2024, day 12

Author: FelipeCortez

2024/12.clj

@@ -0,0 +1,71 @@
+(require '[clojure.string :as str]
+ '[clojure.set :as set])
+
+(def the-map (str/split-lines (slurp "2024/12.in")))
+(def unvisited
+ (set
+ (for [i (range (count the-map)), j (range (count the-map))]
+ [i j])))
+
+(def deltas [[0 1] [0 -1] [1 0] [-1 0]])
+
+(defn flood [unvisited position]
+ (loop [to-visit [position], unvisited unvisited, visited #{}]
+ (if (empty? to-visit)
+ visited
+ (let [visiting (peek to-visit)
+ to-visit (pop to-visit)]
+ (recur (into to-visit
+ (keep (fn [delta]
+ (let [neighbor (mapv + visiting delta)]
+ (when (and (contains? unvisited neighbor)
+ (= (get-in the-map visiting)
+ (get-in the-map neighbor)))
+ neighbor)))
+ deltas))
+ (disj unvisited visiting)
+ (conj visited visiting))))))
+
+(def regions
+ (loop [unvisited unvisited, regions []]
+ (if (empty? unvisited)
+ regions
+ (let [plant (first unvisited), plant-region (flood unvisited plant)]
+ (recur (set/difference unvisited plant-region)
+ (conj regions plant-region))))))
+
+(defn perimeter [region]
+ (reduce (fn [s position]
+ (into s (keep (fn [delta]
+ (let [neighbor (mapv + position delta)]
+ (when-not (contains? region neighbor)
+ (mapv + position (mapv (partial * 1/4) delta)))))
+ deltas)))
+ #{}
+ region))
+
+;; part 1
+(time
+ (reduce + (map (fn [region] (* (count region) (count (perimeter region)))) regions)))
+
+(defn side->orientation [[i _j]] (if (= (type i) clojure.lang.Ratio) :v :h))
+
+(defn side-from-pos [start perimeter]
+ (if (= (side->orientation start) :h)
+ (into #{} cat [(take-while #(contains? perimeter %) (iterate #(update % 0 inc) start))
+ (take-while #(contains? perimeter %) (iterate #(update % 0 dec) start))])
+ (into #{} cat [(take-while #(contains? perimeter %) (iterate #(update % 1 inc) start))
+ (take-while #(contains? perimeter %) (iterate #(update % 1 dec) start))])))
+
+(defn sides [perimeter]
+ (loop [unvisited perimeter, sides #{}]
+ (if (empty? unvisited)
+ sides
+ (let [start (first unvisited), side (side-from-pos start perimeter)]
+ (recur (set/difference unvisited side)
+ (conj sides side))))))
+
+;; part 2
+(time
+ (reduce + (map (fn [region] (* (count region)
+ (count (sides (perimeter region))))) regions)))