Day09

days/src/main/kotlin/Day09.kt

@@ -0,0 +1,51 @@
+package be.vandewalleh.aoc.days
+
+import be.vandewalleh.aoc.utils.input.Day
+import be.vandewalleh.aoc.utils.input.Input
+import be.vandewalleh.aoc.utils.input.Lines
+import be.vandewalleh.aoc.utils.input.createDay
+
+@Day(9)
+class Day09(@Lines val input: Input<LongArray>) {
+
+    fun part1() = input.value
+        .toList()
+        .windowed(size = 26)
+        .find { !isValid(it) }
+        ?.last()
+
+    private fun isValid(items: List<Long>): Boolean {
+        val preamble = items.take(25)
+        val last = items.last()
+
+        for (e in preamble) {
+            for (f in preamble) {
+                if (e + f == last && e != f) {
+                    return true
+                }
+            }
+        }
+        return false
+    }
+
+    fun part2(): Long {
+        val a = 731031916L
+        var size = 2
+        while (true) {
+            for (startIndex in input.value.indices) {
+                val lastIndex = input.value.size - 1 - size
+                if (startIndex + size > lastIndex) break
+                val slice = input.value.sliceArray(startIndex..startIndex + size)
+                val sum = slice.reduce { acc, l -> acc + l }
+                if (sum == a) return slice.minOrNull()!! + slice.maxOrNull()!!
+            }
+            size++
+        }
+    }
+
+}
+
+fun main() = with(createDay<Day09>()) {
+    println(part1())
+    println(part2())
+}