Day 9.

src/aoc2024/Day09.kt

@@ -0,0 +1,117 @@
+package aoc2024
+
+import println
+import readInput
+
+/*
+--- Day 9: Disk Fragmenter ---
+https://adventofcode.com/2024/day/9
+*/
+fun main() {
+
+    val inlineTestInput = """2333133121414131402"""
+
+    fun part1(input: List<String>): Long {
+        val diskSize = input[0].sumOf { it - '0' }
+        val bitmap = IntArray(diskSize) { -1 }
+        var pos = 0
+        var isFile = true
+        var fileId = 0
+        var usedBlocks = 0
+        for (c in input[0]) {
+            val size = c - '0'
+            if (isFile) {
+                usedBlocks += size
+                for (p in 1..size) {
+                    bitmap[pos++] = fileId
+                }
+                fileId++
+            } else {
+                pos += size
+            }
+            isFile = !isFile
+        }
+        var backPos = diskSize - 1
+        for (i in 0 until usedBlocks) {
+            if (bitmap[i] == -1) {
+                while (bitmap[backPos] < 0) {
+                    backPos--
+                }
+                bitmap[i] = bitmap[backPos]
+                bitmap[backPos--] = -1
+            }
+        }
+        return bitmap.take(usedBlocks).map(Int::toLong).reduceIndexed { index, acc, i -> acc + index * i }
+    }
+
+    fun part2(input: List<String>): Long {
+        val diskSize = input[0].sumOf { it - '0' }
+        val bitmap = IntArray(diskSize) { 0 }
+        var pos = 0
+        var isFile = true
+        var fileId = 0
+        val freeList = HashMap<Int, MutableList<Int>>()
+        val fileList = ArrayList<Pair<Int, Pair<Int, Int>>>()
+        val newFileList = ArrayList<Pair<Int, Pair<Int, Int>>>()
+        for (c in input[0]) {
+            val size = c - '0'
+            if (isFile) {
+                if (size == 0) println("Narf!")
+                fileList.add(fileId to (pos to size))
+                fileId++
+            } else {
+                if (size > 0) {
+                    freeList.getOrPut(size) { ArrayDeque(0) }.add(pos)
+                }
+            }
+            pos += size
+            isFile = !isFile
+        }
+
+        for (file in fileList.reversed()) {
+            val (filePos, size) = file.second
+            var bestList: MutableList<Int>? = null
+            var bestListSize = 0
+            for (s in size..9) {
+                val list = freeList[s]
+                if (list?.isNotEmpty() == true && list[0] < filePos) {
+                    if (bestList == null || list[0] < bestList[0]) {
+                        bestList = list
+                        bestListSize = s
+                    }
+                }
+            }
+            if (bestList != null) {
+                val newPos = bestList.removeAt(0)
+                if (bestListSize > size) {
+                    val remBucket = freeList.getOrPut(bestListSize - size) { ArrayDeque(0) }
+                    remBucket.add(newPos + size)
+                    remBucket.sort()
+                }
+                newFileList.add(file.first to (newPos to size))
+            } else {
+                newFileList.add(file)
+            }
+        }
+        for (file in newFileList) {
+            for (i in 0 until file.second.second) {
+                bitmap[i + file.second.first] = file.first
+            }
+        }
+        return bitmap.map(Int::toLong).reduceIndexed { index, acc, i -> acc + index * i }
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    //val testInput = readInput("aoc2024/Day09_test")
+    val testInputPart1Result = part1(testInput)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 1928L)
+    check(testInputPart2Result == 2858L)
+
+    val input = readInput("aoc2024/Day09")
+    part1(input).println()
+    part2(input).println()
+}