Improve solution 2025-04 (Printing Department)

* Avoid marking paper rolls makes the code cleaner but loses the
  ability to print the grid's state with paper rolls to be removed.
* We don't need a new grid for each iteration.

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2025/day04/PrintingDepartment.kt

@@ -1,54 +1,39 @@
 package de.ronny_h.aoc.year2025.day04
 
 import de.ronny_h.aoc.AdventOfCode
-import de.ronny_h.aoc.extensions.asList
 import de.ronny_h.aoc.extensions.grids.SimpleCharGrid
 import de.ronny_h.aoc.year2025.day04.PaperGrid.Companion.EMPTY
-import de.ronny_h.aoc.year2025.day04.PaperGrid.Companion.MARKED_PAPER
 
 fun main() = PrintingDepartment().run(1397, 8758)
 
 class PrintingDepartment : AdventOfCode<Int>(2025, 4) {
-    override fun part1(input: List<String>): Int = PaperGrid(input).countAndMarkAccessiblePaperRolls()
+    override fun part1(input: List<String>): Int = PaperGrid(input).accessiblePaperRollsCoordinates().count()
 
     override fun part2(input: List<String>): Int {
-        var paperGrid = PaperGrid(input)
+        val grid = PaperGrid(input)
         var total = 0
-        var accessiblePaperRolls = paperGrid.countAndMarkAccessiblePaperRolls()
-        while (accessiblePaperRolls > 0) {
-            total += accessiblePaperRolls
-            paperGrid = paperGrid.copyReplacing(MARKED_PAPER, EMPTY)
-            accessiblePaperRolls = paperGrid.countAndMarkAccessiblePaperRolls()
+        while (true) {
+            grid.accessiblePaperRollsCoordinates().toList().apply {
+                if (isEmpty()) return total
+                forEach { grid[it] = EMPTY }
+                total += size
+            }
         }
-        return total
     }
-
 }
 
 class PaperGrid(input: List<String>) : SimpleCharGrid(input, EMPTY) {
 
     companion object {
         private const val PAPER = '@'
-        const val MARKED_PAPER = 'x'
         const val EMPTY = '.'
     }
 
-    fun countAndMarkAccessiblePaperRolls() =
-        forEachCoordinates { position, element ->
-            if (element.isAPaperRoll() && position.neighboursIncludingDiagonals()
-                    .count { this[it].isAPaperRoll() } < 4
-            ) {
-                this[position] = MARKED_PAPER
-                1
-            } else {
-                0
-            }
-        }.sum()
-
-    fun copyReplacing(oldElement: Char, newElement: Char): PaperGrid {
-        val newGridString = toString().replace(oldElement, newElement)
-        return PaperGrid(newGridString.asList())
-    }
-
-    private fun Char.isAPaperRoll(): Boolean = this == PAPER || this == MARKED_PAPER
+    fun accessiblePaperRollsCoordinates() = forEachCoordinates { position, element ->
+        if (element == PAPER && position.neighboursIncludingDiagonals().count { this[it] == PAPER } < 4) {
+            position
+        } else {
+            null
+        }
+    }.filterNotNull()
 }