code: public void clear() {
          _file_mode = false;
          _write_pos = 0;
          _read_pos = 0;
          _window_low = 0;
          _window_high = _window_size;
          _file_high = 0;
          _mark_pos = 0;
      }
docs: Erases all unread buffered data and prepares for next use cycle.
  If temporary file was used, it is not closed/deleted yet as it may be needed again.