Solution 2017-17 (Spinlock)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2017/day17/Spinlock.kt

@@ -0,0 +1,34 @@
+package de.ronny_h.aoc.year2017.day17
+
+import de.ronny_h.aoc.AdventOfCode
+
+fun main() = Spinlock().run(2000, 10242889)
+
+class Spinlock : AdventOfCode<Int>(2017, 17) {
+    override fun part1(input: List<String>): Int = insertIntoCircularBuffer(input.single().toInt())
+    override fun part2(input: List<String>): Int = simulateInsertIntoCircularBuffer(input.single().toInt())
+
+    private fun insertIntoCircularBuffer(steps: Int): Int {
+        val circularBuffer = mutableListOf<Int>()
+        var index = 0
+        repeat(2017) { i ->
+            circularBuffer.add(index, i)
+            index = (index + steps) % circularBuffer.size + 1
+        }
+        return circularBuffer[index]
+    }
+
+    private fun simulateInsertIntoCircularBuffer(steps: Int): Int {
+        var bufferSize = 0
+        var index = 0
+        var atIndex1 = -1
+        repeat(50_000_000) { i ->
+            if (index == 1) {
+                atIndex1 = i
+            }
+            bufferSize++
+            index = (index + steps) % bufferSize + 1
+        }
+        return atIndex1
+    }
+}

src/test/kotlin/de/ronny_h/aoc/year2017/day17/SpinlockTest.kt

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+package de.ronny_h.aoc.year2017.day17
+
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.matchers.shouldBe
+
+class SpinlockTest : StringSpec({
+
+    "part 1: the value after the last value written (2017)" {
+        Spinlock().part1(listOf("3")) shouldBe 638
+    }
+
+    "part 2: the value on index 1 (the one after value 0) the moment 50000000 is inserted" {
+        Spinlock().part2(listOf("3")) shouldBe 1222153
+    }
+})