WIP Day 15 Part 2

2024/src/main/kotlin/Direction.kt

@@ -12,10 +12,10 @@ enum class Direction {
     }
 }
 
-fun Grid.Coordinate.step(direction: Direction) =
+fun Grid.Coordinate.step(direction: Direction, steps: Int = 1) =
     when (direction) {
-        Direction.North -> copy(y = y - 1)
-        Direction.East -> copy(x = x + 1)
-        Direction.South -> copy(y = y + 1)
-        Direction.West -> copy(x = x - 1)
+        Direction.North -> copy(y = y - steps)
+        Direction.East -> copy(x = x + steps)
+        Direction.South -> copy(y = y + steps)
+        Direction.West -> copy(x = x - steps)
     }

2024/src/main/kotlin/day15.kt

@@ -1,9 +1,19 @@
 fun main() {
-    val iterator = readInput("day15.txt").iterator()
+    val iterator = readInput("day15-sample.txt").iterator()
     val input = Sequence { iterator }
 
     val grid = CharGrid(input.takeWhile { it.isNotEmpty() }.toList())
-    var robot = grid.find('@')!!
+
+    val grid2 = CharGrid(grid.rows.map {
+        it.map {
+            when (it) {
+                '@' -> "@."
+                'O' -> "[]"
+                else -> "$it$it"
+            }
+        }.joinToString(separator = "")
+    })
+
 
     val program = input.flatMap { line ->
         line.map {
@@ -15,8 +25,9 @@ fun main() {
                 else -> error("Invalid direction $it")
             }
         }
-    }
+    }.toList()
 
+    var robot = grid.find('@')!!
     grid[robot] = '.'
     for (direction in program) {
         val next = robot.step(direction)
@@ -32,6 +43,25 @@ fun main() {
         }
 
     println("Part 1: $part1")
+
+    robot = grid2.find('@')!!
+    grid2[robot] = '.'
+    for (direction in program) {
+        val next = robot.step(direction)
+        if (grid2.pushWide(next, direction)) {
+            robot = next
+        }
+    }
+
+    val part2 = grid2.coordinates()
+        .filter { grid2[it] == '[' }
+        .fold(0) { acc, coord ->
+            acc + coord.x + 100 * coord.y
+        }
+
+    println(grid2.rows.joinToString(separator = "\n"))
+
+    println("Part 2: $part2")
 }
 
 fun CharGrid.push(position: Grid.Coordinate, direction: Direction): Boolean {
@@ -53,3 +83,60 @@ fun CharGrid.push(position: Grid.Coordinate, direction: Direction): Boolean {
 
     return true
 }
+
+fun CharGrid.pushWide(position: Grid.Coordinate, direction: Direction): Boolean {
+    if (!canPushWide(position, direction)) return false
+
+    doPushWide(position, direction)
+
+    return true
+}
+
+fun CharGrid.doPushWide(position: Grid.Coordinate, direction: Direction, fill: Char = '.') {
+    if (this[position] != '[' && this[position] != ']') return
+
+    when (direction) {
+        Direction.East, Direction.West -> {
+            val next = position.step(direction)
+            doPushWide(next, direction, this[position])
+            this[position] = fill
+        }
+
+        Direction.South, Direction.North -> {
+            val boxStart = if (this[position] == '[') position else position.step(Direction.West)
+            val boxEnd = boxStart.step(Direction.East)
+
+            val nextStart = boxStart.step(direction)
+            val nextEnd = boxStart.step(direction)
+
+            doPushWide(nextStart, direction, this[boxStart])
+            doPushWide(nextEnd, direction, this[boxEnd])
+
+            this[boxStart] = fill
+            this[boxEnd] = fill
+        }
+    }
+}
+
+fun CharGrid.canPushWide(position: Grid.Coordinate, direction: Direction): Boolean {
+    val boxStart = when (this[position]) {
+        '.' -> return true
+        '[' -> position
+        ']' -> {
+            val start = position.copy(x = position.x - 1)
+            require(this[start] == '[')
+            start
+        }
+
+        else -> return false
+    }
+
+    val boxEnd = boxStart.step(Direction.East)
+
+    return when (direction) {
+        Direction.East -> canPushWide(boxEnd.step(Direction.East), direction)
+        Direction.West -> canPushWide(boxStart.step(Direction.West), direction)
+        Direction.North, Direction.South -> canPushWide(boxStart.step(direction), direction) &&
+                canPushWide(boxEnd.step(direction), direction)
+    }
+}