src/day03.lisp

   1 (asdf:load-system :adventofcode2020)
   2 (in-package #:adventofcode2020)
   3 (named-readtables:in-readtable fn-reader)
   4
   5 (defun tree-collisions (slope tree-map)
   6   (loop with (rise run) = slope
   7         with proper-map = (loop for j upfrom 0
   8                                 for line in tree-map
   9                                 when (->> rise (mod j) (= 0))
  10                                 collecting line)
  11         with terrain-width = (-> tree-map first length)
  12         for i upfrom 0 by run
  13         for terrain-line in proper-map
  14         counting (->> terrain-width
  15                    (mod i)
  16                    (char terrain-line)
  17                    (char= #\#))))
  18
  19 (day 03 input
  20   (let ((tree-map (list-from input)))
  21     (part1 (tree-collisions '(1 3) tree-map))
  22     (part2 (->> '((1 1) (1 3) (1 5) (1 7) (2 1))
  23              (mapcar λ(tree-collisions _ tree-map))
  24              (reduce #'*)))))
  25
  26 (def-suite day03)
  27 (in-suite day03)
  28
  29 (test single-collision-check
  30   (is (equal
  31         7
  32         (tree-collisions '(1 3) '("..##......."
  33                                   "#...#...#.."
  34                                   ".#....#..#."
  35                                   "..#.#...#.#"
  36                                   ".#...##..#."
  37                                   "..#.##....."
  38                                   ".#.#.#....#"
  39                                   ".#........#"
  40                                   "#.##...#..."
  41                                   "#...##....#"
  42                                   ".#..#...#.#")))))
  43
  44
  45 (test multiple-collision-check
  46   (is (equal
  47         '(2 7 3 4 2)
  48         (mapcar λ(tree-collisions _ '("..##......."
  49                                         "#...#...#.."
  50                                         ".#....#..#."
  51                                         "..#.#...#.#"
  52                                         ".#...##..#."
  53                                         "..#.##....."
  54                                         ".#.#.#....#"
  55                                         ".#........#"
  56                                         "#.##...#..."
  57                                         "#...##....#"
  58                                         ".#..#...#.#"))
  59                 '((1 1) (1 3) (1 5) (1 7) (2 1))))))
  60
  61 (run! 'day03)