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adventofcode 2025 Day 12
In hindsight that 'check the tree has enough spaces' optimisation is obvious. GitOrigin-RevId: cfd049959bac722af671c8346e858e3989654001
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‎advent-of-code/src/main/kotlin/com/willmolloy/adventofcode/_2025/Day12.kt‎

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package com.willmolloy.adventofcode._2025
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import com.willmolloy.adventofcode.common.Day
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import com.willmolloy.adventofcode.common.Input
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import com.willmolloy.adventofcode.common.extensions.debug
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import com.willmolloy.adventofcode.common.grid.CharGrid
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import com.willmolloy.adventofcode.common.grid.MutableCharGrid
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import com.willmolloy.adventofcode.common.grid.Point
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import java.util.concurrent.atomic.AtomicInteger
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/** https://adventofcode.com/2025/day/12 */
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object Day12 : Day(2025, 12) {
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private data class Present(val grid: CharGrid, val combinations: List<CharGrid>, val index: Int)
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private data class Tree(val width: Int, val length: Int, val presentCounts: List<Int>)
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private fun parsePresents(input: Input): List<Present> {
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var presentIndex = 0
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return input
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.readLines()
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.filter { it.contains('#') || it.contains('.') }
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.filter { it.isNotEmpty() }
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// they're all 3x3
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.chunked(3)
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.map {
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val grid = it.map { it.toCharArray() }.toTypedArray().let { CharGrid(it) }
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Present(grid, grid.combinations(), presentIndex++)
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}
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}
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private fun parseTrees(input: Input): List<Tree> {
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return input
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.readLines()
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.filter { it.contains('x') }
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.map { line ->
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val match = Regex("(\\d+)x(\\d+): (.*)").matchEntire(line)!!
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val (width, length, counts) = match.destructured
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val presentCounts = counts.split(" ").map { it.toInt() }
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Tree(width.toInt(), length.toInt(), presentCounts)
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}
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}
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override fun part1(input: Input): Any {
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val presents = parsePresents(input)
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val trees = parseTrees(input)
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val solveCounter = AtomicInteger()
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fun solve(tree: Tree): Boolean {
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"Solving ${solveCounter.incrementAndGet()}/${trees.size}: $tree".debug()
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// first ensure the grid has enough spaces
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val treeSpaces = tree.length * tree.width
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val totalPresentSpaces =
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presents.sumOf { present ->
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val presentSpaces = present.grid.count { present.grid[it] == '#' }
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presentSpaces * tree.presentCounts[present.index]
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}
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if (totalPresentSpaces > treeSpaces) {
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return false
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}
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val treeGrid = CharGrid(Array(tree.length) { CharArray(tree.width) { '.' } }).toMutable()
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var presentId = 'A' // debugging
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// attempt to place the presents - recursively - it's like the sudoku solver problem
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fun dfs(presentCounts: MutableList<Int>): Boolean {
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// treeGrid.debug()
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// presentCounts.debug()
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for (present in presents) {
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if (presentCounts[present.index] == 0) {
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continue
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}
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for (presentGrid in present.combinations) {
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for (x in 0..tree.width - presentGrid.width()) {
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for (y in 0..tree.length - presentGrid.height()) {
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val offset = Point(x, y)
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if (tryPlacePresent(treeGrid, presentGrid, offset, presentId)) {
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// "Placed: ${present.id} as $presentId".debug()
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presentCounts[present.index]--
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presentId++
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if (dfs(presentCounts)) {
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return true
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} else {
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// back track
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removePresent(treeGrid, presentGrid, offset)
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presentCounts[present.index]++
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presentId--
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}
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}
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}
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}
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}
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}
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return presentCounts.all { it == 0 }
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}
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return dfs(tree.presentCounts.toMutableList())
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}
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return trees.parallelStream().filter { solve(it) }.count()
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}
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private fun tryPlacePresent(
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treeGrid: MutableCharGrid,
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presentGrid: CharGrid,
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offset: Point,
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presentId: Char,
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): Boolean {
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val placed = mutableListOf<Point>()
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for (presentLoc in presentGrid) {
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// TODO precompute this? i.e. really just need the List<Point> which contain '#'
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if (presentGrid[presentLoc] != '#') {
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continue
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}
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val treeLoc = presentLoc.move(offset)
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if (treeGrid[treeLoc] == '.') {
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treeGrid[treeLoc] = presentId
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placed.add(treeLoc)
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} else {
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// need to clear any partially placed present
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placed.forEach { treeGrid[it] = '.' }
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return false
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}
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}
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return true
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}
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private fun removePresent(treeGrid: MutableCharGrid, presentGrid: CharGrid, offset: Point) {
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for (presentLoc in presentGrid) {
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if (presentGrid[presentLoc] != '#') {
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continue
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}
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val treeLoc = presentLoc.move(offset)
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if (treeGrid[treeLoc] != '.') {
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treeGrid[treeLoc] = '.'
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}
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}
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}
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// TODO move these into Grid class?
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private fun CharGrid.combinations(): List<CharGrid> {
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val rotate90 = this.rotate90()
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val rotate180 = rotate90.rotate90()
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val rotate270 = rotate180.rotate90()
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val rotations = listOf(this, rotate90, rotate180, rotate270)
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return rotations.flatMap { listOf(it, it.flip()) }.distinct()
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}
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private fun CharGrid.rotate90(): CharGrid {
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val rotated = Array(width()) { CharArray(height()) }
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// transpose
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for (row in 0 until height()) {
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for (col in 0 until width()) {
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rotated[col][row] = this[row, col]
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}
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}
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// reverse each row
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for (chars in rotated) {
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chars.reverse()
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}
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return CharGrid(rotated)
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}
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private fun CharGrid.flip(): CharGrid {
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val flipped = Array(height()) { CharArray(width()) }
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// copy
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for (row in 0 until height()) {
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for (col in 0 until width()) {
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flipped[row][col] = this[row, col]
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}
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}
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// reverse each row
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for (chars in flipped) {
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chars.reverse()
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}
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return CharGrid(flipped)
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}
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override fun part2(input: Input): Any {
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return 2222222222222222
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}
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}

‎advent-of-code/src/main/kotlin/com/willmolloy/adventofcode/common/extensions/Collections.kt‎

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val rows = size
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val cols = if (isEmpty()) 0 else first().size
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val transpose = MutableList(cols) { MutableList<T?>(rows) { null } }
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val transposed = MutableList(cols) { MutableList<T?>(rows) { null } }
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for (row in 0 until rows) {
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for (col in 0 until cols) {
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transpose[col][row] = this[row][col]
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transposed[col][row] = this[row][col]
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}
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}
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@Suppress("UNCHECKED_CAST")
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return transpose as List<List<T>>
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return transposed as List<List<T>>
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}
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fun <T> Iterable<T>.pairs(): List<Pair<T, T>> {

‎advent-of-code/src/main/kotlin/com/willmolloy/adventofcode/common/grid/CharGrid.kt‎

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@Throws(ArrayIndexOutOfBoundsException::class)
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operator fun get(point: Point): Char = array[point.y.toInt()][point.x.toInt()]
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@Throws(ArrayIndexOutOfBoundsException::class)
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operator fun get(row: Int, col: Int): Char = array[row][col]
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override fun toString(): String = array.joinToString("\n") { String(it) }
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override fun equals(other: Any?): Boolean {
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if (this === other) return true
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if (javaClass != other?.javaClass) return false
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other as CharGrid
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return array.contentDeepEquals(other.array)
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}
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override fun hashCode(): Int {
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return array.contentDeepHashCode()
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}
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}

‎advent-of-code/src/main/kotlin/com/willmolloy/adventofcode/common/grid/Point.kt‎

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return dx * dx + dy * dy
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}
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fun move(p: Point): Point = Point(x + p.x, y + p.y)
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fun move(dx: Int, dy: Int): Point = Point(x + dx, y + dy)
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/**
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* Move the point.
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*

‎advent-of-code/src/main/resources/input/2025/example_input/day12.txt‎

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0:
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###
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##.
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##.
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1:
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###
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##.
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.##
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2:
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.##
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###
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##.
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3:
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##.
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###
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##.
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4:
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###
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#..
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###
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5:
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###
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.#.
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###
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4x4: 0 0 0 0 2 0
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12x5: 1 0 1 0 2 2
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12x5: 1 0 1 0 3 2

‎advent-of-code/src/test/kotlin/com/willmolloy/adventofcode/_2025/Day12Test.kt‎

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package com.willmolloy.adventofcode._2025
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import com.willmolloy.adventofcode.common.DayTest
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import org.junit.jupiter.api.Assumptions.abort
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object Day12Test : DayTest(Day12) {
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// today the example is harder - save build minutes
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override fun part1() = Answer({ abort() }, { 490 })
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override fun part2() = Answer(2222222222222222, 2222222222222222)
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}

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