Solve Day 11

2024/src/main/kotlin/aoc/year2024/Day11.kt

@@ -0,0 +1,50 @@
+package aoc.year2024
+
+import aoc.library.Puzzle
+
+object Day11 : Puzzle<Long, Long>(day = 11) {
+
+  override fun solvePart1(input: String): Long = solve(input, blinks = 25)
+
+  override fun solvePart2(input: String): Long = solve(input, blinks = 75)
+
+  private fun solve(
+    input: String,
+    blinks: Int,
+  ): Long {
+    var numbers = parse(input)
+    repeat(blinks) {
+      numbers = numbers.blink()
+    }
+    return numbers.values.sum()
+  }
+
+  private fun parse(input: String): Map<Long, Long> = input.split(" ")
+    .groupingBy(String::toLong)
+    .eachCount()
+    .mapValues { it.value.toLong() }
+
+  private fun Map<Long, Long>.blink(): Map<Long, Long> {
+    val afterBlink = mutableMapOf<Long, Long>()
+    forEach { (number, count) ->
+      number.blink().forEach { newNumber ->
+        val current = afterBlink[newNumber] ?: 0
+        afterBlink[newNumber] = current + count
+      }
+    }
+    return afterBlink
+  }
+
+  private fun Long.blink(): List<Long> = when {
+    this == 0L -> listOf(1)
+    toString().length % 2 == 0 -> {
+      val asString = this.toString()
+      val center = asString.length / 2
+      listOf(
+        asString.take(center).toLong(),
+        asString.drop(center).toLong(),
+      )
+    }
+    else -> listOf(this * 2024)
+  }
+}

2024/src/test/kotlin/aoc/year2024/Day11Test.kt

@@ -0,0 +1,24 @@
+package aoc.year2024
+
+import aoc.library.solvePart1
+import aoc.library.solvePart2
+import io.kotest.matchers.longs.shouldBeExactly
+import org.junit.jupiter.api.Test
+
+class Day11Test {
+
+  @Test
+  fun part1TestInput() {
+    Day11.solvePart1("125 17") shouldBeExactly 55312
+  }
+
+  @Test
+  fun part1() {
+    Day11.solvePart1() shouldBeExactly 194557
+  }
+
+  @Test
+  fun part2() {
+    Day11.solvePart2() shouldBeExactly 231532558973909
+  }
+}