code: private int figureChunksNumber(String fileName) throws IOException {
        long fileLength = directory.fileLength(fileName);
        return figureChunksNumber(fileName, fileLength, autoChunkSize);
     }
docs: Guess in how many chunks we should split this file. Should return the same value consistently
  for the same file (segments are immutable) so that a full segment can be rebuilt from the upper
  layers without anyone actually specifying the chunks numbers.