Day 15 part 1

2024/src/main/kotlin/Grid.kt

@@ -16,6 +16,14 @@ operator fun <T> Grid<T>.set(coordinate: Grid.Coordinate, value: T) = set(coordi
 operator fun <T> Grid<T>.get(coordinate: Grid.Coordinate) = get(coordinate.x, coordinate.y)
 operator fun <T> Grid<T>.contains(coordinate: Grid.Coordinate) = inside(coordinate)
 
+fun <T> Grid<T>.coordinates() = sequence {
+    for (y in 0..<height) {
+        for (x in 0..<width) {
+            yield(Grid.Coordinate(x, y))
+        }
+    }
+}
+
 class ListGrid<T>(override val width: Int, override val height: Int,  content: List<T>) : Grid<T> {
     init {
         assert(width * height == content.count())

2024/src/main/kotlin/day15.kt

@@ -0,0 +1,55 @@
+fun main() {
+    val iterator = readInput("day15.txt").iterator()
+    val input = Sequence { iterator }
+
+    val grid = CharGrid(input.takeWhile { it.isNotEmpty() }.toList())
+    var robot = grid.find('@')!!
+
+    val program = input.flatMap { line ->
+        line.map {
+            when (it) {
+                '^' -> Direction.North
+                '>' -> Direction.East
+                'v' -> Direction.South
+                '<' -> Direction.West
+                else -> error("Invalid direction $it")
+            }
+        }
+    }
+
+    grid[robot] = '.'
+    for (direction in program) {
+        val next = robot.step(direction)
+        if (grid.push(next, direction)) {
+            robot = next
+        }
+    }
+
+    val part1 = grid.coordinates()
+        .filter { grid[it] == 'O' }
+        .fold(0) { acc, coord ->
+            acc + coord.x + 100 * coord.y
+        }
+
+    println("Part 1: $part1")
+}
+
+fun CharGrid.push(position: Grid.Coordinate, direction: Direction): Boolean {
+    if (this[position] == '.') {
+        return true
+    }
+
+    var next = position
+    while (this[next] == 'O') {
+        next = next.step(direction)
+    }
+
+    if (this[next] == '#') {
+        return false
+    }
+
+    this[next] = 'O'
+    this[position] = '.'
+
+    return true
+}