Day 6 part 1

2024/src/main/kotlin/Grid.kt

@@ -61,4 +61,19 @@ class Grid(val rows: List<String>) {
 
     fun get(x: Int, y: Int): Char = rows[y][x]
 
+    fun find(char: Char): Coordinate? {
+        for (y in rows.indices) {
+            val index = rows[y].indexOf(char)
+            if (index != -1) {
+                return Coordinate(index, y)
+            }
+        }
+        return null
+    }
+
+    fun inside(coordinate: Coordinate) = coordinate.x in 0..<width && coordinate.y in 0..<height
+
+    operator fun get(coordinate: Coordinate) = get(coordinate.x, coordinate.y)
+
+    data class Coordinate(val x: Int, val y: Int)
 }

2024/src/main/kotlin/day6.kt

@@ -0,0 +1,44 @@
+enum class Direction {
+    North,
+    East,
+    South,
+    West;
+
+    fun turnClockwise() = when(this) {
+        North -> East
+        East -> South
+        South -> West
+        West -> North
+    }
+}
+
+fun Grid.Coordinate.step(direction: Direction) =
+    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)
+    }
+
+fun main() {
+    val grid = Grid.read("day6.txt")
+
+    var position = grid.find('^') ?: error("Guard not found")
+    var direction = Direction.North
+    val visitedPositions = mutableSetOf<Grid.Coordinate>()
+
+    while (true) {
+        visitedPositions.add(position)
+
+        val next = position.step(direction)
+        if (!grid.inside(next)) break
+
+        if (grid[next] == '#') {
+            direction = direction.turnClockwise()
+        } else {
+            position = next
+        }
+    }
+
+    println("Part 1: ${visitedPositions.count()}")
+}