private static int fieldOffset(int fileId,
                                   int fieldNo) {
      if (fileId <= 0) {
        throw new IllegalArgumentException("invalid fileId: " + fileId);
      }
      assert (0 <= fieldNo && fieldNo < INT_FIELDS_COUNT) : "fieldNo(=" + fieldNo + ") must be in [0," + INT_FIELDS_COUNT + ")";
      return ((fileId & OFFSET_MASK) * INT_FIELDS_COUNT) + fieldNo;
    }
private static int fieldOffset(int fileId,
                                   int fieldNo)