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+;; Advent of Code 2020 - Day 2
+
+(require 'cl-lib)
+(require 'f)
+(require 'ht)
+(require 's)
+(require 'seq)
+
+(defvar day2/input
+  ;; This one was too large to inline.
+  (s-lines (f-read "/tmp/aoc/day2.txt")))
+
+(defun day2/count-letters (password)
+  (let ((table (ht-create)))
+    (cl-loop for char across password
+             for current = (ht-get table char)
+             do (ht-set table char
+                        (if current (+ 1 current) 1)))
+    table))
+
+(defun day2/parse (input)
+  (let* ((split (s-split " " input))
+         (range (s-split "-" (car split))))
+    (list (string-to-number (car range))
+          (string-to-number (cadr range))
+          (string-to-char (cadr split))
+          (caddr split))))
+
+(defun day2/count-with-validation (func)
+  (length (-filter
+           (lambda (password)
+             (and (not (seq-empty-p password))
+                  (apply func (day2/parse password))))
+           day2/input)))
+
+;; Puzzle 1
+
+(defun day2/validate-oldjob (min max char password)
+  (let ((count (ht-get (day2/count-letters password) char)))
+    (when count
+      (and (>= count min)
+           (<= count max)))))
+
+(message "Solution to day2/1: %s"
+         (day2/count-with-validation #'day2/validate-oldjob))
+
+;; Puzzle 2
+
+(defun day2/validate-toboggan (pos1 pos2 char password)
+  (xor (= char (aref password (- pos1 1)))
+       (= char (aref password (- pos2 1)))))
+
+(message "Solution to day2/2: %s"
+         (day2/count-with-validation #'day2/validate-toboggan))