private int readHeaderInt(int offset) {
    assert (0 <= offset && offset <= HeaderLayout.HEADER_SIZE - Integer.BYTES)
      : "header offset(=" + offset + ") must be in [0," + (HeaderLayout.HEADER_SIZE - Integer.BYTES) + "]";
    return headerPage.rawPageBuffer().getInt(offset);
  }
private int readHeaderInt(int offset)