Day 4.

src/Utils.kt

@@ -152,6 +152,9 @@ class CharGrid {
     fun applyWithPos(op: (grid: CharGrid, pos: RelPos) -> Char?) =
         generateGridPos().forEach { this[it] = op(this, it) }
 
+    fun applyWithPos(relposes: Iterable<RelPos>, op: (grid: CharGrid, pos: RelPos) -> Char?) =
+        relposes.forEach { this[it] = op(this, it) }
+
     fun apply(op: (content: Char) -> Char?) =
         applyWithPos { grid: CharGrid, pos -> op(grid[pos]) }
 
@@ -201,7 +204,7 @@ class CharGrid {
     fun matchRelative(c: Int, r: Int, relposes: Iterable<RelPos>, predicate: (char: Char) -> Boolean): List<RelPos> =
         relposes.filter { predicate(get(c + it.dc, r + it.dr)) }
 
-    fun matchRelativeRelPos(pos: RelPos, relposes: Iterable<RelPos>, predicate: (char: Char) -> Boolean): List<RelPos> =
+    fun matchRelative(pos: RelPos, relposes: Iterable<RelPos>, predicate: (char: Char) -> Boolean): List<RelPos> =
         relposes.filter { predicate(get(pos.translate(it))) }
 
     fun matchAbsoluteRelPos(pos: RelPos, relposes: Iterable<RelPos>, predicate: (char: Char) -> Boolean): List<RelPos> =

src/aoc2025/Day04.kt

@@ -0,0 +1,56 @@
+package aoc2025
+
+import CharGrid
+import println
+import readInput
+
+/*
+--- Day 4: Printing Department ---
+https://adventofcode.com/2025/day/4
+*/
+fun main() {
+
+    val inlineTestInput = """
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.
+"""
+
+    fun part1(input: List<String>): Int {
+        val grid = CharGrid(input)
+        val matches = grid.findMatchesRelPos { it == '@' }.filter { grid.matchRelative(it, CharGrid.BOX_POS) { it == '@' }.size < 4 }
+        return matches.count()
+    }
+
+    fun part2(input: List<String>): Int {
+        val grid = CharGrid(input)
+        var removed = 0
+        do {
+            val matches = grid.findMatchesRelPos { it == '@' }.filter { grid.matchRelative(it, CharGrid.BOX_POS) { it == '@' }.size < 4 }
+            grid.applyWithPos(matches) { _, _ -> 'x' }
+            removed += matches.size
+        } while (matches.isNotEmpty())
+        return removed
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    //val testInput = readInput("aoc2025/Day04_test")
+    val testInputPart1Result = part1(testInput)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 13)
+    check(testInputPart2Result == 43)
+
+    val input = readInput("aoc2025/Day04")
+    part1(input).println()
+    part2(input).println()
+}