3 files changed, 140 insertions, 0 deletions
diff --git a/users/wpcarro/emacs/pkgs/bag/bag.el b/users/wpcarro/emacs/pkgs/bag/bag.el
new file mode 100644
index 0000000000..502f567253
--- /dev/null
+++ b/users/wpcarro/emacs/pkgs/bag/bag.el
@@ -0,0 +1,78 @@
+;;; bag.el --- Working with bags (aka multi-sets) -*- lexical-binding: t -*-
+
+;; Author: William Carroll <wpcarro@gmail.com>
+;; Version: 0.0.1
+;; Package-Requires: ((emacs "24.3"))
+
+;;; Commentary:
+;; What is a bag?  A bag should be thought of as a frequency table.  It's a way
+;; to convert a list of something into a set that allows duplicates.  Isn't
+;; allowing duplicates the whole thing with Sets?  Kind of.  But the interface
+;; of Sets is something that bags resemble, so multi-set isn't as bad of a name
+;; as it may first seem.
+;;
+;; If you've used Python's collections.Counter, the concept of a bag should be
+;; familiar already.
+;;
+;; Interface:
+;; - add        :: x -> Bag(x) -> Bag(x)
+;; - remove     :: x -> Bag(x) -> Bag(x)
+;; - union      :: Bag(x) -> Bag(x) -> Bag(x)
+;; - difference :: Bag(x) -> Bag(x) -> Bag(x)
+
+;;; Code:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'al)
+(require 'list)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(cl-defstruct bag xs)
+
+(defun bag-new ()
+  "Create an empty bag."
+  (make-bag :xs (al-new)))
+
+(defun bag-from-list (xs)
+  "Map a list of `XS' into a bag."
+  (->> xs
+       (list-reduce (bag-new) #'bag-add)))
+
+(defun bag-add (x xs)
+  "Add X to XS."
+  (if (bag-contains? x xs)
+      (struct-update
+       bag xs (lambda (xs) (al-update x (lambda (x) (+ 1 x)) xs)) xs)
+    (struct-update bag xs (lambda (xs) (al-set x 1 xs)) xs)))
+
+(defun bag-remove (x xs)
+  "Remove X from XS.
+This is a no-op is X doesn't exist in XS."
+  (when (bag-contains? x xs)
+    (struct-update bag xs (lambda (xs) (al-delete x xs)) xs)))
+
+(defun bag-count (x xs)
+  "Return the number of occurrences of X in XS."
+  (al-get x (bag-xs xs) 0))
+
+(defun bag-total (xs)
+  "Return the total number of elements in XS."
+  (->> (bag-xs xs)
+       (al-reduce 0 (lambda (_key v acc) (+ acc v)))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Predicates
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defun bag-contains? (x xs)
+  "Return t if XS has X."
+  (al-has-key? x (bag-xs xs)))
+
+(provide 'bag)
+;;; bag.el ends here
diff --git a/users/wpcarro/emacs/pkgs/bag/default.nix b/users/wpcarro/emacs/pkgs/bag/default.nix
new file mode 100644
index 0000000000..3dedc27286
--- /dev/null
+++ b/users/wpcarro/emacs/pkgs/bag/default.nix
@@ -0,0 +1,26 @@
+{ pkgs, depot, ... }:
+
+let
+  bag = pkgs.callPackage
+    ({ emacsPackages }:
+      emacsPackages.trivialBuild {
+        pname = "bag";
+        version = "1.0.0";
+        src = ./bag.el;
+        packageRequires =
+          (with depot.users.wpcarro.emacs.pkgs; [
+            al
+            list
+          ]);
+      })
+    { };
+
+  emacs = (pkgs.emacsPackagesFor pkgs.emacs28).emacsWithPackages (epkgs: [ bag ]);
+in
+bag.overrideAttrs (_old: {
+  doCheck = true;
+  checkPhase = ''
+    ${emacs}/bin/emacs -batch \
+      -l ert -l ${./tests.el} -f ert-run-tests-batch-and-exit
+  '';
+})
diff --git a/users/wpcarro/emacs/pkgs/bag/tests.el b/users/wpcarro/emacs/pkgs/bag/tests.el
new file mode 100644
index 0000000000..0cc2a6029e
--- /dev/null
+++ b/users/wpcarro/emacs/pkgs/bag/tests.el
@@ -0,0 +1,36 @@
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'ert)
+(require 'bag)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Tests
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(setq fixture (bag-from-list '(1 1 1 2 2 3)))
+
+(ert-deftest bag-add ()
+  (and
+   (not (bag-contains? 4 fixture))
+   (bag-contains? 4 (bag-add 4 fixture))))
+
+(ert-deftest bag-remove ()
+  (and
+   (bag-contains? 1 fixture)
+   (not (bag-contains? 3 (bag-remove 3 fixture)))))
+
+(ert-deftest bag-count ()
+  (and
+   (= 3 (bag-count 1 fixture))
+   (= 2 (bag-count 2 fixture))
+   (= 1 (bag-count 3 fixture))))
+
+(ert-deftest bag-total ()
+  (= 6 (bag-total fixture)))
+
+(ert-deftest bag-contains? ()
+  (and
+   (bag-contains? 1 fixture)
+   (not (bag-contains? 4 fixture))))