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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)))
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