// start from center

static LongIterator pixelSpiral_longIterator(int w, int h) {

  ret pixelSpiral_longIterator(w/2, h/2, w, h);

}

// thanks to https://stackoverflow.com/questions/398299/looping-in-a-spiral

// firstIntFromLong(l) = x, secondIntFromLong(l) = y

static LongIterator pixelSpiral_longIterator(int sx, int sy, int imageWidth, int imageHeight, int maxPixels default Int.MAX_VALUE) {

  ret new LongIterator {

    int x, y, dx, dy = -1;

    int maxPixels2 = min(maxPixels, sqr(max4(sx+1, imageWidth-sx, sy+1, imageHeight-sy)*2));

    int i;

    long pixel;

    bool done;

    void fetch {

      while (i++ < maxPixels2) {

        int px = sx+x, py = sy+y;

        if ((x == y) || ((x < 0) && (x == -y)) || ((x > 0) && (x == 1-y))) {

          int t=dx; dx=-dy; dy=t;

        }   

        x += dx; y += dy;

        if (px >= 0 && py >= 0 && px < imageWidth && py < imageHeight)

          ret with pixel = twoIntsToLong(px, py);

       }

       set done;

     }

    { fetch(); }

    public bool hasNext() { ret !done; }

    public long next() {

      long p = pixel;

      fetch();

      ret p;

    }

  };

}