amalloy-4clojure-solution117.clj

;; amalloy's solution to For Science!
;; https://4clojure.com/problem/117

(fn [board]
  (let [names {\space :open
               \# :wall
               \M :goal
               \C :start}
        board (vec (for [row board]
                     (vec (for [col row]
                            (names col)))))
        h (count board)
        w (count (board 0))
        [start goal] (for [token [:start :goal]
                           y (range h)
                           :let [row (board y)]
                           x (range w)
                           :when (= token (row x))]
                       [y x])
        valid? (fn [[y x :as pos]]
                 (and (not-any? neg? pos)
                      (< y h) (< x w)))
        deltas [[0 1] [1 0] [0 -1] [-1 0]]
        neighbors (fn [pos]
                    (filter valid?
                            (for [d deltas]
                              (map + pos d))))
        reachable? (fn reachable? [board pos]
                     (and (not= :wall (get-in board pos))
                          (or (= goal pos)
                              (let [new-board (assoc-in board pos :wall)]
                                (some #(reachable? new-board %) (neighbors pos))))))]
    (boolean (reachable? board start))))