code: private void readFromTempFile(int at_offset, byte[] data, int offset, int len) throws IOException {
          if (_file_pos != at_offset) {
              _tempfile.seek((long)at_offset);
          }
          _tempfile.readFully(data,offset,len);
          _file_pos = at_offset+len;
      }
docs: Read chunk of data from specified offset in tempfile