// an indexing structure for grayscale images
// -all images have the same size
// -images are sorted by
//   1. average brightness
//   2. "lexical" scanline ordering (within same brightness)
// -every image can have an arbitrary value associated (type "A")

sclass ScaledGrayscaleCache<A> extends Meta is WidthAndHeight {
  int w, h;
  new TreeSet<Entry> entries;
  
  class Entry is Comparable<Entry> {
    double brightness;
    BWImage image;
    A value;
    
    public int compareTo(Entry e) {
      if (brightness != e.brightness)
        ret cmp(brightness, e.brightness);
      ret compareBWImagesByScanlineOrdering(image, e.image);
    }
  }
  
  *(int *w, int *h) {}
  
  public int getWidth() { ret w; }
  public int getHeight() { ret h; }
}