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