srecord noeq BWImage_FastRegions(BWImage image) is Runnable {
  int w, h, size, runner;
  
  new IntBuffer stack; // locations as y*w+x
  int[] regionMatrix; // for each pixel: region index (starting at 1)
  new IntStack regionFirstPixel; // for each region: index of first pixel found
  int regionCounter;
  bool verbose;
  
  double regionStep = .1; // for rendering in regionsImage
  
  int x(int pos) { ret pos % w; }
  int y(int pos) { ret pos / w; }
  int pos(int x, int y) { ret y*w+x; }
  
  int getColor(int pos) { ret image.getInt(x(pos), y(pos)); }

  run {
    w = image.getWidth(); h = image.getHeight();
    size = w*h;
    regionMatrix = new int[size];
    regionFirstPixel.add(0); // unused
    
    while (runner < size) {
      if (regionMatrix[runner] == 0) {
        // make a new region, get color
        int region = ++regionCounter;
        regionFirstPixel[region] = runner;
        stack.add(runner);
        int color = getColor(runner);

        // flood-fill region
        while (nempty(stack)) {
          int pos = stack.popLast();
          if (regionMatrix[pos] != 0) continue; // touching myself (or someone else)
          if (getColor(pos) != color) continue; // wrong color
        
          // new pixel found, mark as ours
          regionMatrix[pos] = region;
        
          // explore neighborhood
          int x = x(pos), y = y(pos);
          if (x > 0)   stack.add(pos-1);
          if (x < w-1) stack.add(pos+1);
          if (y > 0)   stack.add(pos-w);
          if (y < h-1) stack.add(pos+w);
        }
      }
      
      ++runner;
    }
  }
  
  IBWImage regionsImage() {
    ret iBWImageFromFunction((x, y) -> {
      var region = regionMatrix[pos(x, y)];
      ret ((region-1)*regionStep) % (1.0+regionStep-0.0001);
    }, w, h);
  }
  
  int regionCount() { ret regionCounter; }
}