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+;; -*- lexical-binding: t; -*-
+;; Advent of Code 2019 - Day 4
+
+(require 'cl-lib)
+(require 'dash)
+
+;; Puzzle 1
+
+(defun day4/to-digits (num)
+  "Convert NUM to a list of its digits."
+  (cl-labels ((steps (n digits)
+                     (if (= n 0) digits
+                       (steps (/ n 10) (cons (% n 10) digits)))))
+    (steps num '())))
+
+(defvar day4/input (-map #'day4/to-digits (number-sequence 128392 643281)))
+
+(defun day4/filter-password (digits)
+  "Determines whether the given rules match the supplied
+  number."
+
+  (and
+   ;; It is a six digit number
+   (= 6 (length digits))
+
+   ;; Value is within the range given in puzzle input
+   ;; (noop because the range is generated from the input)
+
+   ;; Two adjacent digits are the same (like 22 in 122345).
+   (car (-reduce-from (-lambda ((acc . prev) next)
+                        (cons (or acc (= prev next)) next))
+                      '(nil . 0) digits))
+
+   ;; Going from left to right, the digits never decrease; they only
+   ;; ever increase or stay the same (like 111123 or 135679).
+   (car (-reduce-from (-lambda ((acc . prev) next)
+                        (cons (and acc (>= next prev)) next))
+                      '(t . 0) digits))))
+
+;; Puzzle 2
+;;
+;; Additional criteria: If there's matching digits, they're not in a group.
+
+(cl-defstruct day4/acc state prev count)
+
+(defun day4/filter-longer-groups (digits)
+  (let ((res (-reduce-from
+              (lambda (acc next)
+                (cond ;; sequence is broken and count was at 1 ->
+                 ;; match!
+                 ((and (= (day4/acc-count acc) 2)
+                       (/= (day4/acc-prev acc) next))
+                  (setf (day4/acc-state acc) t))
+
+                 ;; sequence continues, counter increment!
+                 ((= (day4/acc-prev acc) next)
+                  (setf (day4/acc-count acc) (+ 1 (day4/acc-count acc))))
+
+                 ;; sequence broken, reset counter
+                 ((/= (day4/acc-prev acc) next)
+                  (setf (day4/acc-count acc) 1)))
+
+                (setf (day4/acc-prev acc) next)
+                acc)
+              (make-day4/acc :prev 0 :count 0) digits)))
+    (or (day4/acc-state res)
+        (= 2 (day4/acc-count res)))))
+
+(let* ((simple (-filter #'day4/filter-password day4/input))
+       (complex (-filter #'day4/filter-longer-groups simple)))
+  (message "Solution to day4/1: %d" (length simple))
+  (message "Solution to day4/2: %d" (length complex)))
+