[2024/20] Race Condition (Part 1)

pfolta · Dec 20, 2024 · 37047c2 · 37047c2
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diff --git a/README.md b/README.md
@@ -28,7 +28,7 @@
 |     2021 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★    |   ★    |        |   ★    |   ★    |        |        |        |        |        |        |        |        |        |        |        |    26 |
 |     2022 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★    |   ★    |   ★    |   ★    |        |        |        |        |        |        |        |   ☆    |        |        |        |   ☆    |    28 |
 |     2023 |   ★   |   ★   |   ★   |   ★   |   ☆   |   ★   |   ☆   |       |       |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |    12 |
-|     2024 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★    |   ★    |   ★    |   ★    |   ★    |        |   ☆    |   ★    |        |   ★    |        |        |        |        |        |        |    33 |
+|     2024 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★    |   ★    |   ★    |   ★    |   ★    |        |   ☆    |   ★    |        |   ★    |   ☆    |        |        |        |        |        |    34 |
 
 ## 🛷 How to run
 
@@ -181,6 +181,7 @@ e.g. `HandyHaversacks`)*
 |                                           |  16 | [Reindeer Maze](https://adventofcode.com/2024/day/16)                                                  | [[Code](src/main/kotlin/adventofcode/year2024/Day16ReindeerMaze.kt)]&nbsp;[[Test](src/test/kotlin/adventofcode/year2024/Day16ReindeerMazeSpec.kt)]                                           | `98484`                |                                                  |
 |                                           |  17 | [Chronospatial Computer](https://adventofcode.com/2024/day/17)                                         | [[Code](src/main/kotlin/adventofcode/year2024/Day17ChronospatialComputer.kt)]&nbsp;[[Test](src/test/kotlin/adventofcode/year2024/Day17ChronospatialComputerSpec.kt)]                         | `5,1,3,4,3,7,2,1,7`    | `216584205979245`                                |
 |                                           |  19 | [Linen Layout](https://adventofcode.com/2024/day/19)                                                   | [[Code](src/main/kotlin/adventofcode/year2024/Day19LinenLayout.kt)]&nbsp;[[Test](src/test/kotlin/adventofcode/year2024/Day19LinenLayoutSpec.kt)]                                             | `267`                  | `796449099271652`                                |
+|                                           |  20 | [Race Condition](https://adventofcode.com/2024/day/20)                                                 | [[Code](src/main/kotlin/adventofcode/year2024/Day20RaceCondition.kt)]&nbsp;[[Test](src/test/kotlin/adventofcode/year2024/Day20RaceConditionSpec.kt)]                                         | `1426`                 |                                                  |
 
 ## 🕯️ Useful commands
 

diff --git a/src/main/kotlin/adventofcode/common/spatial/Point2d.kt b/src/main/kotlin/adventofcode/common/spatial/Point2d.kt
@@ -1,5 +1,7 @@
 package adventofcode.common.spatial
 
+import kotlin.math.absoluteValue
+
 data class Point2d(
     val x: Long,
     val y: Long,
@@ -44,6 +46,8 @@ data class Point2d(
         }
             .map { direction -> this + direction }
             .toSet()
+
+    infix fun distanceTo(other: Point2d): Long = (x - other.x).absoluteValue + (y - other.y).absoluteValue
 }
 
 val NORTH = Point2d(0, -1)

diff --git a/src/main/kotlin/adventofcode/year2024/Day20RaceCondition.kt b/src/main/kotlin/adventofcode/year2024/Day20RaceCondition.kt
@@ -0,0 +1,41 @@
+package adventofcode.year2024
+
+import adventofcode.Puzzle
+import adventofcode.PuzzleInput
+import adventofcode.common.spatial.Grid2d
+import adventofcode.common.spatial.Point2d
+
+class Day20RaceCondition(customInput: PuzzleInput? = null) : Puzzle(customInput) {
+    private val racetrack by lazy { input.racketrack() }
+
+    override fun partOne() = racetrack.findCheats(2)
+
+    companion object {
+        private const val MINIMUM_SAVINGS = 100
+
+        private fun String.racketrack(): List<Point2d> {
+            val grid = Grid2d(this)
+            val path = mutableListOf(grid['S'])
+
+            while (path.last() != grid['E']) {
+                val next =
+                    grid.neighborsOf(path.last())
+                        .filterNot { neighbor -> neighbor == path.getOrNull(path.lastIndex - 1) }
+                        .filterNot { neighbor -> grid[neighbor] == '#' }
+                        .first()
+
+                path.add(next)
+            }
+
+            return path.toList()
+        }
+
+        private fun List<Point2d>.findCheats(time: Int) =
+            indices.sumOf { start ->
+                (start + MINIMUM_SAVINGS..lastIndex).count { end ->
+                    val distance = this[start] distanceTo this[end]
+                    (distance <= time) && (distance <= end - start - MINIMUM_SAVINGS)
+                }
+            }
+    }
+}