Day 16 part 1

2024/src/main/kotlin/day16.kt

@@ -1,3 +1,64 @@
-fun main() {
+import java.util.PriorityQueue
 
+data class Position(val coordinate: Grid.Coordinate, val direction: Direction)
+
+fun main() {
+    val grid = CharGrid.read("day16.txt")
+    val start = grid.find('S')!!
+    val goal = grid.find('E')!!
+
+    val startPosition = Position(start, Direction.East)
+
+    val part1 = dijkstra(startPosition, goal = { it.coordinate == goal }) { current ->
+        Direction.entries
+            .asSequence()
+            .map { Position(current.coordinate.step(it), it) }
+            .filter { grid[it.coordinate] != '#' }
+            .map { it to 1 + current.direction.turns(it.direction) * 1000 }
+    }
+    println("Part 1: $part1")
+}
+
+fun Direction.turns(to: Direction): Int = when {
+    this == to -> 0
+    this == Direction.North && (to == Direction.East || to == Direction.West) -> 1
+    this == Direction.North && to == Direction.South -> 2
+    this == Direction.East && (to == Direction.North || to == Direction.South) -> 1
+    this == Direction.East && to == Direction.West -> 2
+    this == Direction.South && (to == Direction.West || to == Direction.East) -> 1
+    this == Direction.South && to == Direction.North -> 2
+    this == Direction.West && (to == Direction.North || to == Direction.South) -> 1
+    this == Direction.West && to == Direction.East -> 2
+    else -> error("Missing directions")
+}
+
+fun <T> dijkstra(start: T, goal: (T) -> Boolean, neighbors: (T) -> Sequence<Pair<T, Int>>): Int? {
+    val distanceFromStart = mutableMapOf(start to 0)
+    val visited = mutableSetOf<T>()
+    val queue = PriorityQueue<Pair<T, Int>> { a, b -> a.second.compareTo(b.second) }
+
+    queue.add(start to 0)
+
+    while (true) {
+        val (current, totalDistance) = queue.poll() ?: break
+        if (goal(current)) {
+            return totalDistance
+        }
+
+        visited.add(start)
+
+        for ((neighbor, distance) in neighbors(current)) {
+            if (neighbor in visited) continue;
+
+            val newDistance = totalDistance + distance
+
+            val currentDistance = distanceFromStart[neighbor]
+            if (currentDistance == null || newDistance < currentDistance) {
+                distanceFromStart[neighbor] = newDistance
+                queue.add(neighbor to newDistance)
+            }
+        }
+    }
+
+    return null
 }