Solution 2017-15 (Dueling Generators)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2017/day15/DuelingGenerators.kt

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+package de.ronny_h.aoc.year2017.day15
+
+import de.ronny_h.aoc.AdventOfCode
+
+fun main() = DuelingGenerators().run(594, 328)
+
+class DuelingGenerators : AdventOfCode<Int>(2017, 15) {
+    override fun part1(input: List<String>): Int = judge(
+        Generator(seed(input, 'A', 0), 16807),
+        Generator(seed(input, 'B', 1), 48271),
+    )
+
+    override fun part2(input: List<String>): Int = judge(
+        Generator(seed(input, 'A', 0), 16807, 4),
+        Generator(seed(input, 'B', 1), 48271, 8),
+        5_000_000,
+    )
+
+    private fun seed(input: List<String>, generator: Char, index: Int): Long = input[index]
+        .substringAfter("Generator $generator starts with ").toLong()
+
+    private fun judge(
+        generatorA: Generator,
+        generatorB: Generator,
+        iterations: Int = 40_000_000,
+    ): Int {
+        val bitmask = "00000000000000001111111111111111".toLong(2)
+
+        var equalLowestBitsCount = 0
+        repeat(iterations) {
+            val lowestABits = generatorA.next() and bitmask
+            val lowestBBits = generatorB.next() and bitmask
+            if (lowestABits == lowestBBits) {
+                equalLowestBitsCount++
+            }
+        }
+        return equalLowestBitsCount
+    }
+}
+
+class Generator(seed: Long, private val factor: Int, private val provideOnlyMultiplesOf: Int = 1) {
+    private var value = seed
+    private val divider = 2147483647
+
+    fun next(): Long {
+        do {
+            value = (value * factor) % divider
+        } while (value % provideOnlyMultiplesOf != 0L)
+        return value
+    }
+}

src/test/kotlin/de/ronny_h/aoc/year2017/day15/DuelingGeneratorsTest.kt

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+package de.ronny_h.aoc.year2017.day15
+
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.data.forAll
+import io.kotest.data.row
+import io.kotest.matchers.shouldBe
+
+class DuelingGeneratorsTest : StringSpec({
+
+    "generators A and B" {
+        val generatorA = Generator(65, 16807)
+        val generatorB = Generator(8921, 48271)
+        forAll(
+            row(1092455, 430625591),
+            row(1181022009, 1233683848),
+            row(245556042, 1431495498),
+            row(1744312007, 137874439),
+            row(1352636452, 285222916),
+        ) { a, b ->
+            generatorA.next() shouldBe a
+            generatorB.next() shouldBe b
+        }
+    }
+
+    "part 1: The number of equal lowest 16 bit values in a sample of 40 million pairs" {
+        val input = listOf(
+            "Generator A starts with 65",
+            "Generator B starts with 8921",
+        )
+        DuelingGenerators().part1(input) shouldBe 588
+    }
+
+    "generators A and B only providing multiples" {
+        val generatorA = Generator(65, 16807, 4)
+        val generatorB = Generator(8921, 48271, 8)
+        forAll(
+            row(1352636452, 1233683848),
+            row(1992081072, 862516352),
+            row(530830436, 1159784568),
+            row(1980017072, 1616057672),
+            row(740335192, 412269392),
+        ) { a, b ->
+            generatorA.next() shouldBe a
+            generatorB.next() shouldBe b
+        }
+    }
+
+    "part 2: The number of equal lowest 16 bit values generated by modified generators in a sample of 5 million pairs" {
+        val input = listOf(
+            "Generator A starts with 65",
+            "Generator B starts with 8921",
+        )
+        DuelingGenerators().part2(input) shouldBe 309
+    }
+})