class ChecksumFileSystem extends FilterFileSystem {

  public static long getChecksumLength(long size, int bytesPerSum) {
    //the checksum length is equal to size passed divided by bytesPerSum +
    //bytes written in the beginning of the checksum file.  
    return ((size + bytesPerSum - 1) / bytesPerSum) * 4 +
             CHECKSUM_VERSION.length + 4;  
  }

  public  ChecksumFileSystem(FileSystem fs);

}