Solution 2025-12 (Christmas Tree Farm)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2025/day12/ChristmasTreeFarm.kt

@@ -0,0 +1,74 @@
+package de.ronny_h.aoc.year2025.day12
+
+import de.ronny_h.aoc.AdventOfCode
+import de.ronny_h.aoc.extensions.collections.split
+import de.ronny_h.aoc.extensions.grids.SimpleCharGrid
+
+fun main() = ChristmasTreeFarm().run(454, 0)
+
+class ChristmasTreeFarm : AdventOfCode<Int>(2025, 12) {
+    override fun part1(input: List<String>): Int {
+        val presents = input.parsePresents()
+        return presents.regions.count { PresentsSpace(it, presents.shapes).allShapesFit() }
+    }
+
+    override fun part2(input: List<String>): Int {
+        return 0
+    }
+}
+
+fun List<String>.parsePresents(): Presents {
+    val chunks = split()
+    val shapes = chunks.dropLast(1).map { it.parseShape() }
+    val regions = chunks.last().map { it.parseRegion() }
+    return Presents(shapes, regions)
+}
+
+private fun List<String>.parseShape() = PresentShape(
+    index = first().dropLast(1).toInt(),
+    input = drop(1),
+)
+
+private fun String.parseRegion(): Region {
+    val (dimensions, presents) = split(": ")
+    val (width, length) = dimensions.split("x").map(String::toInt)
+    return Region(width, length, presents.split(" ").map(String::toInt))
+}
+
+data class Presents(val shapes: List<PresentShape>, val regions: List<Region>)
+
+data class Region(val width: Int, val length: Int, val presents: List<Int>) {
+    val area: Int
+        get() = width * length
+}
+
+class PresentShape(private val index: Int, input: List<String>) : SimpleCharGrid(input, nullElement = BACKGROUND) {
+
+    companion object {
+        const val BACKGROUND = '.'
+    }
+
+    val numberOfTiles
+        get() = forEachElement { _, _, element ->
+            if (element != BACKGROUND) 1 else 0
+        }.sum()
+
+    override fun equals(other: Any?): Boolean {
+        if (other !is PresentShape) return false
+        return index == other.index && toString() == other.toString()
+    }
+
+    override fun hashCode(): Int = index
+}
+
+class PresentsSpace(private val region: Region, private val shapes: List<PresentShape>) {
+
+    fun allShapesFit(): Boolean {
+        val presentsToAdd = region.presents.mapIndexed { i, times ->
+            if (times == 0) null else shapes[i] to times
+        }.filterNotNull()
+
+        return presentsToAdd.sumOf { (shape, times) -> times * shape.numberOfTiles } <= region.area
+    }
+
+}

src/test/kotlin/de/ronny_h/aoc/year2025/day12/ChristmasTreeFarmTest.kt

@@ -0,0 +1,93 @@
+package de.ronny_h.aoc.year2025.day12
+
+import de.ronny_h.aoc.extensions.asList
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.data.forAll
+import io.kotest.data.row
+import io.kotest.matchers.shouldBe
+
+class ChristmasTreeFarmTest : StringSpec({
+
+    val input = """
+        0:
+        ###
+        ##.
+        ##.
+
+        1:
+        ###
+        ##.
+        .##
+
+        2:
+        .##
+        ###
+        ##.
+
+        3:
+        ##.
+        ###
+        ##.
+
+        4:
+        ###
+        #..
+        ###
+
+        5:
+        ###
+        .#.
+        ###
+
+        4x4: 0 0 0 0 2 0
+        12x5: 1 0 1 0 2 2
+        12x5: 1 2 1 5 3 2
+    """.asList()
+
+    "parse presents" {
+        val input = """
+            0:
+            ###
+            ##.
+            ##.
+    
+            1:
+            ###
+            ##.
+            .##
+    
+            4x4: 0 0
+            12x5: 1 0
+        """.asList()
+
+        input.parsePresents() shouldBe Presents(
+            shapes = listOf(
+                PresentShape(0, listOf("###", "##.", "##.")),
+                PresentShape(1, listOf("###", "##.", ".##")),
+            ),
+            regions = listOf(
+                Region(4, 4, listOf(0, 0)),
+                Region(12, 5, listOf(1, 0)),
+            )
+        )
+    }
+
+    "PresentShape's number of tiles" {
+        forAll(
+            row(listOf("...", "...", "..."), 0),
+            row(listOf("#.#", ".##", ".#."), 5),
+            row(listOf("###", "##.", "##."), 7),
+            row(listOf("###", "###", "###"), 9),
+        ) { input, expected ->
+            PresentShape(0, input).numberOfTiles shouldBe expected
+        }
+    }
+
+    "part 1: the number of regions that can fit all of the presents listed" {
+        ChristmasTreeFarm().part1(input) shouldBe 2
+    }
+
+    "part 2 was to solve all of the year's previous puzzles" {
+        ChristmasTreeFarm().part2(listOf()) shouldBe 0
+    }
+})