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diff --git a/users/tazjin/aoc2020/solution-day5.el b/users/tazjin/aoc2020/solution-day5.el new file mode 100644 index 000000000000..9bba322902b0 --- /dev/null +++ b/users/tazjin/aoc2020/solution-day5.el @@ -0,0 +1,61 @@ +;; 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)))))) |