Day 04 2025 (#298)

* Day 04 2025 ... messy

* split out input parsing

* more cleanup

Author: peckb1

src/main/kotlin/me/peckb/aoc/_2025/calendar/day04/Day04.kt

@@ -0,0 +1,74 @@
+package me.peckb.aoc._2025.calendar.day04
+
+import javax.inject.Inject
+import me.peckb.aoc.generators.InputGenerator.InputGeneratorFactory
+import kotlin.text.forEach
+
+class Day04 @Inject constructor(
+  private val generatorFactory: InputGeneratorFactory,
+) {
+  fun partOne(filename: String) = generatorFactory.forFile(filename).read { input ->
+    val printingDepartment = mapPaper(input)
+
+    var removable = 0
+
+    printingDepartment.indices.forEach { r ->
+      printingDepartment[r].indices.forEach { c ->
+        if (printingDepartment[r][c] == '@' && printingDepartment.paperNeighbors(r, c) < 4) { removable++ }
+      }
+    }
+
+    removable
+  }
+
+  fun partTwo(filename: String) = generatorFactory.forFile(filename).read { input ->
+    val printingDepartment = mapPaper(input)
+
+    var totalRemovedCount = 0
+
+    do {
+      val toRemove = mutableListOf<Pair<Int, Int>>()
+
+      printingDepartment.indices.forEach { r ->
+        printingDepartment[r].indices.forEach { c ->
+          if (printingDepartment[r][c] == '@' && printingDepartment.paperNeighbors(r, c) < 4) { toRemove.add(r to c) }
+        }
+      }
+
+      toRemove.forEach { (r, c) -> printingDepartment[r][c] = '.' }
+      totalRemovedCount += toRemove.size
+    } while (toRemove.isNotEmpty())
+
+    totalRemovedCount
+  }
+
+  private fun mapPaper(input: Sequence<String>): MutableList<MutableList<Char>> {
+    val workshop = mutableListOf<MutableList<Char>>()
+
+    input.forEach { r ->
+      val row = mutableListOf<Char>()
+      r.forEach { c -> row.add(c) }
+      workshop.add(row)
+    }
+
+    return workshop
+  }
+
+  private fun MutableList<MutableList<Char>>.paperNeighbors(r: Int, c: Int): Int {
+    return (-1 .. 1).sumOf { rDelta ->
+      // ensure row bounds
+      val rIndex = rDelta + r
+      if (rIndex !in 0 until size) return@sumOf 0
+
+      (-1 .. 1).count { cDelta ->
+        // don't count ourselves
+        if (rDelta == 0 && cDelta == 0) return@count false
+        // ensure column bounds
+        val cIndex = cDelta + c
+        if (cIndex !in 0 until this[rIndex].size) return@count false
+
+        this[rIndex][cIndex] == '@'
+      }
+    }
+  }
+}

src/main/kotlin/me/peckb/aoc/_2025/calendar/day04/README.md

@@ -0,0 +1 @@
+## [Day 4: Printing Department](https://adventofcode.com/2025/day/4)

src/test/kotlin/me/peckb/aoc/_2025/TestDayComponent.kt

@@ -3,6 +3,7 @@ package me.peckb.aoc._2025
 import dagger.Component
 import me.peckb.aoc._2025.calendar.day02.Day02Test
 import me.peckb.aoc._2025.calendar.day03.Day03Test
+import me.peckb.aoc._2025.calendar.day04.Day04Test
 import javax.inject.Singleton
 import me.peckb.aoc.DayComponent
 import me.peckb.aoc.InputModule
@@ -14,4 +15,5 @@ internal interface TestDayComponent : DayComponent {
   fun inject(day01Test: Day01Test)
   fun inject(day02Test: Day02Test)
   fun inject(day03Test: Day03Test)
+  fun inject(day04Test: Day04Test)
 }

src/test/kotlin/me/peckb/aoc/_2025/calendar/day04/Day04Test.kt

@@ -0,0 +1,32 @@
+package me.peckb.aoc._2025.calendar.day04
+
+import javax.inject.Inject
+
+import me.peckb.aoc._2025.DaggerTestDayComponent
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+
+internal class Day04Test {
+  @Inject
+  lateinit var day04: Day04
+
+  @BeforeEach
+  fun setup() {
+    DaggerTestDayComponent.create().inject(this)
+  }
+
+  @Test
+  fun testDay04PartOne() {
+    assertEquals(1508, day04.partOne(DAY_04))
+  }
+
+  @Test
+  fun testDay04PartTwo() {
+    assertEquals(8538, day04.partTwo(DAY_04))
+  }
+
+  companion object {
+    private const val DAY_04: String = "advent-of-code-input/2025/day04.input"
+  }
+}