[2025/7] Laboratories (Part 2)

Author: pfolta

README.md

@@ -29,7 +29,7 @@
 |     2022 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★    |   ★    |   ★    |   ★    |        |        |        |        |        |        |        |   ☆    |        |        |        |   ☆    |    28 |
 |     2023 |   ★   |   ★   |   ★   |   ★   |   ☆   |   ★   |   ☆   |       |       |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |    12 |
 |     2024 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★    |   ★    |   ★    |   ★    |   ★    |        |   ☆    |   ★    |   ★    |   ★    |   ★    |        |   ★    |        |        |   ☆    |    40 |
-|     2025 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ☆   |       |       |        |        |        |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |    13 |
+|     2025 |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |   ★   |       |       |        |        |        |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |   –    |    14 |
 
 ## 🛷 How to run
 
@@ -212,7 +212,7 @@ e.g. `HandyHaversacks`)*
 |                                           |   4 | [Printing Department](https://adventofcode.com/2025/day/4)                                             | [[Code](src/main/kotlin/adventofcode/year2025/Day04PrintingDepartment.kt)] [[Test](src/test/kotlin/adventofcode/year2025/Day04PrintingDepartmentSpec.kt)]                               | `1411`                 | `8557`                                           |
 |                                           |   5 | [Cafeteria](https://adventofcode.com/2025/day/5)                                                       | [[Code](src/main/kotlin/adventofcode/year2025/Day05Cafeteria.kt)] [[Test](src/test/kotlin/adventofcode/year2025/Day05CafeteriaSpec.kt)]                                                 | `601`                  | `367899984917516`                                |
 |                                           |   6 | [Trash Compactor](https://adventofcode.com/2025/day/6)                                                 | [[Code](src/main/kotlin/adventofcode/year2025/Day06TrashCompactor.kt)] [[Test](src/test/kotlin/adventofcode/year2025/Day06TrashCompactorSpec.kt)]                                       | `5733696195703`        | `10951882745757`                                 |
-|                                           |   7 | [Laboratories](https://adventofcode.com/2025/day/7)                                                    | [[Code](src/main/kotlin/adventofcode/year2025/Day07Laboratories.kt)] [[Test](src/test/kotlin/adventofcode/year2025/Day07LaboratoriesSpec.kt)]                                           | `1711`                 |                                                  |
+|                                           |   7 | [Laboratories](https://adventofcode.com/2025/day/7)                                                    | [[Code](src/main/kotlin/adventofcode/year2025/Day07Laboratories.kt)] [[Test](src/test/kotlin/adventofcode/year2025/Day07LaboratoriesSpec.kt)]                                           | `1711`                 | `36706966158365`                                 |
 
 ## 🕯️ Useful commands
 

src/main/kotlin/adventofcode/year2025/Day07Laboratories.kt

@@ -7,31 +7,56 @@ import adventofcode.common.spatial.Grid2d
 class Day07Laboratories(
     customInput: PuzzleInput? = null,
 ) : Puzzle(customInput) {
-    private fun parseInput() = Grid2d(input)
-
-    override fun partOne(): Int {
-        val grid = parseInput()
-        val start = grid[START].x.toInt()
+    private fun parseInput(): Pair<Long, List<Set<Long>>> {
+        val grid = Grid2d(input)
+        val start = grid[START].x
         val splittersByRow =
             grid
                 .rows()
                 .map { row ->
                     row
                         .mapIndexedNotNull { x, char ->
                             when (char) {
-                                SPLITTER -> x
+                                SPLITTER -> x.toLong()
                                 else -> null
                             }
                         }.toSet()
                 }
 
+        return Pair(start, splittersByRow)
+    }
+
+    override fun partOne(): Int {
+        val (start, splittersByRow) = parseInput()
+
         return splittersByRow
             .fold(Pair(setOf(start), 0)) { (beam, splitCount), splittersInRow ->
                 val reachedSplitters = beam intersect splittersInRow
                 Pair(beam - reachedSplitters + reachedSplitters.flatMap { x -> setOf(x - 1, x + 1) }, splitCount + reachedSplitters.size)
             }.second
     }
 
+    override fun partTwo(): Long {
+        val (start, splittersByRow) = parseInput()
+        val beam = mutableMapOf(start to 1L)
+
+        for (splittersInRow in splittersByRow) {
+            val reachedSplitters = splittersInRow intersect beam.keys
+
+            for (reachedSplitter in reachedSplitters) {
+                val pathCount = beam[reachedSplitter] ?: 0
+
+                if (pathCount > 0) {
+                    beam[reachedSplitter - 1] = (beam[reachedSplitter - 1] ?: 0) + pathCount
+                    beam[reachedSplitter + 1] = (beam[reachedSplitter + 1] ?: 0) + pathCount
+                    beam[reachedSplitter] = 0
+                }
+            }
+        }
+
+        return beam.values.sum()
+    }
+
     companion object {
         private const val START = 'S'
         private const val SPLITTER = '^'

src/test/kotlin/adventofcode/year2025/Day07LaboratoriesSpec.kt

@@ -2,4 +2,4 @@ package adventofcode.year2025
 
 import adventofcode.PuzzleBaseSpec
 
-class Day07LaboratoriesSpec : PuzzleBaseSpec(21)
+class Day07LaboratoriesSpec : PuzzleBaseSpec(21, 40)