Day15

days/src/main/kotlin/Day15.kt

@@ -0,0 +1,53 @@
+package be.vandewalleh.aoc.days
+
+import be.vandewalleh.aoc.utils.input.Csv
+import be.vandewalleh.aoc.utils.input.Day
+import be.vandewalleh.aoc.utils.input.Input
+import be.vandewalleh.aoc.utils.input.createDay
+import kotlin.math.abs
+import org.eclipse.collections.impl.factory.primitive.IntObjectMaps
+
+@Day(15)
+class Day15(@Csv val input: Input<IntArray>) {
+
+    private fun run(until: Int): Int {
+        val start = input.value
+
+        val map = IntObjectMaps.mutable.empty<IntArray>()
+
+        var last = -1
+
+        fun say(round: Int, number: Int) {
+            val lastValues = map.get(number)
+            val values = lastValues
+                ?.let { intArrayOf(round, it.maxOrNull()!!) }
+                ?: intArrayOf(round)
+
+            map.put(number, values)
+            last = number
+        }
+
+        for (i in 1..until) {
+            if (i <= start.size) {
+                say(i, start[i - 1])
+            } else {
+                val lastValues = map.get(last)
+                if (lastValues?.size ?: 0 == 1) say(i, 0)
+                else say(i, abs(lastValues[0] - lastValues[1]))
+            }
+        }
+        return last
+    }
+
+    fun part1() = run(until = 2020)
+
+    fun part2() = run(until = 30000000)
+}
+
+fun main() {
+    // val input = Input(intArrayOf(3, 1, 2))
+    with(createDay<Day15>()) {
+        println(part1())
+        println(part2())
+    }
+}

days/src/main/resources/day15.txt

@@ -0,0 +1 @@
+0,1,5,10,3,12,19