Day 10 complete

src/main/kotlin/common/intpos2d/intpos2d.kt

@@ -16,3 +16,31 @@ operator fun Int.times(pos: IntPos2D): IntPos2D =
 
 operator fun IntPos2D.times(factor: Int) =
     factor * this
+
+enum class Direction(val delta: IntPos2D) {
+    NORTH(IntPos2D(-1, 0)),
+    EAST(IntPos2D(0, 1)),
+    SOUTH(IntPos2D(1, 0)),
+    WEST(IntPos2D(0, -1));
+
+    fun clockwise(): Direction = when (this) {
+        NORTH -> EAST
+        EAST -> SOUTH
+        SOUTH -> WEST
+        WEST -> NORTH
+    }
+
+    fun counterClockwise(): Direction = when (this) {
+        NORTH -> WEST
+        WEST -> SOUTH
+        SOUTH -> EAST
+        EAST -> NORTH
+    }
+
+    fun opposite(): Direction = when (this) {
+        NORTH -> SOUTH
+        EAST -> WEST
+        SOUTH -> NORTH
+        WEST -> EAST
+    }
+}

src/main/kotlin/day06/day06.kt

@@ -9,23 +9,6 @@ package day06
 import common.aocreader.fetchAdventOfCodeInput
 import common.intpos2d.*
 
-/**
- * The direction that the guard is facing and moves in.
- */
-private enum class Direction(val delta: IntPos2D) {
-    NORTH(IntPos2D(-1, 0)),
-    SOUTH(IntPos2D(1, 0)),
-    EAST(IntPos2D(0, 1)),
-    WEST(IntPos2D(0, -1));
-
-    fun clockwise(): Direction = when (this) {
-        NORTH -> EAST
-        EAST -> SOUTH
-        SOUTH -> WEST
-        WEST -> NORTH
-    }
-}
-
 private typealias Orientation = Pair<Direction, IntPos2D>
 
 private data class MapGrid(val rows: Int,

src/main/kotlin/day09/day09.kt

@@ -191,7 +191,7 @@ fun answer2(input: String): BigInteger =
 fun main() {
     val input = fetchAdventOfCodeInput(2024, 9)
 
-    println("--- Day 9: Resonant Collinearity ---")
+    println("--- Day 9: Disk Fragmenter ---")
 
     // Part 1: 6384282079460
     println("Part 1: ${answer1(input)}")

src/main/kotlin/day10/day10.kt

@@ -0,0 +1,68 @@
+// Advent of Code 2024, Day 09.
+// By Sebastian Raaphorst, 2024.
+
+package day10
+
+import common.aocreader.fetchAdventOfCodeInput
+import common.intpos2d.*
+
+private typealias Trail = List<IntPos2D>
+private typealias Trails = Set<List<IntPos2D>>
+
+private fun parse(input: String): List<List<Int>> =
+    input.trim().lines()
+        .map { line -> line.trim().toList().map { it.digitToIntOrNull() ?: -1 } }
+
+private fun findTrails(grid: List<List<Int>>): Map<IntPos2D, Trails> {
+    val height = grid.size
+    val width = grid[0].size
+
+    val zeros = grid.flatMapIndexed { rowIdx, row ->
+        row.mapIndexedNotNull { colIdx, height ->
+            if (height == 0) IntPos2D(rowIdx, colIdx) else null
+        }
+    }.toSet()
+
+    // For each 9, we want to find the trails that lead to the peak.
+    fun aux(trailSoFar: Trail): Trails {
+        val currentPos = trailSoFar.last()
+        val (currY, currX) = currentPos
+        val currHeight = grid[currY][currX]
+        if (currHeight == 9) return setOf(trailSoFar)
+
+        // Try all the valid neighbours.
+        val neighbours = Direction.entries
+            .map { currentPos + it.delta }
+            .filter { coords -> coords.first in 0 until height && coords.second in 0 until width }
+            .filter { coords -> grid[coords.first][coords.second] == currHeight + 1 }
+        if (neighbours.isEmpty()) return emptySet()
+
+        return neighbours.flatMap { pos -> aux(trailSoFar + pos) }.toSet()
+    }
+
+    return zeros.associate { zero -> zero to aux(listOf(zero)) }
+}
+
+private fun countTrailheads(trails: Map<IntPos2D, Trails>): Int =
+    trails.values.sumOf { it.map { it.last() }.toSet().size }
+
+private fun countTrails(trails: Map<IntPos2D, Trails>): Int =
+    trails.values.sumOf { it.count() }
+
+fun answer1(input: String): Int =
+    parse(input).let(::findTrails).let(::countTrailheads)
+
+fun answer2(input: String): Int =
+    parse(input).let(::findTrails).let(::countTrails)
+
+fun main() {
+    val input = fetchAdventOfCodeInput(2024, 10)
+
+    println("--- Day 10: Hoof It ---")
+
+    // Part 1: 719
+    println("Part 1: ${answer1(input)}")
+
+    // Part 2: 1530
+    println("Part 2: ${answer2(input)}")
+}

src/test/kotlin/day10/day10.kt

@@ -0,0 +1,31 @@
+// Advent of Code 2024, Day 10.
+// By Sebastian Raaphorst, 2024.
+
+package day10
+
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+class Day10Test {
+    private companion object {
+        val input =
+            """
+            89010123
+            78121874
+            87430965
+            96549874
+            45678903
+            32019012
+            01329801
+            10456732
+            """.trimIndent().trim()
+    }
+
+    @Test
+    fun `Problem 1 example`() =
+        assertEquals(36, answer1(input))
+
+    @Test
+    fun `Problem 2 example`() =
+        assertEquals(81, answer2(input))
+}