remove secondary mutable structure

Author: peckb1

src/main/kotlin/me/peckb/aoc/_2025/calendar/day07/Day07.kt

@@ -9,69 +9,63 @@ class Day07 @Inject constructor(
   private val generatorFactory: InputGeneratorFactory,
 ) {
   fun partOne(filename: String) = generatorFactory.forFile(filename).read { input ->
-    var beamIndices = mutableSetOf<Int>()
+    val beamIndices = mutableSetOf<Int>()
     val manifold = mutableListOf<MutableList<Char>>()
 
     input.forEachIndexed { index, line ->
-      if (index == 0) beamIndices.add(line.indexOf('S'))
+      if (index == 0) { beamIndices.add(line.indexOf('S')) }
       manifold.add(line.toCharArray().toMutableList())
     }
 
     var depth = 1
     var splits = 0
     while (depth < manifold.size) {
-      val updateBeamIndices = mutableSetOf<Int>()
       val line = manifold[depth]
 
-      beamIndices.forEach { beamIndex ->
+      beamIndices.toList().also { beamIndices.clear() }.forEach { beamIndex ->
         if (line[beamIndex] == '^') {
           splits++
-          updateBeamIndices.add(beamIndex - 1)
-          updateBeamIndices.add(beamIndex + 1)
+          beamIndices.add(beamIndex - 1)
+          beamIndices.add(beamIndex + 1)
         } else {
-          updateBeamIndices.add(beamIndex)
+          beamIndices.add(beamIndex)
         }
       }
 
-      beamIndices = updateBeamIndices
       depth++
     }
 
     splits
   }
 
   fun partTwo(filename: String) = generatorFactory.forFile(filename).read { input ->
-    var beams = mutableMapOf<Location, Long>()
+    val beams = mutableMapOf<Location, Long>().withDefault { 0 }
     val manifold = mutableListOf<MutableList<Char>>()
 
     input.forEachIndexed { index, line ->
-      if (index == 0) {
-        beams[Location(0, line.indexOf('S'))] = 1
-      }
+      if (index == 0) { beams[Location(0, line.indexOf('S'))] = 1 }
       manifold.add(line.toCharArray().toMutableList())
     }
 
     var depth = 1
     while (depth < manifold.size) {
-      val newBeams = mutableMapOf<Location, Long>().withDefault { 0 }
       val line = manifold[depth]
 
-      beams.forEach { (location, count) ->
+      beams.toList().also { beams.clear() }.forEach { (location, count) ->
         if (location.depth != depth - 1) { throw IllegalStateException("We should only be tracking the last row.") }
 
         if (line[location.index] == '^') {
           val left = Location(depth, location.index - 1)
           val right = Location(depth, location.index + 1)
 
-          newBeams[left] = newBeams.getValue(left) + count
-          newBeams[right] = newBeams.getValue(right) + count
+          beams[left] = beams.getValue(left) + count
+          beams[right] = beams.getValue(right) + count
         } else {
           val below = Location(depth, location.index)
-          newBeams[below] = newBeams.getValue(below) + count
+          beams[below] = beams.getValue(below) + count
         }
       }
 
-      beams = newBeams
       depth++
     }