srecord noeq PicturesByMD5(File dir) {

  settable S extension = ".jpeg";

  transient Lock lock = lock();

  // current count of images (calculated lazily)

  transient new VarWithNotify<Int> imageCount;

  File imageFile(S md5) {

    ret newFile(dir, assertMD5(md5) + extension);

  }

  bool has(S md5) { ret isFile(imageFile(md5)); }

  BufferedImage get(S md5) {

    ret decodeImage(imageFile(md5));

  }

  swappable BufferedImage decodeImage(File f) {

    ret loadImage2(f);

  }

  bool put(S md5, byte[] imageData) {

    // double-checked locking, yay!

    if (has(md5)) false;

    lock lock;

    if (has(md5)) false;

    saveBinaryFile(imageFile(md5), imageData);

    if (imageCount.has()) imageCount.set(imageCount()+1);

    true;

  }

  // -actually scans the directory

  // -only sees files with the correct extension

  int freshImageCount() {

    lock lock;

    ret setAndReturn(imageCount, l(freshImageFiles()));

  }

  L<File> freshImageFiles() {

    ret listFilesWithExtension(dir, extension);

  }

  // get current image count in an efficient way

  // -does not notice when other objects/processes

  //  add or remove images in the directory

  int imageCount() {

    lock lock;

    if (!imageCount.has())

      freshImageCount();

    ret imageCount!;

  }

  IVarWithNotify varImageCount() { ret imageCount; }

}