[2024/18] RAM Run (Part 2)

pfolta · Dec 21, 2024 · a9873fe · a9873fe
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diff --git a/README.md b/README.md
@@ -28,7 +28,7 @@
 |     2021 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★    |   ★    |        |   ★    |   ★    |        |        |        |        |        |        |        |        |        |        |        |    26 |
 |     2022 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★    |   ★    |   ★    |   ★    |        |        |        |        |        |        |        |   ☆    |        |        |        |   ☆    |    28 |
 |     2023 |   ★   |   ★   |   ★   |   ★   |   ☆   |   ★   |   ☆   |       |       |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |    12 |
-|     2024 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★    |   ★    |   ★    |   ★    |   ★    |        |   ☆    |   ★    |   ☆    |   ★    |   ★    |        |        |        |        |        |    36 |
+|     2024 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★    |   ★    |   ★    |   ★    |   ★    |        |   ☆    |   ★    |   ★    |   ★    |   ★    |        |        |        |        |        |    37 |
 
 ## 🛷 How to run
 
@@ -180,7 +180,7 @@ e.g. `HandyHaversacks`)*
 |                                           |  14 | [Restroom Redoubt](https://adventofcode.com/2024/day/14)                                               | [[Code](src/main/kotlin/adventofcode/year2024/Day14RestroomRedoubt.kt)]&nbsp;[[Test](src/test/kotlin/adventofcode/year2024/Day14RestroomRedoubtSpec.kt)]                                     | `224357412`            | `7083`                                           |
 |                                           |  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`                                |
-|                                           |  18 | [RAM Run](https://adventofcode.com/2024/day/18)                                                        | [[Code](src/main/kotlin/adventofcode/year2024/Day18RamRun.kt)]&nbsp;[[Test](src/test/kotlin/adventofcode/year2024/Day18RamRunSpec.kt)]                                                       | `302`                  |                                                  |
+|                                           |  18 | [RAM Run](https://adventofcode.com/2024/day/18)                                                        | [[Code](src/main/kotlin/adventofcode/year2024/Day18RamRun.kt)]&nbsp;[[Test](src/test/kotlin/adventofcode/year2024/Day18RamRunSpec.kt)]                                                       | `302`                  | `24,32`                                          |
 |                                           |  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`                 | `1000697`                                        |
 

diff --git a/src/main/kotlin/adventofcode/year2024/Day18RamRun.kt b/src/main/kotlin/adventofcode/year2024/Day18RamRun.kt
@@ -6,44 +6,57 @@ import adventofcode.common.spatial.Point2d
 import adventofcode.common.spatial.Point2d.Companion.Origin
 
 class Day18RamRun(customInput: PuzzleInput? = null) : Puzzle(customInput) {
+    override val name = "RAM Run"
+
     private val corruptedBytes by lazy {
         input.lines().map { line ->
             val (x, y) = line.split(",").map(String::toLong)
             Point2d(x, y)
         }
     }
 
-    override val name = "RAM Run"
+    override fun partOne() = shortestPath(Origin, corruptedBytes.take(KILOBYTE_IN_BYTES))!!
 
-    override fun partOne() = shortestPath(Origin, Point2d(GRID_SIZE, GRID_SIZE), corruptedBytes.take(1024).toSet())
+    override fun partTwo(): String {
+        val (x, y) =
+            generateSequence(KILOBYTE_IN_BYTES + 1, Int::inc)
+                .first { bytes -> shortestPath(Origin, corruptedBytes.take(bytes)) == null }
+                .let { byte -> corruptedBytes[byte - 1] }
+
+        return "$x,$y"
+    }
 
     companion object {
         private const val GRID_SIZE = 70
+        private const val KILOBYTE_IN_BYTES = 1024
+        private val end = Point2d(GRID_SIZE, GRID_SIZE)
 
         private fun shortestPath(
             start: Point2d,
-            end: Point2d,
-            corruptedBytes: Set<Point2d>,
-        ): Int {
-            val visited = mutableSetOf<Pair<Point2d, Int>>()
+            corruptedBytes: List<Point2d>,
+        ): Int? {
+            val visited = mutableSetOf<Point2d>()
             val queue = ArrayDeque(setOf(start to 0))
 
             while (queue.isNotEmpty()) {
                 val (position, cost) = queue.removeFirst()
 
+                if (position == end) {
+                    return cost
+                }
+
                 val neighbors =
                     position
                         .neighbors()
                         .filter { (x, y) -> x in 0..end.x && y in 0..end.y }
-                        .filterNot { neighbor -> neighbor in visited.map { it.first } }
+                        .filterNot { neighbor -> neighbor in visited }
                         .filterNot { neighbor -> neighbor in corruptedBytes }
-                        .map { neighbor -> neighbor to cost + 1 }
 
                 visited.addAll(neighbors)
-                queue.addAll(neighbors)
+                queue.addAll(neighbors.map { neighbor -> neighbor to cost + 1 })
             }
 
-            return visited.first { (position) -> position == end }.second
+            return null
         }
     }
 }