day 20 init pt 1

2024/src/day20.clj

@@ -0,0 +1,79 @@
+(ns day20
+  (:require
+   [core :as c]
+   input-manager))
+
+(def maze (input-manager/get-input 2024 20)
+  #_(c/split-whitespace "###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############"))
+(def grid (->> maze
+               (map-indexed (fn [row-idx row]
+                              (map-indexed (fn [col-idx c]
+                                             [[row-idx col-idx] c])
+                                           row)))
+               (mapcat identity)
+               (into {})))
+(def start (->> grid
+                (filter (comp #{\S} second))
+                ffirst))
+(def end (->> grid
+              (filter (comp #{\E} second))
+              ffirst))
+
+(def direction-offsets #{[0 1] [1 0] [0 -1] [-1 0]})
+
+(defn find-search-path []
+  (loop [[[pos steps] & rst] [[start 0]]
+         visited #{start}
+         shortest {start 0}]
+    (cond
+      (= pos end) shortest
+
+      :else
+      (let [searches (->> direction-offsets
+                          (map (partial mapv + pos))
+                          (filter grid)
+                          (filter (comp not visited))
+                          (filter (comp not #{\#} grid)))
+            searches-w-path-lengths (mapv #(vector % (inc steps)) searches)]
+        (recur (into rst searches-w-path-lengths)
+               (into visited searches)
+               (into shortest searches-w-path-lengths))))))
+
+(defn find-best-cheat []
+  (let [search-path (find-search-path)
+        ; no-cheat-solution (search-path end)
+        walls (->> grid
+                   (filter (comp #{\#} second))
+                   (map first))
+        cheats (->> walls
+                    (map (fn [pos]
+                           (->> direction-offsets
+                                (map (partial mapv + pos))
+                                (map search-path)
+                                (filter identity))))
+                    (filter not-empty)
+                    (filter (comp (partial <= 2) count))
+                    (map sort)
+                    (mapcat (fn [[a & bs]]
+                              (map #(- % a) bs))))]
+    (frequencies cheats)))
+
+(->> (find-best-cheat)
+     (filter (comp #(> % 100) first))
+     (map second)
+     (apply +))
+