Day 9: Disk Defragmenter

README.md

@@ -13,4 +13,4 @@ Development occurs in language-specific directories:
 |[Day6.hs](hs/src/Day6.hs)|[Day6.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day6.kt)|[day6.py](py/aoc2024/day6.py)|[day6.rs](rs/src/day6.rs)|
 |[Day7.hs](hs/src/Day7.hs)|[Day7.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day7.kt)|[day7.py](py/aoc2024/day7.py)|[day7.rs](rs/src/day7.rs)|
 |[Day8.hs](hs/src/Day8.hs)|[Day8.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day8.kt)|[day8.py](py/aoc2024/day8.py)|[day8.rs](rs/src/day8.rs)|
-|[Day9.hs](hs/src/Day9.hs)||||
+|[Day9.hs](hs/src/Day9.hs)|[Day9.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day9.kt)|||

kt/aoc2024-exe/src/commonBench/kotlin/com/github/ephemient/aoc2024/exe/Day9Bench.kt

@@ -0,0 +1,31 @@
+package com.github.ephemient.aoc2024.exe
+
+import com.github.ephemient.aoc2024.Day9
+import kotlinx.benchmark.Benchmark
+import kotlinx.benchmark.Blackhole
+import kotlinx.benchmark.Scope
+import kotlinx.benchmark.Setup
+import kotlinx.benchmark.State
+
+@State(Scope.Benchmark)
+class Day9Bench {
+    private lateinit var input: String
+
+    @Setup
+    fun setup() {
+        input = getDayInput(9)
+    }
+
+    @Benchmark
+    fun part1() = Day9(input).part1()
+
+    @Benchmark
+    fun part2() = Day9(input).part2()
+
+    @Benchmark
+    fun solve(bh: Blackhole) {
+        val day9 = Day9(input)
+        bh.consume(day9.part1())
+        bh.consume(day9.part2())
+    }
+}

kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day9.kt

@@ -0,0 +1,56 @@
+package com.github.ephemient.aoc2024
+
+class Day9(input: String) {
+    private val input = input.mapNotNull { it.digitToIntOrNull() }
+
+    fun part1(): Long {
+        val chunks = input.toIntArray()
+        var total = 0L
+        var i = 0
+        var j = chunks.lastIndex
+        var offset = 0
+        while (i <= j) {
+            if (i % 2 == 0) {
+                val size = chunks[i]
+                total += i / 2L * triRange(offset, size)
+                offset += size
+                i++
+            } else if (j % 2 == 0) {
+                val free = chunks[i]
+                val size = chunks[j]
+                total += j / 2L * triRange(offset, minOf(free, size))
+                offset += minOf(free, size)
+                if (free <= size) i++ else chunks[i] = free - size
+                if (free >= size) j-- else chunks[j] = size - free
+            } else {
+                j--
+            }
+        }
+        return total
+    }
+
+    fun part2(): Long {
+        val files = IntArray((input.size + 1) / 2) { input[2 * it] }
+        val frees = IntArray(input.size / 2) { input[2 * it + 1] }
+        val offsets = IntArray(input.size)
+        for (i in 0..offsets.size - 2) offsets[i + 1] = offsets[i] + input[i]
+        var total = 0L
+        outer@for (i in files.lastIndex downTo 0) {
+            val size = files[i]
+            for (j in 0 until i) {
+                if (frees[j] >= size) {
+                    total += i.toLong() * triRange(offsets[2 * j + 1], size)
+                    frees[j] -= size
+                    offsets[2 * j + 1] += size
+                    continue@outer
+                }
+            }
+            total += i.toLong() * triRange(offsets[2 * i], size)
+        }
+        return total
+    }
+
+    companion object {
+        private fun triRange(offset: Int, size: Int) = (2 * offset + size - 1) * size / 2
+    }
+}

kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Days.kt

@@ -9,6 +9,7 @@ val days: List<Day> = listOf(
     Day(6, ::Day6, Day6::part1, Day6::part2),
     Day(7, ::Day7, Day7::part1, Day7::part2),
     Day(8, ::Day8, Day8::part1, Day8::part2),
+    Day(9, ::Day9, Day9::part1, Day9::part2),
 )
 
 data class Day(

kt/aoc2024-lib/src/commonTest/kotlin/com/github/ephemient/aoc2024/Day9Test.kt

@@ -0,0 +1,23 @@
+package com.github.ephemient.aoc2024
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class Day9Test {
+    @Test
+    fun part1() {
+        assertEquals(1928L, Day9(example).part1())
+    }
+
+    @Test
+    fun part2() {
+        assertEquals(2858L, Day9(example).part2())
+    }
+
+    companion object {
+        private val example =
+            """
+            |2333133121414131402
+            |""".trimMargin()
+    }
+}