day12.clj

(ns aoc.day12
  [:require [clojure.string :as str]])

(def input "abcccaaaaaaccccccccaaaaaccccccaaaaaaccccccaaaaaaaacccaaaaaaaccaaaacccccccccccccccccccccccccaaaaaacccccccccccccccccccccccccccccaaaaaa\nabcccaaaaaacccccccaaaaaaccccaaaaaaaacccccccaaaaaaaaaaaaaaaaccaaaaacccccccccccccccccccccccccaaaaaacccccccccccccccccccccccccccccaaaaaa\nabccccaaaaacaaaccaaaaaaaacccaaaaaaaaacccccccaaaaaaaaaaaaaaaacaaaaaacccccccccaaacccccccccccaaaaaaaaccccccccccaaccccccccccccccccaaaaaa\nabccccaaaaccaaaaaaaaaaaaacccaaaaaaaaaacccccaaaaaaaaaaaaaaaaaaacaaaacccccccccaaaacccccccccaaaaaaaaaacccccccccaaaccccccccccccccccccaaa\nabcccccccccaaaaaacccaacccccccccaaacaaaccccccaacccccccaaaaaaaaacaacccccccccccaaaacccccccccaaaaaaaaaacccccccccaaaccacaaccccccccccccaaa\nabcccccccccaaaaaacccaacccccccccaaacccccccccccccccccccaaaacaaaacccccccaacaaccaaaccccccccccaccaaaaacacccccccccaaaacaaaaccccccccccccaac\nabccccccccccaaaaacccccccccccccccacccaaaacccccccccccccaaaacccccccccccccaaaacccccccccccaacccccaaaaccccccccjjjjaaaaaaaaaccccccccccccccc\nabccccccccccaaaacccccccccccccccccccaaaaacccccccccccccaaaccccccccccccccaaaaacccccccccaaaaaacccaaccccccccjjjjjjkkaaaacccccccccaacccccc\nabcccccaaccccccccccccccccccccccccccaaaaaacccccccccccccaacccccccccccccaaaaaaccccccccccaaaaaccccccccccccjjjjjjjkkkkaacccccaacaaacccccc\nabccaaaacccccccccccccccccccccccccccaaaaaaccccccccccccccccccccccccccccaaaacaccccccccaaaaaaaccccaacccccjjjjoooookkkkkkkklllaaaaaaacccc\nabccaaaaaacccccccccccccccccccccccccaaaaacccccccccccccccccccccccccccccccaaccccccccccaaaaaaaaccaaaaccccjjjoooooookkkkkkkllllaaaaaacccc\nabcccaaaaacccccccccccccccccccccccccccaaaccccccccaaaacccccccccccccccccccccccccccccccaaaaaaaaccaaaaccccjjooooooooppkkppplllllaccaacccc\nabccaaaaaccccccccccccaccccccccccccccccccccccccccaaaacccccccccccccccccccccccccccccccccaaacacccaaaacccijjooouuuuoppppppppplllccccccccc\nabcccccaacccccccccccaaaaaaaaccccccccccccccccccccaaaaccccaaccccccccaaacccccccccccccaacaaccccccccccccciijoouuuuuuppppppppplllcccaccccc\nabcccccccccccccccccccaaaaaaccccccccccccccccccccccaaccccaaaacccccccaaaaccccccccccaaaaaaccccccccccccciiiiootuuuuuupuuuvvpppllccccccccc\nabcccccccccccccccccccaaaaaaccaaaaacccccccccccccccccccccaaaacccccccaaaaccccccccccaaaaaaccccccccccccciiinnotuuxxxuuuuvvvpppllccccccccc\nabccccccccccccccacccaaaaaaaacaaaaaaacccccccccccccccccccaaaacccccccaaacccccaaaaccaaaaaccccaaccccccciiiinnnttxxxxuuyyyvvqqqllccccccccc\nabcccccccccccaaaaccaaaaaaaaaaaaaaaaaaccaacccccccccccccccccccccccccccccccccaaaacccaaaaaccaaacccccciiinnnnnttxxxxxyyyyvvqqqllccccccccc\nabaaaacccccccaaaaaaaaaaaaaaaaaaaaaaaaaaaacccccccccccccccccccccccccccccccccaaaacccaaaaaacaaaccccciiinnnnttttxxxxxyyyyvvqqmmmccccccccc\nabaaaaccccccccaaaaacccaaaaacaaaaaacaaaaaaccccccccccccccccaaccccccccccccccccaacccccccaaaaaaaaaaciiinnnnttttxxxxxyyyyvvqqqmmmccccccccc\nSbaaaacccccccaaaaaccccaaaaaccaaaaaaaaaaaccccccccccccccccaaacaacccccccccccccccccccccccaaaaaaaaachhhnnntttxxxEzzzzyyvvvqqqmmmccccccccc\nabaaaacccccccaacaacccccaaaaaaaacaaaaaaaaaccccccccccccccccaaaaaccccccccccccccccccccccccaaaaaaacchhhnnntttxxxxxyyyyyyvvvqqmmmdddcccccc\nabaaaacccccccccccccccccccaaaaaacaaaaaaaaaacccccccccccccaaaaaaccccccccaaaccccccccccccccaaaaaaccchhhnnntttxxxxywyyyyyyvvvqqmmmdddccccc\nabaacccccccccccccccccccaaaaaaacccccaaaaaaacccccccccccccaaaaaaaacccccaaaacccccccccccccaaaaaaacaahhhmmmttttxxwwyyyyyyyvvvqqmmmdddccccc\nabcccccccccccccccccccccaaaaaaacaaccaaacccccccccccccccccaacaaaaacccccaaaacccccccccccccaaacaaaaaahhhmmmmtsssswwyywwwwvvvvqqqmmdddccccc\nabcccccccccccccccaaaccccaaaaaaaaaacaaccaaccccccccccccccccaaacaccccccaaaacccccccccccccccccaaaaacahhhmmmmmsssswwywwwwwvvrrqqmmdddccccc\nabcccccccccccccaaaaaaccccaaaaaaaaaccaaaacccccccccccccccccaacccccccccccccccccccccccaaaccccaaaaaaahhhhhmmmmssswwwwwrrrrrrrrmmmmddccccc\nabcccccccccccccaaaaaaccccaaaaaaaaaaaaaaaaaccccccccccccccccccccccccccccccccccccccaaaaaacccccaaaaachhhhhmmmmsswwwwrrrrrrrrrkkmdddccccc\nabccccccccccccccaaaaaccccccaaaaaaaaaaaaaaaccccccccccccccccccccccccccccccccccccccaaaaaaccccaaaaacccchhggmmmssswwrrrrrkkkkkkkkdddacccc\nabccaaaacccccccaaaaacccccccccaaaaaacaaaaacccccccccccccccccccccccccccccccccccccccaaaaaaccccaacaaaccccggggmmsssssrrlkkkkkkkkkdddaccccc\nabccaaaacccccccaaaaacccccccccaaaaaaccccaacccccccccccccccccccccccccccccccccccccccaaaaaccccccccaaccccccgggmllssssrllkkkkkkkeeeddaccccc\nabccaaaacccccccaaacccccccccccaaaaaacccccccccccccccccccaacccccccccccccccccccccccaaaaaacccccccccccccccccggllllssslllkkeeeeeeeeeaaacccc\nabcccaaccccccccaaacaaaccccccaaaaaaaaaaacccccccccccccaaaaaacccccccccccccccccccccaaacaaacccccaacccccccccggglllllllllfeeeeeeeeaaaaacccc\nabccccccccccaaaaaaaaaaccccccccccccaccaaaccacccccccccaaaaaaccccaaccaacccaaccccccaaaaaaacccccaaccccccccccggglllllllfffeeecccaaaaaacccc\nabccccccccccaaaaaaaaacccccccccccccccaaaaaaaccccccccccaaaaaccccaaaaaacccaaaaaaccaaaaaacccaaaaaaaacccccccggggllllfffffccccccaacccccccc\nabcccccccccccaaaaaaacccccccccccccccccaaaaaaccaacccccaaaaaccccccaaaaacccaaaaaacaaaaaaacccaaaaaaaaccccccccgggffffffffccccccccccccccccc\nabccccccccccccaaaaaaacccccccccccccaaaaaaaaacaaaaccccaaaaacaaaaaaaaaacaaaaaaacaaaaaaaaaccccaaaacccccccccccggffffffacccccccccccccccaaa\nabccccccccccccaaaaaaacaaccccccccccaaaaaaaaacaaaacccccaaaaaaaaaaaaaaaaaaaaaaacaaaaaaaaaacccaaaaacccccccccccaffffaaaaccccccccccccccaaa\nabccccccccccccaaacaaaaaacccccccccccaaaaaaaacaaaaaaaaaaaaaaaaaaaaaaaaacaaaaaaacccaaacaaaccaaaaaacccccccccccccccccaaaccccccccccccccaaa\nabccccccccccccaaccaaaaaccccccccccccccaaaaaaaccccaaaaaaaaaaaaccccaacccccaaaaaacccaaaccccccaaccaacccccccccccccccccaaacccccccccccaaaaaa\nabcccccccccccccccaaaaaaaaccccccccccccaacccacccccccaaaaaaaaaaccccaacccccaaccccccccaccccccccccccccccccccccccccccccccccccccccccccaaaaaa\n")

(defn parse-input [input]
  (let [heightmap (->> (str/split-lines input)
                       (map (fn [s]
                              (->> (map #(- (int (case %
                                                   \S \a
                                                   \E \z
                                                   %))
                                            (int \a))
                                        s)
                                   (vec))))
                       (vec))
        width (count (first heightmap))
        idx->coord (fn [idx]
                     [(int (/ idx (inc width)))
                      (mod idx (inc width))])]
    {:heightmap   heightmap
     :start-coord (idx->coord (str/index-of input \S))
     :end-coord   (idx->coord (str/index-of input \E))
     :a-coords    (loop [from-index 0, idxs []]
                    (if-let [idx (str/index-of input \a from-index)]
                      (recur (inc idx) (conj idxs idx))
                      (map idx->coord idxs)))}))

(defn neighbours [heightmap [row col]]
  (for [[row' col'] [[(inc row) col]
                     [row (inc col)]
                     [(dec row) col]
                     [row (dec col)]]
        :when (and (<= 0 row' (dec (count heightmap)))
                   (<= 0 col' (dec (count (first heightmap)))))]
    [row' col']))

(defn reachable-neighbours [heightmap from]
  (let [from-height (if (int? (get-in heightmap from))
                      (get-in heightmap from)
                      0)
        neighbours (neighbours heightmap from)]
    (filter #(let [neighbour-height (get-in heightmap %)]
               (if (int? neighbour-height)
                 (<= (- neighbour-height from-height) 1)
                 true))
            neighbours)))

(defn steps-to-end [state from-coords]
  (let [[end-row end-col] (:end-coord state)]
    (loop [open (into (sorted-set) (map #(vector 0 %) from-coords))
           g-score (into {} (map #(vector % 0) from-coords))]
      (if (seq open)
        (let [[current-cost current-coord :as curr] (first open)]
          (if (= current-coord (:end-coord state))
            current-cost
            (let [tentative-g-score (inc (g-score current-coord))
                  neighbours (filter #(or (not (contains? g-score %))
                                          (< tentative-g-score (g-score %)))
                                     (reachable-neighbours (:heightmap state) current-coord))]
              (recur (into (disj open curr)
                           (map (fn [[row col :as neighbour]]
                                  [(+ tentative-g-score
                                      (abs (- end-row row))
                                      (abs (- end-col col)))
                                   neighbour])
                                neighbours))
                     (into g-score
                           (map (fn [neighbour]
                                  [neighbour tentative-g-score])
                                neighbours))))))
        nil))))

(defn part1 []
  (let [state (parse-input input)]
    (steps-to-end state [(:start-coord state)])))

(defn part2 []
  (let [state (parse-input input)]
    (steps-to-end state (:a-coords state))))