⚡️cache walking paths

src/main/scala/org/lemon/advent/year2024/Day06.scala

@@ -21,19 +21,33 @@ private object Day06:
     val start = grid.find(_._2 == '^').get._1
     walk(grid, start, Direction('^')).size
 
+  type Paths = Map[Coord, Map[Direction, Coord]]
+  def routes(grid: Map[Coord, Char]): Paths =
+    val paths =
+      for
+        (start, ch) <- grid.iterator
+        if ch != '#'
+        dir <- Direction.values
+      yield
+        val end = Iterator.iterate(start)(_ + dir)
+          .find(coord => !grid.contains(coord) || grid(coord) == '#')
+          .get
+        start -> (dir -> (if !grid.contains(end) then end else end - dir))
+    paths.toSeq.groupBy(_._1).mapValues(_.map(_._2).toMap).toMap
+
   @annotation.tailrec
-  def isLoop(grid: Map[Coord, Char], cameFrom: Coord, dir: Direction, seen: Set[(Coord, Direction)] = Set()): Boolean =
-    val goingTo = cameFrom + dir
-    if !grid.contains(goingTo) then false
-    else if seen.contains((goingTo, dir)) then true
+  def isLoop(paths: Paths, wall: Coord)(at: Coord, dir: Direction, seen: Set[(Coord, Direction)] = Set()): Boolean =
+    if seen.contains((at, dir)) then true
+    else if !paths.contains(at) then false
     else
-      grid(goingTo) match
-        case '#' => isLoop(grid, cameFrom, dir.turnRight, seen + ((cameFrom, dir)))
-        case _ => isLoop(grid, goingTo, dir, seen)
+      val dest = paths(at)(dir)
+      if dest == wall || at.directionTo(wall).contains(dir) && dest.directionTo(wall).contains(dir.turnAround) then
+        isLoop(paths, wall)(wall - dir, dir.turnRight, seen + ((at, dir)))
+      else isLoop(paths, wall)(dest, dir.turnRight, seen + ((at, dir)))
 
   def part2(input: String) =
     val grid = parse(input)
+    val paths = routes(grid)
     val start = grid.find(_._2 == '^').get._1
     val dir = Direction('^')
-    val path = walk(grid, start, dir)
-    path.par.count(c => isLoop(grid + (c -> '#'), start, dir))
+    walk(grid, start, dir).count(isLoop(paths, _)(start, dir))