static S md5OfFile(File f) ctex {
  if (!f.exists()) ret "-";
  
  MessageDigest md5 = MessageDigest.getInstance("MD5");
  temp FileInputStream in = new FileInputStream(f);

  byte buf[] = new byte[65536];
  int l;
  while (true) {
    l = in.read(buf);
    if (l <= 0) break;
    md5.update(buf, 0, l);
  }
  
  ret bytesToHex(md5.digest());
}