"
-    "${src}/src/prove.lisp"
-    "${src}/src/report.lisp"
-    "${src}/src/reporter.lisp"
-    "${src}/src/test.lisp"
-    "${src}/src/reporter/dot.lisp"
-    "${src}/src/reporter/fiveam.lisp"
-    "${src}/src/reporter/list.lisp"
-    "${src}/src/reporter/tap.lisp"
-  ];
-}
diff --git a/third_party/lisp/anaphora.nix b/third_party/lisp/anaphora.nix
new file mode 100644
index 000000000000..04a1dd847feb
--- /dev/null
+++ b/third_party/lisp/anaphora.nix
@@ -0,0 +1,17 @@
+{ depot ? import <depot> {}, ... }:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/tokenrove/anaphora.git";
+    rev = "aeace4c68cf55098a67112750b28f8f2dc6d0e30";
+  };
+in depot.nix.buildLisp.library {
+  name = "anaphora";
+  deps = [];
+  srcs = [
+    "${src}/packages.lisp"
+    "${src}/early.lisp"
+    "${src}/symbolic.lisp"
+    "${src}/anaphora.lisp"
+  ];
+}
diff --git a/third_party/lisp/cl-colors.nix b/third_party/lisp/cl-colors.nix
new file mode 100644
index 000000000000..2217e68ce70b
--- /dev/null
+++ b/third_party/lisp/cl-colors.nix
@@ -0,0 +1,24 @@
+{
+  depot ? import <depot> {},
+  universe ? import <universe> {},
+  ...
+}:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/tpapp/cl-colors.git";
+    rev = "827410584553f5c717eec6182343b7605f707f75";
+  };
+in depot.nix.buildLisp.library {
+  name = "cl-colors";
+  deps = [
+    depot.third_party.lisp.alexandria
+    universe.third_party.lisp.let-plus
+  ];
+  srcs = [
+    "${src}/package.lisp"
+    "${src}/colors.lisp"
+    "${src}/colornames.lisp"
+    "${src}/hexcolors.lisp"
+  ];
+}
diff --git a/third_party/lisp/let-plus.nix b/third_party/lisp/let-plus.nix
new file mode 100644
index 000000000000..a3a15776bf86
--- /dev/null
+++ b/third_party/lisp/let-plus.nix
@@ -0,0 +1,23 @@
+{
+  depot ? import <depot> {},
+  universe ? import <universe> {},
+  ...
+}:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/tpapp/let-plus.git";
+    rev = "7cf18b29ed0fe9c667a9a6a101b08ab9661a59e9";
+  };
+in depot.nix.buildLisp.library {
+  name = "let-plus";
+  deps = [
+    depot.third_party.lisp.alexandria
+    universe.third_party.lisp.anaphora
+  ];
+  srcs = [
+    "${src}/package.lisp"
+    "${src}/let-plus.lisp"
+    "${src}/extensions.lisp"
+  ];
+}
diff --git a/third_party/lisp/prove.nix b/third_party/lisp/prove.nix
new file mode 100644
index 000000000000..d6c0fe7413cb
--- /dev/null
+++ b/third_party/lisp/prove.nix
@@ -0,0 +1,35 @@
+{
+  depot ? import <depot> {},
+  universe? import <universe> {},
+  ...
+}:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/fukamachi/prove.git";
+    rev = "5d71f02795b89e36f34e8c7d50e69b67ec6ca2de";
+  };
+in depot.nix.buildLisp.library {
+  name = "prove";
+  deps = [
+    depot.third_party.lisp.cl-ppcre
+    depot.third_party.lisp.cl-ansi-text
+    depot.third_party.lisp.alexandria
+    depot.third_party.lisp.uiop
+    universe.third_party.lisp.cl-colors
+  ];
+  srcs = [
+    "${src}/src/asdf.lisp"
+    "${src}/src/suite.lisp"
+    "${src}/src/color.lisp"
+    "${src}/src/output.lisp"
+    "${src}/src/prove.lisp"
+    "${src}/src/report.lisp"
+    "${src}/src/reporter.lisp"
+    "${src}/src/test.lisp"
+    "${src}/src/reporter/dot.lisp"
+    "${src}/src/reporter/fiveam.lisp"
+    "${src}/src/reporter/list.lisp"
+    "${src}/src/reporter/tap.lisp"
+  ];
+}
-- 
cgit 1.4.1


From 7f37acf548307448c92bd567de096d5734ac6f0d Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 16:33:04 +0000
Subject: Prefer <depot> and <universe>

Consuming the aliases that I defined in `NIX_PATH` in `.envrc`.
---
 common-lisp/default.nix | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/common-lisp/default.nix b/common-lisp/default.nix
index b3c51eea704c..0969c7abf7e2 100644
--- a/common-lisp/default.nix
+++ b/common-lisp/default.nix
@@ -1,10 +1,13 @@
-{ tpkgs ? (import (builtins.fetchGit "https://git.tazj.in/") {}), ... }:
+{ depot ? import <depot> {},
+  universe ? import <universe> {},
+  ...
+}:
 
-tpkgs.nix.buildLisp.program {
+depot.nix.buildLisp.program {
   name = "unit-testing";
 
-  deps = with tpkgs.third_party.lisp; [
-    (import ./prove.nix {})
+  deps = with universe.third_party.lisp; [
+    prove
   ];
 
   srcs = [
-- 
cgit 1.4.1


From 2c28f85946538397e0a32ef334b525e5db29712c Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 16:34:07 +0000
Subject: Start working on a blog

Attempting to write a blog where:

- The server is Common Lisp. Why? I'd like to learn Common Lisp.
- The blog posts can be written in Markdown.
- The package is developed and deployed using Nix.

Most of this is a learning exercise. The blog itself is something that I'd like
to use instead of Medium and other forums.
---
 blog/default.nix     | 15 +++++++++++++++
 blog/posts/test.md   |  4 ++++
 blog/src/server.lisp | 10 ++++++++++
 3 files changed, 29 insertions(+)
 create mode 100644 blog/default.nix
 create mode 100644 blog/posts/test.md
 create mode 100644 blog/src/server.lisp

diff --git a/blog/default.nix b/blog/default.nix
new file mode 100644
index 000000000000..7005cc648d00
--- /dev/null
+++ b/blog/default.nix
@@ -0,0 +1,15 @@
+{
+  depot ? import <depot> {},
+  universe ? import <universe> {},
+  ...
+}:
+
+depot.nix.buildLisp.program {
+  name = "server";
+  deps = with depot.third_party.lisp; [
+    hunchentoot
+  ];
+  srcs = [
+    ./src/server.lisp
+  ];
+}
diff --git a/blog/posts/test.md b/blog/posts/test.md
new file mode 100644
index 000000000000..ec2e030b2824
--- /dev/null
+++ b/blog/posts/test.md
@@ -0,0 +1,4 @@
+# Testing
+
+The goal here is to be able to write markdown files and have a server that can
+render the markdown into HTML and serve them to the clients.
diff --git a/blog/src/server.lisp b/blog/src/server.lisp
new file mode 100644
index 000000000000..63323e933bd3
--- /dev/null
+++ b/blog/src/server.lisp
@@ -0,0 +1,10 @@
+(in-package #:cl-user)
+(defpackage #:server
+  (:documentation "Robot condemned to a life of admin work for my blog.")
+  (:use #:cl)
+  (:export :main))
+(in-package #:server)
+
+(defun main ()
+  "This is the main entrypoint for our application."
+  (hunchentoot))
-- 
cgit 1.4.1


From 336a1fdf9f72ef9559fa71c073d5e076e34466d5 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 18:24:20 +0000
Subject: Add <nixpkgs> to NIX_PATH

Lest we forget our roots.
---
 .envrc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.envrc b/.envrc
index b56819ed8cdf..bf79e325a121 100644
--- a/.envrc
+++ b/.envrc
@@ -1 +1 @@
-NIX_PATH=depot=$HOME/depot:universe=$HOME/mono
+export NIX_PATH=nixpkgs=$HOME/.nix-defexpr/nixpkgs:depot=$HOME/depot:universe=$HOME/mono
-- 
cgit 1.4.1


From b25d06db7e924effcb304645ab6121aad4b18ce5 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 18:25:10 +0000
Subject: Attempt to inject dependencies into blog/server

- We need the markdown files, to be in the /nix/store and the server needs to be
  aware of there location.
- Since we're dependending on `pandoc`, our server needs to know about it too.

For both of these cases -- especially for the latter case -- I imagine there may
be a more idiomatic way of doing this.
---
 blog/default.nix | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/blog/default.nix b/blog/default.nix
index 7005cc648d00..ed1e405ddca4 100644
--- a/blog/default.nix
+++ b/blog/default.nix
@@ -1,15 +1,27 @@
 {
+  pkgs ? import <nixpkgs> {},
   depot ? import <depot> {},
   universe ? import <universe> {},
   ...
 }:
 
-depot.nix.buildLisp.program {
+let
+  injectedPosts = pkgs.writeText "posts.lisp" ''
+    (in-package #:server)
+    (setq *path-to-posts* "${./posts}")
+  '';
+  injectedExecutables = pkgs.writeText "executables.lisp" ''
+    (in-package #:server)
+    (setq *pandoc-bin* "${pkgs.pandoc}/bin/pandoc")
+  '';
+in depot.nix.buildLisp.program {
   name = "server";
   deps = with depot.third_party.lisp; [
     hunchentoot
   ];
   srcs = [
     ./src/server.lisp
+    injectedPosts
+    injectedExecutables
   ];
 }
-- 
cgit 1.4.1


From 81d7bc405eaf631bf58377c629af6d461021bb67 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 18:27:11 +0000
Subject: Extend blog server to consume nix injections

All of this is still a work-in-progress. Just checking in my work.

Also:
- Write function `render-post` to convert a markdown file into HTML. This is
  still a work-in-progress since we need to capture the output and not just have
  it printed to *standard-out*.
- Return dummy data in /posts
---
 blog/src/server.lisp | 39 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/blog/src/server.lisp b/blog/src/server.lisp
index 63323e933bd3..7f669ddd77bf 100644
--- a/blog/src/server.lisp
+++ b/blog/src/server.lisp
@@ -5,6 +5,43 @@
   (:export :main))
 (in-package #:server)
 
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Nix-injected dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defvar *path-to-posts* "/tmp"
+  "File path pointing to the posts directory.")
+
+(defvar *pandoc-bin* "/usr/bin/pandoc")
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defun render-post (path)
+  "Render the markdown file stored at PATH to HTML using pandoc."
+  (uiop:run-program (list *pandoc-bin* path "--to" "html")
+                    :output t))
+
+;; TODO: Figure out how to handle this with Nix.
+(defvar *posts* (uiop:directory-files *path-to-posts*)
+  "List of the paths to the blog posts.")
+
+(hunchentoot:define-easy-handler
+    (get-latest :uri "/latest") ()
+  (print (parameter "name"))
+  (uiop:read-file-string (car *posts*)))
+
+(hunchentoot:define-easy-handler
+    (get-posts :uri "/posts") ()
+  "Working!")
+
 (defun main ()
   "This is the main entrypoint for our application."
-  (hunchentoot))
+  (hunchentoot:start (make-instance 'hunchentoot:easy-acceptor :port 4242))
+  (print "Listing on port 4242...")
+  (sb-thread:join-thread
+   (find-if (lambda (th)
+              (string= (sb-thread:thread-name th)
+                       "hunchentoot-listener-*:4242"))
+            (sb-thread:list-all-threads))))
-- 
cgit 1.4.1


From 2e3bb0435f2849e2a5d4da0ed8408c2993ca9052 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 21:56:24 +0000
Subject: Package cl-arrows package

Using Nix to package this library of Clojure-inspired threading macros.

TODO: Send this patch to tazjin.
---
 third_party/lisp/cl-arrows.nix | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
 create mode 100644 third_party/lisp/cl-arrows.nix

diff --git a/third_party/lisp/cl-arrows.nix b/third_party/lisp/cl-arrows.nix
new file mode 100644
index 000000000000..60cd8a375b1c
--- /dev/null
+++ b/third_party/lisp/cl-arrows.nix
@@ -0,0 +1,15 @@
+{ depot ? import <depot> {}, ... }:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/nightfly19/cl-arrows.git";
+    rev = "cbb46b69a7de40f1161c9caaf6cef93b3af9994f";
+  };
+in depot.nix.buildLisp.library {
+  name = "cl-arrows";
+  deps = [];
+  srcs = [
+    "${src}/packages.lisp"
+    "${src}/arrows.lisp"
+  ];
+}
-- 
cgit 1.4.1


From 6108f8df0140e2f71c9f9b50d530d16400d9cdcb Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 23 Jan 2020 21:58:31 +0000
Subject: Support proof-of-concept blog

After some toil, I have a working proof-of-concept blog. The idea is simple:
write blog posts in markdown and store the posts in the `./posts`
directory. Then use the server and `pandoc` to convert these markdown files into
HTML at request time. I'm using nix to package everything together. It's far
from perfect, but it works at the moment, which is encouraging.
---
 blog/default.nix     | 11 ++++++-----
 blog/src/server.lisp | 10 ++++++----
 default.nix          |  5 ++++-
 3 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/blog/default.nix b/blog/default.nix
index ed1e405ddca4..5359a0ab6f37 100644
--- a/blog/default.nix
+++ b/blog/default.nix
@@ -1,23 +1,24 @@
 {
-  pkgs ? import <nixpkgs> {},
+  nixpkgs ? import <nixpkgs> {},
   depot ? import <depot> {},
   universe ? import <universe> {},
   ...
 }:
 
 let
-  injectedPosts = pkgs.writeText "posts.lisp" ''
+  injectedPosts = nixpkgs.writeText "posts.lisp" ''
     (in-package #:server)
     (setq *path-to-posts* "${./posts}")
   '';
-  injectedExecutables = pkgs.writeText "executables.lisp" ''
+  injectedExecutables = nixpkgs.writeText "executables.lisp" ''
     (in-package #:server)
-    (setq *pandoc-bin* "${pkgs.pandoc}/bin/pandoc")
+    (setq *pandoc-bin* "${nixpkgs.pandoc}/bin/pandoc")
   '';
 in depot.nix.buildLisp.program {
   name = "server";
-  deps = with depot.third_party.lisp; [
+  deps = with depot.third_party.lisp; with universe.third_party.lisp; [
     hunchentoot
+    cl-arrows
   ];
   srcs = [
     ./src/server.lisp
diff --git a/blog/src/server.lisp b/blog/src/server.lisp
index 7f669ddd77bf..ad8169fa1af9 100644
--- a/blog/src/server.lisp
+++ b/blog/src/server.lisp
@@ -2,6 +2,7 @@
 (defpackage #:server
   (:documentation "Robot condemned to a life of admin work for my blog.")
   (:use #:cl)
+  (:import-from #:cl-arrows #:->>)
   (:export :main))
 (in-package #:server)
 
@@ -9,7 +10,9 @@
 ;; Nix-injected dependencies
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-(defvar *path-to-posts* "/tmp"
+;; TODO: Wrap this in an assert or ensure that there's a trailing slash so it's
+;; treated as a directory.
+(defvar *path-to-posts* "/tmp/"
   "File path pointing to the posts directory.")
 
 (defvar *pandoc-bin* "/usr/bin/pandoc")
@@ -21,7 +24,7 @@
 (defun render-post (path)
   "Render the markdown file stored at PATH to HTML using pandoc."
   (uiop:run-program (list *pandoc-bin* path "--to" "html")
-                    :output t))
+                    :output :string))
 
 ;; TODO: Figure out how to handle this with Nix.
 (defvar *posts* (uiop:directory-files *path-to-posts*)
@@ -29,8 +32,7 @@
 
 (hunchentoot:define-easy-handler
     (get-latest :uri "/latest") ()
-  (print (parameter "name"))
-  (uiop:read-file-string (car *posts*)))
+  (render-post (concatenate 'string *path-to-posts* "/" "test.md")))
 
 (hunchentoot:define-easy-handler
     (get-posts :uri "/posts") ()
diff --git a/default.nix b/default.nix
index d1fb88e04f93..ff31279941ae 100644
--- a/default.nix
+++ b/default.nix
@@ -15,8 +15,11 @@ let
 
   readTree' = import /home/wpcarro/depot/nix/readTree {};
 
+  # TODO: Find a better way to expose entire monorepo without introducing
+  # "infinite recursion".
   localPkgs = readTree: {
-    third_party   = readTree ./third_party;
+    blog = readTree ./blog;
+    third_party = readTree ./third_party;
   };
 in fix(self: {
   config = config self;
-- 
cgit 1.4.1


From b6fa3941b3731e959a7f54bf8e68e43e9077bf90 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Fri, 24 Jan 2020 10:50:05 +0000
Subject: Rename common-lisp directory to lisp

I could have renamed common-lisp to common_lisp. I think Nix prefers underscores
to hyphens.
---
 common-lisp/default.nix       | 16 ----------------
 common-lisp/main.lisp         | 15 ---------------
 common-lisp/unit-testing.lisp | 13 -------------
 default.nix                   |  1 +
 lisp/default.nix              | 16 ++++++++++++++++
 lisp/main.lisp                | 15 +++++++++++++++
 lisp/unit-testing.lisp        | 13 +++++++++++++
 7 files changed, 45 insertions(+), 44 deletions(-)
 delete mode 100644 common-lisp/default.nix
 delete mode 100644 common-lisp/main.lisp
 delete mode 100644 common-lisp/unit-testing.lisp
 create mode 100644 lisp/default.nix
 create mode 100644 lisp/main.lisp
 create mode 100644 lisp/unit-testing.lisp

diff --git a/common-lisp/default.nix b/common-lisp/default.nix
deleted file mode 100644
index 0969c7abf7e2..000000000000
--- a/common-lisp/default.nix
+++ /dev/null
@@ -1,16 +0,0 @@
-{ depot ? import <depot> {},
-  universe ? import <universe> {},
-  ...
-}:
-
-depot.nix.buildLisp.program {
-  name = "unit-testing";
-
-  deps = with universe.third_party.lisp; [
-    prove
-  ];
-
-  srcs = [
-    ./unit-testing.lisp
-  ];
-}
diff --git a/common-lisp/main.lisp b/common-lisp/main.lisp
deleted file mode 100644
index 2c4a5a411635..000000000000
--- a/common-lisp/main.lisp
+++ /dev/null
@@ -1,15 +0,0 @@
-(in-package #:cl-user)
-(defpackage #:utils
-  (:documentation "Some utility functions and macros to wet my beak.")
-  (:use #:cl)
-  (:shadow #:type))
-(in-package #:utils)
-
-(defmacro type (name in out)
-  `(declaim (ftype (function ,in ,out) ,name)))
-
-(defmacro comment (&rest _forms) nil)
-
-(type add (int int) int)
-(defun add (a b)
-  (+ a b))
diff --git a/common-lisp/unit-testing.lisp b/common-lisp/unit-testing.lisp
deleted file mode 100644
index c0b3be9b4515..000000000000
--- a/common-lisp/unit-testing.lisp
+++ /dev/null
@@ -1,13 +0,0 @@
-(in-package #:cl-user)
-(defpackage #:my-test
-  (:documentation "Unit testing in Common Lisp.")
-  (:use #:cl))
-(in-package #:my-test)
-
-(plan 3)
-
-(ok (not (find 4 '(1 2 3))))
-(is 4 4)
-(isnt 1 #\1)
-
-(finalize)
diff --git a/default.nix b/default.nix
index ff31279941ae..f4eda0d5ea26 100644
--- a/default.nix
+++ b/default.nix
@@ -19,6 +19,7 @@ let
   # "infinite recursion".
   localPkgs = readTree: {
     blog = readTree ./blog;
+    lisp = readTree ./lisp;
     third_party = readTree ./third_party;
   };
 in fix(self: {
diff --git a/lisp/default.nix b/lisp/default.nix
new file mode 100644
index 000000000000..0969c7abf7e2
--- /dev/null
+++ b/lisp/default.nix
@@ -0,0 +1,16 @@
+{ depot ? import <depot> {},
+  universe ? import <universe> {},
+  ...
+}:
+
+depot.nix.buildLisp.program {
+  name = "unit-testing";
+
+  deps = with universe.third_party.lisp; [
+    prove
+  ];
+
+  srcs = [
+    ./unit-testing.lisp
+  ];
+}
diff --git a/lisp/main.lisp b/lisp/main.lisp
new file mode 100644
index 000000000000..2c4a5a411635
--- /dev/null
+++ b/lisp/main.lisp
@@ -0,0 +1,15 @@
+(in-package #:cl-user)
+(defpackage #:utils
+  (:documentation "Some utility functions and macros to wet my beak.")
+  (:use #:cl)
+  (:shadow #:type))
+(in-package #:utils)
+
+(defmacro type (name in out)
+  `(declaim (ftype (function ,in ,out) ,name)))
+
+(defmacro comment (&rest _forms) nil)
+
+(type add (int int) int)
+(defun add (a b)
+  (+ a b))
diff --git a/lisp/unit-testing.lisp b/lisp/unit-testing.lisp
new file mode 100644
index 000000000000..c0b3be9b4515
--- /dev/null
+++ b/lisp/unit-testing.lisp
@@ -0,0 +1,13 @@
+(in-package #:cl-user)
+(defpackage #:my-test
+  (:documentation "Unit testing in Common Lisp.")
+  (:use #:cl))
+(in-package #:my-test)
+
+(plan 3)
+
+(ok (not (find 4 '(1 2 3))))
+(is 4 4)
+(isnt 1 #\1)
+
+(finalize)
-- 
cgit 1.4.1


From 95d03facb3af93323101737d76470f190e7ca2ae Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Fri, 24 Jan 2020 10:52:53 +0000
Subject: Support prelude.lisp

Using this module to support utility functions that I have not classified
further than being miscellaneous.
---
 lisp/main.lisp    | 15 ---------------
 lisp/prelude.lisp | 14 ++++++++++++++
 lisp/prelude.nix  |  8 ++++++++
 3 files changed, 22 insertions(+), 15 deletions(-)
 delete mode 100644 lisp/main.lisp
 create mode 100644 lisp/prelude.lisp
 create mode 100644 lisp/prelude.nix

diff --git a/lisp/main.lisp b/lisp/main.lisp
deleted file mode 100644
index 2c4a5a411635..000000000000
--- a/lisp/main.lisp
+++ /dev/null
@@ -1,15 +0,0 @@
-(in-package #:cl-user)
-(defpackage #:utils
-  (:documentation "Some utility functions and macros to wet my beak.")
-  (:use #:cl)
-  (:shadow #:type))
-(in-package #:utils)
-
-(defmacro type (name in out)
-  `(declaim (ftype (function ,in ,out) ,name)))
-
-(defmacro comment (&rest _forms) nil)
-
-(type add (int int) int)
-(defun add (a b)
-  (+ a b))
diff --git a/lisp/prelude.lisp b/lisp/prelude.lisp
new file mode 100644
index 000000000000..3522567ea0f7
--- /dev/null
+++ b/lisp/prelude.lisp
@@ -0,0 +1,14 @@
+(in-package #:cl-user)
+(defpackage #:prelude
+  (:documentation "Supporting miscellaneous utility functions and macros.")
+  (:use #:cl)
+  (:shadow #:type)
+  (:export #:type #:comment))
+(in-package #:prelude)
+
+;; TODO: Add documentation to these macros.
+
+(defmacro type (name in out)
+  `(declaim (ftype (function ,in ,out) ,name)))
+
+(defmacro comment (&rest _forms) nil)
diff --git a/lisp/prelude.nix b/lisp/prelude.nix
new file mode 100644
index 000000000000..9051f82394ff
--- /dev/null
+++ b/lisp/prelude.nix
@@ -0,0 +1,8 @@
+{ depot ? import <depot> {}, ... }:
+
+depot.nix.buildLisp.library {
+  name = "prelude";
+  srcs = [
+    ./prelude.lisp
+  ];
+}
-- 
cgit 1.4.1


From e2e59c63e7899ea06e4273f0ee58a5842f6e3384 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Fri, 24 Jan 2020 10:53:37 +0000
Subject: Cleanup lisp directory

Removing the default.nix that I used to attempt to build prove, a Common Lisp
unit testing library. Also removing the lisp module with the unit tests
themselves.
---
 lisp/default.nix       | 16 ----------------
 lisp/unit-testing.lisp | 13 -------------
 2 files changed, 29 deletions(-)
 delete mode 100644 lisp/default.nix
 delete mode 100644 lisp/unit-testing.lisp

diff --git a/lisp/default.nix b/lisp/default.nix
deleted file mode 100644
index 0969c7abf7e2..000000000000
--- a/lisp/default.nix
+++ /dev/null
@@ -1,16 +0,0 @@
-{ depot ? import <depot> {},
-  universe ? import <universe> {},
-  ...
-}:
-
-depot.nix.buildLisp.program {
-  name = "unit-testing";
-
-  deps = with universe.third_party.lisp; [
-    prove
-  ];
-
-  srcs = [
-    ./unit-testing.lisp
-  ];
-}
diff --git a/lisp/unit-testing.lisp b/lisp/unit-testing.lisp
deleted file mode 100644
index c0b3be9b4515..000000000000
--- a/lisp/unit-testing.lisp
+++ /dev/null
@@ -1,13 +0,0 @@
-(in-package #:cl-user)
-(defpackage #:my-test
-  (:documentation "Unit testing in Common Lisp.")
-  (:use #:cl))
-(in-package #:my-test)
-
-(plan 3)
-
-(ok (not (find 4 '(1 2 3))))
-(is 4 4)
-(isnt 1 #\1)
-
-(finalize)
-- 
cgit 1.4.1


From 143fc970322237963d5358ae832fd18ea045d67a Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Fri, 24 Jan 2020 10:55:04 +0000
Subject: Start working on f.lisp

I would like to port the `f.el` library to Common Lisp. I'm adding a README,
some build files, and the module itself to get started.
---
 lisp/f/README.md   |  5 +++++
 lisp/f/default.nix | 15 +++++++++++++++
 lisp/f/main.lisp   | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 68 insertions(+)
 create mode 100644 lisp/f/README.md
 create mode 100644 lisp/f/default.nix
 create mode 100644 lisp/f/main.lisp

diff --git a/lisp/f/README.md b/lisp/f/README.md
new file mode 100644
index 000000000000..34e07180d492
--- /dev/null
+++ b/lisp/f/README.md
@@ -0,0 +1,5 @@
+# f.lisp
+
+In this project, I'm attempting to port the Elisp library [`f.el`][1] to Common Lisp.
+
+[1]: https://github.com/rejeep/f.el
diff --git a/lisp/f/default.nix b/lisp/f/default.nix
new file mode 100644
index 000000000000..6911b2102634
--- /dev/null
+++ b/lisp/f/default.nix
@@ -0,0 +1,15 @@
+{
+  depot ? import <depot> {},
+  universe ? import <universe> {},
+  ...
+}:
+
+depot.nix.buildLisp.library {
+  name = "f";
+  deps = with universe.lisp; [
+    prelude
+  ];
+  srcs = [
+    ./main.lisp
+  ];
+}
diff --git a/lisp/f/main.lisp b/lisp/f/main.lisp
new file mode 100644
index 000000000000..a51c38127815
--- /dev/null
+++ b/lisp/f/main.lisp
@@ -0,0 +1,48 @@
+(in-package #:cl-user)
+(defpackage #:main
+  (:documentation "Modern API for working with files and directories.")
+  (:use #:cl)
+  (:shadow #:type))
+(in-package #:main)
+
+;; Common Lisp distinguishes between `namestrings` and `pathnames` as two types
+;; of filename representations.
+;;
+;; A `pathname` is a structured representation of the name of a file, which
+;; consists of six parts:
+;; 1. host
+;; 2. device
+;; 3. directory
+;; 4. name
+;; 5. type
+;; 6. version
+
+;; TODO: Should I be using `string` as a type or `namestring`?
+
+(defmacro type (name in out)
+  `(declaim (ftype (function ,in ,out) ,name)))
+
+(type join (&rest namestring) pathname)
+(defun join (&rest args)
+  "Join ARGS to a single path."
+  (apply #'merge-pathnames args))
+
+(type ext (pathname) string)
+(defun ext (path)
+  "Return the file extension of PATH."
+  (pathname-type path))
+
+;; TODO: Define these tests elsewhere.
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Tests
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; join
+(string= (join "path") "path")
+(string= (join "path" "to") "path/to")
+(string= (join "/" "path" "to" "heaven") "/path/to/heaven")
+
+;; ext
+(string= (ext #p"path/to/file.ext") "ext")
+(string= (ext #p"path/to/directory") nil)
-- 
cgit 1.4.1


From 5a2a1694e3790e97da5933ac66ff68fb3170c8f0 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Sat, 25 Jan 2020 19:39:49 +0000
Subject: Support java directory

Add an example of two java files, Main.java and Another.java, where Main.java
depends on Another.java. This is part of a larger example of attempting to use
Nix to package these.

Also ignoring the *.class files that `javac <filename>` outputs.
---
 .gitignore        | 1 +
 java/Another.java | 5 +++++
 java/Main.java    | 6 ++++++
 3 files changed, 12 insertions(+)
 create mode 100644 java/Another.java
 create mode 100644 java/Main.java

diff --git a/.gitignore b/.gitignore
index 51eb9644acd7..12ecf28fd42f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,3 +4,4 @@
 
 # Python
 __pycache__
+*.class
diff --git a/java/Another.java b/java/Another.java
new file mode 100644
index 000000000000..e431e157336b
--- /dev/null
+++ b/java/Another.java
@@ -0,0 +1,5 @@
+public class Another {
+  public static String whatis() {
+    return "Testing";
+  }
+}
diff --git a/java/Main.java b/java/Main.java
new file mode 100644
index 000000000000..0bfa9ebc2a34
--- /dev/null
+++ b/java/Main.java
@@ -0,0 +1,6 @@
+public class Main {
+  public static void main(String[] args) {
+    System.out.println(System.getProperty("java.class.path"));
+    System.out.println(Another.whatis());
+  }
+}
-- 
cgit 1.4.1


From 130155121a2e67228f96aa557e0ac03c781146f9 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Sat, 25 Jan 2020 19:41:19 +0000
Subject: Sketch idea of a buildClojure nix function

The previous commit that adds Java code is part of a larger project intended to
use Nix to package Clojure. I'd like to build something similar to @tazjin's
buildLisp except for Clojure instead of for Common Lisp. Once building for both
ecosystems is similarly easy, it will be easier for me to compare the two
languages. Right now `buildLisp` is so good that it attracts me to Common Lisp
even when I don't know the language.
---
 clojure/buildClojure.nix | 11 +++++++++++
 1 file changed, 11 insertions(+)
 create mode 100644 clojure/buildClojure.nix

diff --git a/clojure/buildClojure.nix b/clojure/buildClojure.nix
new file mode 100644
index 000000000000..1596279de1c5
--- /dev/null
+++ b/clojure/buildClojure.nix
@@ -0,0 +1,11 @@
+{ universe ? import <universe> {}, ... }:
+
+universe.nix.buildClojure.program {
+  name = "test";
+  deps = with universe.third_party.clojure; [
+
+  ];
+  srcs = [
+    ./main.clj
+  ]
+}
-- 
cgit 1.4.1


From d67993266f33bed8ef8988bccaf77882f25e458e Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Sun, 26 Jan 2020 00:29:34 +0000
Subject: Add index.html with AdSense script to blog

Adds some dummy markup, which includes my Google AdSense `<script>` tag. The
script won't work unless it is loaded from wpcarro.dev.
---
 blog/src/index.html | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
 create mode 100644 blog/src/index.html

diff --git a/blog/src/index.html b/blog/src/index.html
new file mode 100644
index 000000000000..a9d9cf7eaa25
--- /dev/null
+++ b/blog/src/index.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <meta name="description" content="Showcase of AI bots at DeepMind" />
+    <link rel="stylesheet" href="/index.css">
+    <title>AI Showcase</title>
+    <script data-ad-client="ca-pub-6018268443649487" async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
+  </head>
+  <body>
+    <noscript>You need to enable JavaScript to run this app.</noscript>
+    <h1>Welcome</h1>
+    <p>To my blog!</p>
+  </body>
+</html>
-- 
cgit 1.4.1


From be0e05c6694a715beb242c16c91f890e5fbdc08e Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Mon, 27 Jan 2020 14:57:47 +0000
Subject: Prefer home variable rather than hard coded path to readTree

TIL: Nix support the ~ as a home variable, which makes my code more portable
since it's shared between my work laptop and work desktop.
---
 default.nix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/default.nix b/default.nix
index f4eda0d5ea26..ff4f40d8f943 100644
--- a/default.nix
+++ b/default.nix
@@ -13,7 +13,7 @@ let
     inherit pkgs;
   };
 
-  readTree' = import /home/wpcarro/depot/nix/readTree {};
+  readTree' = import ~/depot/nix/readTree {};
 
   # TODO: Find a better way to expose entire monorepo without introducing
   # "infinite recursion".
-- 
cgit 1.4.1


From 7d51b40e661e53ea1904fd1cf5ca407b1497e4f2 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Mon, 27 Jan 2020 16:02:57 +0000
Subject: Rename mono -> universe

Prefer the name universe for my mono-repo. A bit grandiose, sure, but I prefer
the metaphor to no metaphor at all.
---
 .envrc    | 2 +-
 README.md | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/.envrc b/.envrc
index bf79e325a121..365d37dbcabf 100644
--- a/.envrc
+++ b/.envrc
@@ -1 +1 @@
-export NIX_PATH=nixpkgs=$HOME/.nix-defexpr/nixpkgs:depot=$HOME/depot:universe=$HOME/mono
+export NIX_PATH=nixpkgs=$HOME/.nix-defexpr/nixpkgs:depot=$HOME/depot:universe=$HOME/universe
diff --git a/README.md b/README.md
index e7a01960b87c..4fa248554751 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# Mono
+# Universe
 
 This is my mono-repo. Having a personal mono-repo is a new idea for me, so at
 the time of this writing, the state of this repository is fledgling.
-- 
cgit 1.4.1


From af26b6a1818cf9ee8b4fc9764201556f234202cd Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Mon, 27 Jan 2020 16:52:16 +0000
Subject: Partially packages package that I partially want to use

A Poem:

I wanted to try out this package...
30 minutes later after a dozen failed attempts at packaging it...
I no longer want to try this package...
for now.
---
 third_party/lisp/linear-programming.nix | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)
 create mode 100644 third_party/lisp/linear-programming.nix

diff --git a/third_party/lisp/linear-programming.nix b/third_party/lisp/linear-programming.nix
new file mode 100644
index 000000000000..432fedb8bcac
--- /dev/null
+++ b/third_party/lisp/linear-programming.nix
@@ -0,0 +1,26 @@
+{ depot ? import <depot> {}, ... }:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/neil-lindquist/linear-programming.git";
+    rev = "8c8d55e7584773b90c4ba4b225c5f2008f4c474a";
+  };
+in depot.nix.buildLisp.library {
+  name = "linear-programming";
+  deps = [
+    (depot.nix.buildLisp.bundled "uiop")
+    depot.third_party.lisp.iterate
+    depot.third_party.lisp.alexandria
+  ];
+  srcs = [
+    "${src}/src/conditions.lisp"
+    "${src}/src/expressions.lisp"
+    "${src}/src/simplex.lisp"
+    "${src}/src/system-info.lisp"
+    "${src}/src/utils.lisp"
+    "${src}/src/problem.lisp"
+    "${src}/src/solver.lisp"
+    "${src}/src/external-formats.lisp"
+    "${src}/src/all.lisp"
+  ];
+}
-- 
cgit 1.4.1


From bacaa0ca8ad26b4f7ff592746d63d5c6b6c39ab0 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Tue, 28 Jan 2020 00:00:05 +0000
Subject: Add docker/cloud_run.nix

I'm attempting to setup my blog using the following:
- Google Cloud Run: I whitelist a docker image that packages my application and
then Google runs it "statelessly" (i.e. without persistence). The stateless part
should be fine for the time being.
- Nix: Using `<nixpkgs>.dockerTools.buildLayeredImage` to output docker images
from Nix expressions.
- Docker: Upload the output image from the Nix expressions and upload it to
Google Container Registry from which it can be run from Google Cloud Run.

Some helpful commands:

```shell
> sudo gcloud auth login
> nix-build ./docker/cloud_run.nix
> sudo docker image import ./result
> sudo docker tag <name> gcr.io/<google-cloud-project-id>/<name>:<tag>
> sudo docker push gcr.io/<google-cloud-project-id>/<name>:<tag>
```

I'm unsure if Google Cloud Run is my desired end goal, but it may help me
publish a blog faster than setting up a Kubernetes cluster, which is what I'd
ultimately like to do. Cloud Run should be cheaper financially and time-wise.
---
 docker/cloud_run.nix | 9 +++++++++
 1 file changed, 9 insertions(+)
 create mode 100644 docker/cloud_run.nix

diff --git a/docker/cloud_run.nix b/docker/cloud_run.nix
new file mode 100644
index 000000000000..02440df08872
--- /dev/null
+++ b/docker/cloud_run.nix
@@ -0,0 +1,9 @@
+# Attempting to build a Docker image with Nix to run using Google Cloud Run.
+{ pkgs ? import <nixpkgs> {}, ... }:
+
+pkgs.dockerTools.buildLayeredImage {
+  name = "mysql";
+  tag = "latest";
+  config.Cmd = [ "${pkgs.mysql}/bin/mysqld" ];
+  maxLayers = 120;
+}
-- 
cgit 1.4.1


From ff06ffcf9cd8ad78f0dcc7df4e6bbfa684e73f1e Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Tue, 28 Jan 2020 16:47:35 +0000
Subject: Package depot's gemma as a docker image for Cloud Run

Using <depot>'s gemma project with `dockerTools.buildLayeredImage` because I
need access to a nix-packaged server and gemma is the first thing that comes to
mind.
---
 docker/cloud_run.nix | 17 +++++++++++++----
 docker/config.lisp   | 21 +++++++++++++++++++++
 2 files changed, 34 insertions(+), 4 deletions(-)
 create mode 100644 docker/config.lisp

diff --git a/docker/cloud_run.nix b/docker/cloud_run.nix
index 02440df08872..70be4040c36b 100644
--- a/docker/cloud_run.nix
+++ b/docker/cloud_run.nix
@@ -1,9 +1,18 @@
-# Attempting to build a Docker image with Nix to run using Google Cloud Run.
-{ pkgs ? import <nixpkgs> {}, ... }:
+{
+  pkgs ? import <nixpkgs> {},
+  depot ? import <depot> {},
+  ...
+}:
 
 pkgs.dockerTools.buildLayeredImage {
-  name = "mysql";
+  name = "gemma";
   tag = "latest";
-  config.Cmd = [ "${pkgs.mysql}/bin/mysqld" ];
+  config.ExposedPorts = {
+    "4242" = {};
+  };
+  config.Env = [
+    "GEMMA_CONFIG=${./config.lisp}"
+  ];
+  config.Cmd = [ "${depot.fun.gemma}/bin/gemma" ];
   maxLayers = 120;
 }
diff --git a/docker/config.lisp b/docker/config.lisp
new file mode 100644
index 000000000000..54f8e5f34462
--- /dev/null
+++ b/docker/config.lisp
@@ -0,0 +1,21 @@
+;; Example configuration file for Gemma
+
+(config :port 4242
+        :data-dir "/tmp/gemma/")
+
+(deftask bathroom/wipe-mirror 7)
+(deftask bathroom/wipe-counter 7)
+
+;; Bedroom tasks
+(deftask bedroom/change-sheets 7)
+(deftask bedroom/vacuum 10)
+
+;; Kitchen tasks
+(deftask kitchen/normal-trash 3)
+(deftask kitchen/green-trash 5)
+(deftask kitchen/blue-trash 5)
+(deftask kitchen/wipe-counters 3)
+(deftask kitchen/vacuum 5 "Kitchen has more crumbs and such!")
+
+;; Entire place
+(deftask clean-windows 60)
-- 
cgit 1.4.1


From 8ad51b24dd8719840aac47134835ea25cfe1b0b8 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Tue, 28 Jan 2020 16:48:28 +0000
Subject: Document current deployment tactics

Adding a README including my current method for deploying. See the README for
more details. All of this is quite virgin and as such is subject to change.
---
 docker/README.md | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)
 create mode 100644 docker/README.md

diff --git a/docker/README.md b/docker/README.md
new file mode 100644
index 000000000000..34f762cc1970
--- /dev/null
+++ b/docker/README.md
@@ -0,0 +1,59 @@
+# Deployments
+
+I'm documenting how I currently deploy things.
+
+I'd like to automate this workflow as much as possible, and I intend to do just
+that. For now, I'm running things manually until I can design an generalization
+that appeals to me.
+
+## Dependencies
+- `nix-build`
+- `docker`
+- `gcloud`
+
+## Step-by-step
+
+1. Use `nix-build` to create our Docker image for Cloud Run.
+
+```shell
+> nix-build ./cloud_run.nix
+```
+
+This outputs a Docker image at `./result`.
+
+1. Load the built image (i.e. `./result`) into `docker` so that we can tag it
+   and push it to the Google Container Registry (i.e. GCR).
+
+```shell
+> sudo docker load <./result
+```
+
+1. (Optionally) Run the image locally to verify its integrity.
+
+```shell
+> sudo docker run -d <name>:<tag>
+```
+
+1. Tag and push the image to GCR.
+
+```shell
+> sudo docker tag <name>:<label> gcr.io/<google-cloud-project-id>/<name>:<latest>
+```
+
+# TODO: Prefer using a command line tool like `gcloud` for these steps.
+
+1. Visit Google Cloud Run; create a new service with "Create Service"; select
+   the uploaded Docker image from the "Container Image URL" field; click
+   "Create" to deploy.
+
+
+## Notes
+
+You may need to authorize `gcloud` by running the following:
+
+```shell
+> sudo gcloud auth login --no-launch-browser
+```
+
+You must use `sudo` here since the `docker` invocations are prefixed with `sudo`
+as well.
-- 
cgit 1.4.1