do day 9

2023/src/day09.clj

@@ -0,0 +1,37 @@
+(ns day09
+  (:require [core :refer [get-puzzle-input]]
+            [clojure.string :as str]))
+
+(defn parse-line [line]
+  (->> (str/split line #" ")
+       (mapv #(Long/parseLong %))))
+
+(defn generate-pyramid [nums]
+  (loop [pyramid [nums]]
+    (let [last-line (peek pyramid)]
+      (if (every? zero? last-line)
+        pyramid
+        (recur (conj pyramid
+                     (mapv - (rest last-line) last-line)))))))
+
+(defn get-missing-num [nums reduction]
+  (let [pyrimid (generate-pyramid nums)]
+    (loop [acc 0
+           line-idx (dec (count pyrimid))]
+      (if (< line-idx 0)
+        acc
+        (recur (reduction (get pyrimid line-idx) acc)
+               (dec line-idx))))))
+
+(defn solve [reduction]
+  (->> (get-puzzle-input 9)
+       (mapv parse-line)
+       (mapv #(get-missing-num % reduction))
+       (reduce +)))
+
+;; part 1
+(solve (fn [line num] (+ (peek line) num)))
+
+;; part 2
+(solve (fn [line num] (- (first line) num)))
+