persistable sclass FloodFill is Steppable {

  // input

  int w, h;

  swappable bool isConnectable(Pt a, Pt b) { throw unimplemented(); }

  void visited(Pt p) {}

  // internal

  new PtBuffer stack;

  BitMatrix visited;

  *(int *w, int *h) { visited = new BitSetMatrix(w, h); }

  *(int *w, int *h, BitMatrix *visited) {}

  selfType useQuadTree() { visited = new QuadTreeBitMatrix(w, h); this; }

  bool isPointVisited(Pt p) {

    ret visited.get(p);

  }

  void startAt aka addPoint(Pt p) {

    if (!isAddable(p)) ret;

    stack.add(p);

    addedPoint(p);

  }

  bool isValidPoint(Pt p) {

    ret p != null && !(p.x < 0 || p.y < 0 || p.x >= w || p.y >= h);

  }

  bool isAddable(Pt p) {  

    if (!isValidPoint(p)) false;

    if (isPointVisited(p)) false;

    true;

  }

  void addedPoint(Pt p) {}

  public bool step() {

    if (empty(stack)) false;

    Pt p = popLast(stack);

    int x = p.x, y = p.y;

    visited.set(p, true);

    visited(p);

    possibleBridge(p, new Pt(x-1, y));

    possibleBridge(p, new Pt(x+1, y));

    possibleBridge(p, new Pt(x, y-1));

    possibleBridge(p, new Pt(x, y+1));

    true;

  }

  void possibleBridge(Pt a, Pt b) {

    if (isValidPoint(b) && isConnectable(a, b)) {

      addPoint(b);

      createBridge(a, b);

    }

  }

  void createBridge(Pt a, Pt b) {}

  void addAllPoints {  

    for (p : allPointsInRect_virtual(w, h))

      addPoint(p);

  }

  void doAllPoints {

    for (p : allPointsInRect_virtual(w, h)) {

      addPoint(p);

      stepAll(this);

    }

  }

}