Day 20 part 1.

2024/src/main/kotlin/Dijkstra.kt

@@ -1,7 +1,10 @@
 import java.util.*
 
-fun <T> dijkstra(start: T, goal: (T) -> Boolean, neighbors: (T) -> Sequence<Pair<T, Int>>): Int? {
+data class PathResult<T>(val steps: List<Pair<T, Int>>, val totalDistance: Int)
+
+fun <T> dijkstraPath(start: T, goal: (T) -> Boolean, neighbors: (T) -> Sequence<Pair<T, Int>>): PathResult<T>? {
     val distanceFromStart = mutableMapOf(start to 0)
+    val previous = mutableMapOf<T, T>()
     val visited = mutableSetOf<T>()
     val queue = PriorityQueue<Pair<T, Int>> { a, b -> a.second.compareTo(b.second) }
 
@@ -10,7 +13,15 @@ fun <T> dijkstra(start: T, goal: (T) -> Boolean, neighbors: (T) -> Sequence<Pair
     while (true) {
         val (current, totalDistance) = queue.poll() ?: break
         if (goal(current)) {
-            return totalDistance
+            val path = generateSequence(current) { previous[it] }
+                .map { it to (distanceFromStart[it] ?: 0) }
+                .toList()
+                .reversed()
+
+            return PathResult(
+                path,
+                totalDistance
+            )
         }
 
         visited.add(start)
@@ -23,10 +34,15 @@ fun <T> dijkstra(start: T, goal: (T) -> Boolean, neighbors: (T) -> Sequence<Pair
             val currentDistance = distanceFromStart[neighbor]
             if (currentDistance == null || newDistance < currentDistance) {
                 distanceFromStart[neighbor] = newDistance
+                previous[neighbor] = current
                 queue.add(neighbor to newDistance)
             }
         }
     }
 
     return null
+}
+
+fun <T> dijkstra(start: T, goal: (T) -> Boolean, neighbors: (T) -> Sequence<Pair<T, Int>>): Int? {
+    return dijkstraPath(start, goal, neighbors)?.totalDistance
 }

2024/src/main/kotlin/day20.kt

@@ -1,58 +1,29 @@
 fun main() {
-    val maze = CharGrid.read("day20-sample.txt")
+    val maze = CharGrid.read("day20.txt")
     val start = maze.find('S') ?: error("No start position")
     val end = maze.find('E') ?: error("No end position")
 
-    val paths = maze.findPaths(start, end)
-
-    val total = paths.first { !it.cheated }.time
+    val shortest = maze.findShortestPath(start, end) ?: error("No shortest path found")
+    val total = shortest.last().second
+    val toGoal = shortest.associate { (step, fromStart) ->
+        step to total - fromStart
+    }
     println("Time without cheating: $total")
-    val part1 = paths.count { it.time <= total - 100 }
-    println("Part 1: $part1")
 
-    paths.sortedBy { it.time }
-        .groupBy { it.time }
-        .forEach { (time, list) ->
-            val saving = total - time
-            println("${list.count()} x $saving")
-        }
-}
-
-data class MazeResult(val time: Int, val cheated: Boolean)
-
-fun CharGrid.findPaths(
-    position: Grid.Coordinate,
-    goal: Grid.Coordinate,
-    visited: Set<Grid.Coordinate> = emptySet(),
-    time: Int = 0,
-    cheated: Boolean = false
-): List<MazeResult> {
-    if (position == goal) {
-        return listOf(MazeResult(time, cheated))
-    }
-
-    val newVisited = visited + position
-
-    fun canVisit(position: Grid.Coordinate): Boolean =
-        position in this && position !in newVisited && this[position] != '#'
-
-    val paths = position.neighbors()
-        .filter { canVisit(it) }
-        .fold(mutableListOf<MazeResult>()) { list, next ->
-            list.addAll(findPaths(next, goal, newVisited, time + 1, cheated))
-            list
-        }
-
-    return if (cheated) {
-        paths
-    } else {
-        position.neighbors()
-            .filter { it in this && this[it] == '#' }
+    val part1 = shortest.flatMap { (step, fromStart) ->
+        step.neighbors()
+            .filter { it in maze && maze[it] == '#' }
             .flatMap { it.neighbors() }
-            .filter { canVisit(it) }
-            .fold(paths) { list, next ->
-                list.addAll(findPaths(next, goal, newVisited, time + 2, true))
-                list
-            }
-    }
+            .filter { it != step && it in maze && maze[it] != '#' }
+            .mapNotNull { toGoal[it] }
+            .map { fromStart + 2 + it }
+    }.count { it <= total - 100 }
+    println("Part 1: $part1")
 }
+
+fun CharGrid.findShortestPath(start: Grid.Coordinate, end: Grid.Coordinate) =
+    dijkstraPath(start, goal = { it == end }, neighbors = { position ->
+        position.neighbors()
+            .filter { it in this && this[it] != '#' }
+            .map { it to 1 }
+    })?.steps