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