(in-package #:adventofcode2020)

(defun tree-collisions (slope tree-map)
  (destructuring-bind (rise run) slope
    (loop with terrain-width = (-> tree-map first length)
          for i upfrom 0 by run
          for j upfrom 0
          for terrain-line in tree-map
          when (->> rise (mod j) (= 0))
          counting (->> terrain-width
                     (mod i)
                     (char terrain-line)
                     (char= #\#)))))

(day 03 input
  (let ((tree-map (list-from input)))
    (part1 (tree-collisions '(1 3) tree-map))
    (part2 (->> '((1 1) (1 3) (1 5) (1 7) (2 1))
             (mapcar (fn* (tree-collisions _ tree-map)))
             (reduce #'*)))))

(def-suite day03)
(in-suite day03)

(test single-collision-check
  (is (equal
        7
        (tree-collisions '(1 3) '("..##......."
                                  "#...#...#.."
                                  ".#....#..#."
                                  "..#.#...#.#"
                                  ".#...##..#."
                                  "..#.##....."
                                  ".#.#.#....#"
                                  ".#........#"
                                  "#.##...#..."
                                  "#...##....#"
                                  ".#..#...#.#")))))


(test multiple-collision-check
  (is (equal
        '(2 7 3 4 2)
        (mapcar (fn* (tree-collisions _ '("..##......."
                                          "#...#...#.."
                                          ".#....#..#."
                                          "..#.#...#.#"
                                          ".#...##..#."
                                          "..#.##....."
                                          ".#.#.#....#"
                                          ".#........#"
                                          "#.##...#..."
                                          "#...##....#"
                                          ".#..#...#.#")))
                '((1 1) (1 3) (1 5) (1 7) (2 1))))))

(run! 'day03)