2024, day 8

Author: FelipeCortez

2024/08.clj

@@ -0,0 +1,47 @@
+(require '[clojure.string :as str])
+
+(def the-map (str/split-lines (slurp "2024/08.in")))
+
+(defn in-bounds? [position] (every? #(<= 0 % (dec (count the-map))) position))
+(defn p*2 [[i j]] [(* 2 i) (* 2 j)])
+(defn p- [[i1 j1] [i2 j2]] [(- i1 i2) (- j1 j2)])
+
+(def antenna-groups
+ (reduce (fn [m [a i j]] (update m a (fnil conj []) [i j]))
+ {}
+ (for [i (range (count the-map)), j (range (count the-map))
+ :let [antenna (get-in the-map [i j])]
+ :when (not= \. antenna)]
+ [antenna i j])))
+
+(defn antinodes [antenna-group]
+ (into [] (comp cat (filter in-bounds?))
+ (for [a (range (count antenna-group)),
+ b (range (inc a) (count antenna-group))
+ :let [a (nth antenna-group a)
+ b (nth antenna-group b)]]
+ [(p- (p*2 a) b)
+ (p- (p*2 b) a)])))
+
+(count
+ (into #{} cat
+ (vals (update-vals antenna-groups antinodes))))
+
+;; ---
+
+(defn p*n [[i j] n] [(* n i) (* n j)])
+(defn p+ [[i1 j1] [i2 j2]] [(+ i1 i2) (+ j1 j2)])
+
+(defn antinodes+harmonics [antenna-group]
+ (into [] (comp cat (filter in-bounds?))
+ (for [a (range (count antenna-group)),
+ b (range (inc a) (count antenna-group))
+ :let [a (nth antenna-group a)
+ b (nth antenna-group b)]]
+ (concat
+ (take-while in-bounds? (map #(p+ a (p*n (p- b a) %)) (range)))
+ (take-while in-bounds? (map #(p+ b (p*n (p- a b) %)) (range)))))))
+
+(count
+ (into #{} cat
+ (vals (update-vals antenna-groups antinodes+harmonics))))