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;; Advent of Code 2020 - Day 5
(require 'cl-lib)
(require 'dash)
(require 'f)
(require 'ht)
(require 's)
(require 'seq)
(defvar day5/input
(-filter (lambda (s) (not (seq-empty-p s)))
(s-lines (f-read "/tmp/aoc/day5.txt"))))
(defun day5/lower (sequence)
(seq-subseq sequence 0 (/ (length sequence) 2)))
(defun day5/upper (sequence)
(seq-subseq sequence (/ (length sequence) 2)))
(defun day5/seat-id (column row)
(+ column (* 8 row)))
(defun day5/find-seat (boarding-pass)
(let ((rows (number-sequence 0 127))
(columns (number-sequence 0 7)))
(cl-loop for char across boarding-pass
do (pcase char
(?F (setq rows (day5/lower rows)))
(?B (setq rows (day5/upper rows)))
(?R (setq columns (day5/upper columns)))
(?L (setq columns (day5/lower columns))))
finally return (day5/seat-id (car columns) (car rows)))))
;; Puzzle 1
(message "Solution to day5/1: %s"
(cl-loop for boarding-pass in day5/input
maximize (day5/find-seat boarding-pass)))
;; Puzzle 2
(defun day5/all-seats-in (row)
(-map (lambda (column) (day5/seat-id column row))
(number-sequence 0 7)))
(message "Solution to day5/2: %s"
(let ((all-seats (ht-create)))
(-each (-mapcat #'day5/all-seats-in (number-sequence 1 126))
(lambda (seat) (ht-set all-seats seat nil)))
(cl-loop for boarding-pass in day5/input
do (ht-remove all-seats (day5/find-seat boarding-pass))
;; Remove seats that lack adjacent entries, those
;; are missing on the plane.
finally return
(car
(-filter (lambda (seat)
(and (not (ht-contains? all-seats (- seat 1)))
(not (ht-contains? all-seats (+ seat 1)))))
(ht-keys all-seats))))))
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