Solution 2025-04 (Printing Department)

Author: rhaendel

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

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+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 part2(input: List<String>): Int {
+        var paperGrid = PaperGrid(input)
+        var total = 0
+        var accessiblePaperRolls = paperGrid.countAndMarkAccessiblePaperRolls()
+        while (accessiblePaperRolls > 0) {
+            total += accessiblePaperRolls
+            paperGrid = paperGrid.copyReplacing(MARKED_PAPER, EMPTY)
+            accessiblePaperRolls = paperGrid.countAndMarkAccessiblePaperRolls()
+        }
+        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
+}

src/test/kotlin/de/ronny_h/aoc/year2025/day04/PrintingDepartmentTest.kt

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+package de.ronny_h.aoc.year2025.day04
+
+import de.ronny_h.aoc.extensions.asList
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.matchers.shouldBe
+
+class PrintingDepartmentTest : StringSpec({
+
+    val input = """
+        ..@@.@@@@.
+        @@@.@.@.@@
+        @@@@@.@.@@
+        @.@@@@..@.
+        @@.@@@@.@@
+        .@@@@@@@.@
+        .@.@.@.@@@
+        @.@@@.@@@@
+        .@@@@@@@@.
+        @.@.@@@.@.
+    """.asList()
+
+    "part 1: the number of rolls of paper that can be accessed by a forklift" {
+        PrintingDepartment().part1(input) shouldBe 13
+    }
+
+    "part 2: the number of rolls of paper in total that can be removed by forklifts" {
+        PrintingDepartment().part2(input) shouldBe 43
+    }
+})