// A = the objects to be cached (each associated with a BWImage
// which is scaled down to the thumbnail size.)
sclass G22ThumbnailCache<A> extends Meta is WidthAndHeight {
  int w, h;
  int bitsPerPixel, bitsPerImage;
  new MultiMap<Long, A> elementsByThumbnail;
  
  *(int *w, int *h, int *bitsPerPixel) {
    bitsPerImage = safeMul(w, h, bitsPerPixel);
    if (bitsPerImage >= 64)
      fail("Too many bits per thumbnail (64 allowed max)", +w, +h, +bitsPerPixel, +bitsPerImage);
  }
  
  public int getWidth() { ret w; }
  public int getHeight() { ret h; }
  
  void add(A element, IBWIntegralImage image) {
    long thumbnail = getThumbnail(image);
    elementsByCacheImage.add(thumbnail, element);
  }
  
  long getThumbnail(IBWIntegralImage image) {
    
    bitsPerImage
  }
}