Day 16: Use two stacks instead of PQ

aoc/src/dev/ctsk/aoc/days/Day16.scala

@@ -16,26 +16,34 @@ object Day16 extends Solver(16):
     case class State(distance: Int, pose: Pose)
     implicit def ord: Ordering[State] = Ordering.by(-1 * _.distance)
 
-    Direction.Up.ordinal
-
     def search(start: Pose): (Map[Pose, Int], Map[Pose, List[Pose]]) =
       val distance = mutable.Map(start -> 0)
       val ancestor = mutable.Map.empty[Pose, List[Pose]]
-      val heap = mutable.PriorityQueue(State(0, start))
+      var near = mutable.Stack(State(0, start))
+      var far = mutable.Stack.empty[State]
 
-      inline def relax(u: Pose, v: Pose, vDist: Int): Unit =
+      inline def relax(u: Pose, v: Pose, vDist: Int, isFar: Boolean): Unit =
         val oldDist = distance.getOrElse(v, 1_000_000)
         if vDist == oldDist then ancestor(v) +:= u
         if vDist < oldDist then
           ancestor(v) = List(u)
           distance(v) = vDist
-          heap.enqueue(State(vDist, v))
+          if isFar
+          then far.push(State(vDist, v))
+          else near.push(State(vDist, v))
 
-      while heap.nonEmpty do
-        val State(uDist, u) = heap.dequeue()
+      while near.nonEmpty do
+        while near.nonEmpty do
+          val State(uDist, u) = near.pop()
           if uDist == distance(u) then
-          if !grid(u.step.pos).contains('#') then relax(u, u.step, uDist + 1)
-          for (v <- Seq(u.turnLeft, u.turnRight)) do relax(u, v, uDist + 1000)
+            if !grid(u.step.pos).contains('#') then
+              relax(u, u.step, uDist + 1, false)
+            for (v <- Seq(u.turnLeft, u.turnRight)) do
+              relax(u, v, uDist + 1000, true)
+
+        val tmp = near
+        near = far
+        far = tmp
 
       (distance.toMap, ancestor.toMap)