Day 18 part 1

2024/src/main/kotlin/Dijkstra.kt

@@ -0,0 +1,32 @@
+import java.util.*
+
+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
+}

2024/src/main/kotlin/day16.kt

@@ -31,34 +31,3 @@ fun Direction.turns(to: Direction): Int = when {
     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
-}

2024/src/main/kotlin/day18.kt

@@ -0,0 +1,25 @@
+fun main() {
+    val size = 71
+
+    val memory = readInput("day18.txt")
+        .map {
+            val (a, b) = it.split(',', limit = 2)
+            Grid.Coordinate(a.toInt(), b.toInt())
+        }
+        .take(1024)
+        .fold(CharGrid(size, size, '.')) { acc, coordinate ->
+            acc[coordinate] = '#'
+            acc
+        }
+
+    val start = Grid.Coordinate(0, 0)
+    val goal = Grid.Coordinate(size - 1, size - 1)
+
+    val distance = dijkstra(start, goal = { it == goal }, neighbors = { pos ->
+        Direction.entries.asSequence()
+            .map { pos.step(it) }
+            .filter { it in memory && memory[it] == '.' }
+            .map { it to 1 }
+    })
+    println("Part 1: $distance")
+}