day 2

inputs/day2.example

@@ -0,0 +1,6 @@
+7 6 4 2 1
+1 2 7 8 9
+9 7 6 2 1
+1 3 2 4 5
+8 6 4 4 1
+1 3 6 7 9

src/day2.clj

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+(ns day2
+  (:require [clojure.string :as s]))
+
+(defn line->vec [steps]
+  (->>
+   (re-seq #"\d+" steps)
+   (mapv parse-long)))
+
+(def input
+  (->> "inputs/day2"
+       slurp
+       s/split-lines
+       (map line->vec)))
+
+(defn diff [steps]
+  (->>
+   (partition 2 1 steps)
+   (map (partial apply -))))
+
+(defn safe? [steps]
+  (let [diffs (diff steps)
+        sign (pos? (first diffs))]
+    (loop [diffs diffs]
+      (if (empty? diffs)
+        true
+        (if (zero? (first diffs))
+          false
+          (if-not (= sign (pos? (first diffs)))
+            false
+            (if (> (abs (first diffs)) 3)
+              false
+              (recur (rest diffs)))))))))
+
+(defn cut-nth [steps n]
+  (concat (subvec steps 0 n)
+          (subvec steps (inc n))))
+
+(defn safe?-2 [steps]
+  (if (safe? steps)
+    true
+    (not-empty
+     (for [i (range (count steps))
+           :let [safe (safe? (cut-nth steps i))]
+           :when (true? safe)]
+       safe))))
+
+(println "Number of safe reports:"
+         (->> (map safe? input)
+              (filter true?)
+              count))
+
+(println "Number of safe reports (part 2):"
+         (->> (map safe?-2 input)
+              (remove nil?)
+              count))