Day 20 2024 (#283)

Author: peckb1

src/main/kotlin/me/peckb/aoc/_2024/calendar/day16/Day16.kt

@@ -158,7 +158,10 @@ data class Room(
 enum class Space { FULL, EMPTY }
 
 enum class Direction(val yDelta: Int, val xDelta: Int) {
-  N(-1, 0), E(0, 1), S(1, 0), W(0, -1)
+  N(-1, 0),
+  E(0, 1),
+  S(1, 0),
+  W(0, -1)
 }
 
 private fun Direction?.turnCost(direction: Direction): Long {

src/main/kotlin/me/peckb/aoc/_2024/calendar/day20/Day20.kt

@@ -0,0 +1,108 @@
+package me.peckb.aoc._2024.calendar.day20
+
+import javax.inject.Inject
+import me.peckb.aoc.generators.InputGenerator.InputGeneratorFactory
+import me.peckb.aoc.pathing.GenericIntDijkstra
+import kotlin.math.abs
+
+class Day20 @Inject constructor(
+  private val generatorFactory: InputGeneratorFactory,
+) {
+  fun partOne(filename: String) = generatorFactory.forFile(filename).read { input ->
+    findCheats(input, 2)
+  }
+
+  fun partTwo(filename: String) = generatorFactory.forFile(filename).read { input ->
+    findCheats(input, 20)
+  }
+
+  private fun findCheats(input: Sequence<String>, maxCheatTime: Int): Int {
+    lateinit var end: Location
+
+    val maze = mutableListOf<MutableList<Space>>()
+    input.forEachIndexed { y, line ->
+      val row = mutableListOf<Space>()
+      line.forEachIndexed { x, c ->
+        when (c) {
+          '#' -> row.add(Space.FULL)
+          '.' -> row.add(Space.EMPTY)
+          'S' -> row.add(Space.EMPTY)
+          'E' -> row.add(Space.EMPTY.also { end = Location(y, x) } )
+        }
+      }
+      maze.add(row)
+    }
+
+    val solver = object : GenericIntDijkstra<Location>() {}
+
+    val costs = solver.solve(end.withArea(maze))
+    var cheats = 0
+
+    val explorationRange = (-maxCheatTime .. maxCheatTime)
+    val explorationDeltas = explorationRange.flatMapIndexed { y, yStep ->
+      explorationRange.mapIndexedNotNull { x, xStep ->
+        val stepCount = abs(yStep) + abs(xStep)
+        if (stepCount <= maxCheatTime) {
+          yStep to xStep
+        } else {
+          null
+        }
+      }
+    }
+
+    costs.entries.forEach { (cheatStart, _) ->
+      val reachableEmptySpaces = mutableSetOf<Location>()
+
+      val curY = cheatStart.y
+      val curX = cheatStart.x
+
+      explorationDeltas.forEach { (yStep, xStep) ->
+        val y = curY + yStep
+        val x = curX + xStep
+
+        if (y in maze.indices && x in maze[y].indices && maze[y][x] == Space.EMPTY) {
+          reachableEmptySpaces.add(Location(y, x))
+        }
+      }
+
+      reachableEmptySpaces.forEach { endSpace ->
+        val costAtCheatStart = costs[cheatStart]!!
+        val costAtCheatEnd = costs[endSpace]!!
+        val distanceTravelled = cheatStart.distanceFrom(endSpace)
+
+        val timeSaved = costAtCheatStart - costAtCheatEnd - distanceTravelled
+        if (timeSaved >= 100) { cheats++ }
+      }
+    }
+
+    return cheats
+  }
+}
+
+data class Location(val y: Int, val x: Int) : GenericIntDijkstra.DijkstraNode<Location> {
+  fun distanceFrom(other: Location): Int {
+    return abs(other.y - y) + abs(other.x - x)
+  }
+
+  lateinit var area: MutableList<MutableList<Space>>
+
+  fun withArea(area: MutableList<MutableList<Space>>)   = apply { this.area = area }
+
+  override fun neighbors(): Map<Location, Int> {
+    return Direction.entries.mapNotNull { d ->
+      val (newY, newX) = d.yDelta + y to d.xDelta + x
+      if (newY in area.indices && newX in area[newY].indices && area[newY][newX] == Space.EMPTY) {
+        Location(newY, newX).withArea(area)
+      } else { null }
+    }.associateWith { 1 }
+  }
+}
+
+enum class Space { FULL, EMPTY }
+
+enum class Direction(val yDelta: Int, val xDelta: Int) {
+  N(-1, 0),
+  E(0, 1),
+  S(1, 0),
+  W(0, -1);
+}

src/main/kotlin/me/peckb/aoc/_2024/calendar/day20/README.md

@@ -0,0 +1 @@
+## [Day 20: Race Condition](https://adventofcode.com/2024/day/20)

src/test/kotlin/me/peckb/aoc/_2024/TestDayComponent.kt

@@ -19,6 +19,7 @@ import me.peckb.aoc._2024.calendar.day16.Day16Test
 import me.peckb.aoc._2024.calendar.day17.Day17Test
 import me.peckb.aoc._2024.calendar.day18.Day18Test
 import me.peckb.aoc._2024.calendar.day19.Day19Test
+import me.peckb.aoc._2024.calendar.day20.Day20Test
 import javax.inject.Singleton
 import me.peckb.aoc.DayComponent
 import me.peckb.aoc.InputModule
@@ -46,4 +47,5 @@ internal interface TestDayComponent : DayComponent {
   fun inject(day17Test: Day17Test)
   fun inject(day18Test: Day18Test)
   fun inject(day19Test: Day19Test)
+  fun inject(day20Test: Day20Test)
 }

src/test/kotlin/me/peckb/aoc/_2024/calendar/day20/Day20Test.kt

@@ -0,0 +1,32 @@
+package me.peckb.aoc._2024.calendar.day20
+
+import javax.inject.Inject
+
+import me.peckb.aoc._2024.DaggerTestDayComponent
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+
+internal class Day20Test {
+  @Inject
+  lateinit var day20: Day20
+
+  @BeforeEach
+  fun setup() {
+    DaggerTestDayComponent.create().inject(this)
+  }
+
+  @Test
+  fun testDay20PartOne() {
+    assertEquals(1317, day20.partOne(DAY_20))
+  }
+
+  @Test
+  fun testDay20PartTwo() {
+    assertEquals(982474, day20.partTwo(DAY_20))
+  }
+
+  companion object {
+    private const val DAY_20: String = "advent-of-code-input/2024/day20.input"
+  }
+}