sclass HashedBWImage is IBWImage, MakesBWImage {
  BWImage image; // don't change this afterwards...
  long hash;
  bool hashComputed;
  
  *(BufferedImage image) { this(bwImage(image)); }
  *(BWImage *image) { assertNotNull(image); }
  
  long hash() {
    while (!hashComputed) {
      synchronized {
        if (!hashComputed) {
          hash = hashBWImage(image);
          set hashComputed;
        }
      }
    }
    
    ret hash;
  }
  
  equals HashedBWImage o {
    if (hash() != o.hash()) false;
    if (!bwImagesIdentical(image, o.image)) {
      reportHashCollision(this, o);
      false;
    }
    true;
  }
  
  public int hashCode() { ret (int) hash(); }
  
  public BWImage get aka getBWImage() { ret image; }
  
  public int getWidth() { ret image.getWidth(); }
  public int getHeight() { ret image.getHeight(); }
  public BufferedImage getBufferedImage() { ret image.getBufferedImage(); }
  
  // check that BWImage wasn't modified
  void verify {
    if (hash != 0)
      assertEquals(hash, hashBWImage(image));
  }
  
  S classNameForPrinting() { ret cnfp(this, selfType.class, "HashedBWImage"; }
  
  toString { ret formatFunctionCall(classNameForPrinting(),
    w() + "*" + h() + "px", "hash " + longToHex(hash())); }
  
  // implement IBWImage
  
  public float getFloatPixel(int x, int y) { ret image.getFloatPixel(x, y); }
  public int getInt(int x, int y) { ret image.getInt(x, y); }
}