sclass SimpleOutlineWalker is Steppable {
  IIntIntPred pixelIsInside;
  int x, y;
  int startX, startY;
  NESWDirection direction = NESWDirection.right;
  
  event foundPoint(int x, int y);
  
  // x, y must be inside the shape
  *(IIntIntPred *pixelIsInside, int *x, int *y) {
    if (!pixelIsInside.get(x, y))
      fail("pixel is not inside shape: " + x + "/" + y);
      
    // go north until we hit the border
    while (pixelIsInside.get(x, y-1))
      --y;
      
    startX = x; startY = y;
  }
  
  public bool step() {
    foundPoint(x, y);
    
    // try left, straight, right, back
    for (int turn = 3; turn <= 6; turn++) {
      Direction newDirection = direction.plus(turn);
      int x2 = x+newDirection.x, y2 = y+newDirection.y;
      
      bool inside = pixelIsInside.get(x2, y2);
      ifdef SimpleOutlineWalker_debug
        printVars(+x, +y, +direction, +turn, +newDirection, +x2, +y2, +inside);
      endifdef
      if (inside) {
        x = x2; y = y2; dir = newDir;
        true;
      }
    }
    
    if (x == startX && y == startY)
      false;
  }
}