Day 11

src/main/kotlin/xyz/luan/advent/day11/Main.kt

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+package xyz.luan.advent.day11
+
+import xyz.luan.advent.util.readFile
+
+fun main() {
+    val input = parse(readFile("day11/input"))
+
+    val stones = Stones()
+    stones.ingest(input)
+    stones.evolve(75)
+    val result = stones.compute()
+
+    println("B: $result")
+}
+
+private fun parse(input: List<String>): List<Long> {
+    val line = input.filterNot { it.isBlank() }.single()
+    return line
+        .split(" ")
+        .map { it.trim().toLong() }
+}
+
+class Stones {
+    private val stones = mutableMapOf<Long, Long>()
+
+    private fun rule(value: Long): List<Long> {
+        val str = value.toString()
+        return when {
+            value == 0L -> listOf(1)
+            str.length % 2 == 0 -> listOf(
+                str.substring(0..<str.length / 2).toLong(),
+                str.substring(str.length / 2).toLong(),
+            )
+
+            else -> listOf(value * 2024)
+        }
+    }
+
+    private fun inc(stone: Long, amount: Long = 1) {
+        stones[stone] = (stones[stone] ?: 0) + amount
+    }
+
+    fun ingest(stones: List<Long>) {
+        stones.forEach { inc(it) }
+    }
+
+    fun evolve(blinks: Int) {
+        repeat(blinks) { evolveOnce() }
+    }
+
+    fun compute(): Long {
+        return stones.values.sum()
+    }
+
+    private fun evolveOnce() {
+        for ((stone, count) in stones.toList()) {
+            inc(stone, -count)
+            rule(stone).forEach { inc(it, count) }
+        }
+    }
+}

src/main/resources/xyz/luan/advent/day11/input

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+9694820 93 54276 1304 314 664481 0 4

src/test/kotlin/xyz/luan/advent/day11/Day11Test.kt

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+package xyz.luan.advent.day11
+
+import org.junit.jupiter.api.Test
+import xyz.luan.advent.day11.Stones
+import kotlin.test.assertEquals
+
+class Day11Test {
+    @Test
+    fun `simplest case`() {
+        testCase(listOf(125, 17), 6, 22)
+        testCase(listOf(125, 17), 25, 55312)
+    }
+
+    private fun testCase(input: List<Long>, blink: Int, result: Long) {
+        val stones = Stones()
+        stones.ingest(input)
+        stones.evolve(blink)
+        assertEquals(result, stones.compute())
+    }
+}