private long toOffsetInFile(int fileId) {
    checkFileIdValid(fileId);
    long offsetInFile = (fileId - FSRecords.ROOT_FILE_ID) * (long)bytesPerRow + HeaderLayout.HEADER_SIZE;
    if (offsetInFile < 0) {
      throw new AssertionError("fileId(=" + fileId + ") x bytesPerRow(=" + bytesPerRow + ") is too big: " +
                               "offsetInFile(=" + offsetInFile + ") must be positive");
    }
    return offsetInFile;
  }
private long toOffsetInFile(int fileId)