Day 10.

src/Utils.kt

@@ -196,10 +196,17 @@ class CharGrid {
     fun matchRelative(c: Int, r: Int, relposes: Iterable<RelPos>, predicate: (char: Char) -> Boolean): List<RelPos> =
         relposes.filter { predicate(get(c + it.dc, r + it.dr)) }
 
+    fun marchMatching(pos: RelPos, relposes: Iterable<RelPos>, predicate: (char: Char) -> Boolean): List<RelPos> =
+        relposes.map { pos.translate(it) }.filter { predicate(get(it)) }
+
     fun findMatches(predicate: (char: Char) -> Boolean): List<Pair<Int, Int>> {
         return generateGridPos().filter { predicate(get(it)) }.map { it.dc to it.dr }.toList()
     }
 
+    fun findMatchesRelPos(predicate: (char: Char) -> Boolean): List<RelPos> {
+        return generateGridPos().filter { predicate(get(it)) }.toList()
+    }
+
     fun collectMatches(predicate: (char: Char) -> Boolean): List<Pair<Char, RelPos>> {
         return generateGridPos().filter { predicate(get(it)) }.map { get(it) to it }.toList()
     }

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()
+}

src/aoc2024/Day10.kt

@@ -0,0 +1,64 @@
+package aoc2024
+
+import CharGrid
+import RelPos
+import println
+import readInput
+
+/*
+--- Day 10: Hoof It ---
+https://adventofcode.com/2024/day/10
+*/
+fun main() {
+
+    val inlineTestInput = """
+89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732
+"""
+
+    fun part1(input: List<String>): Int {
+        val grid = CharGrid(input)
+        val starts = grid.findMatchesRelPos { it == '0' }
+        var sum = 0
+        for (startPos in starts) {
+            var nodes = listOf(startPos)
+            for (c in '1'..'9') {
+                nodes = nodes.flatMap { grid.marchMatching(it, CharGrid.PLUS_POS) { char -> char == c } }.distinct()
+                if (nodes.isEmpty()) break
+            }
+            sum += nodes.size
+        }
+
+        return sum
+    }
+
+    fun walkTrail(grid: CharGrid, c: Char, p: RelPos): Int {
+        val newNodes = grid.marchMatching(p, CharGrid.PLUS_POS) { it == c }
+        return if (c == '9') newNodes.size else newNodes.sumOf { walkTrail(grid, c + 1, it) }
+    }
+
+    fun part2(input: List<String>): Int {
+        val grid = CharGrid(input)
+        return grid.findMatchesRelPos { it == '0' }.sumOf { walkTrail(grid, '1', it) }
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    //val testInput = readInput("aoc2024/Day10_test")
+    val testInputPart1Result = part1(testInput)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 36)
+    check(testInputPart2Result == 81)
+
+    val input = readInput("aoc2024/Day10")
+    part1(input).println()
+    part2(input).println()
+}