WIP: Day 20 part 1.

Algorithm works, too slow.

2024/src/main/kotlin/Direction.kt

@@ -18,4 +18,9 @@ fun Grid.Coordinate.step(direction: Direction, steps: Int = 1) =
         Direction.East -> copy(x = x + steps)
         Direction.South -> copy(y = y + steps)
         Direction.West -> copy(x = x - steps)
-    }
+    }
+
+fun Grid.Coordinate.neighbors(distance: Int = 1): Sequence<Grid.Coordinate> =
+    Direction.entries
+        .asSequence()
+        .map { step(it, distance) }

2024/src/main/kotlin/day20.kt

@@ -0,0 +1,58 @@
+fun main() {
+    val maze = CharGrid.read("day20-sample.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
+    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] == '#' }
+            .flatMap { it.neighbors() }
+            .filter { canVisit(it) }
+            .fold(paths) { list, next ->
+                list.addAll(findPaths(next, goal, newVisited, time + 2, true))
+                list
+            }
+    }
+}