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; }
}