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); }

}