// 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;
    }
  };
}