// backtracks lead to endless loops - must rework the architecture

sclass SimpleOutlineWalker is Steppable {

  IIntIntPred pixelIsInside;

  int x, y;

  settable bool processBacktracks = false;

  int startX, startY;

  NESWDirection direction = NESWDirection.east;

  gettable int nPixels;

  new PtBuffer pixels;

  new L<BacktrackPoint> backtrackStack;

  // seen points as intPairToLong

  new CompactLongSet seen;

  event foundPoint(int x, int y);

  srecord BacktrackPoint(int x, int y, NESWDirection dir, int turn) {}

  *(IIntIntPred pixelIsInside, Pt p) {

    this(pixelIsInside, p.x, p.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; this.y = y;

  }

  public bool step() {

    ++nPixels;

    pixels?.add(x, y);

    foundPoint(x, y);

    if (walkOneStep()) true;

    if (!processBacktracks) false;

    while (nempty(backtrackStack))

      if (processBacktrack()) true;

    false;

  }

  bool walkOneStep(int turn default 3) {

    // try left, straight, right, back

    for (; turn <= 6; turn++) {

      NESWDirection 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) {

        if (turn < 6)

          backtrackStack.add(new BacktrackPoint(x, y, direction, turn+1));

        x = x2; y = y2; direction = newDirection;

        break;

      }

    }

    ret seen.add(intPairToLong(x, y));

    //ret x != startX || y != startY;

  }

  bool processBacktrack() {

    if (empty(backtrackStack)) false;

    var bt = popLast(backtrackStack);

    x = bt.x; y = bt.y; direction = bt.dir;

    ret walkOneStep(bt.turn);

  }

  Pt currentPt() { ret pt(x, y); }

}