code: func (ms *FileMsgStore) addIndex(buf []byte, seq uint64, offset, timestamp int64, msgSize int) {
  	util.ByteOrder.PutUint64(buf, seq)
  	util.ByteOrder.PutUint64(buf[8:], uint64(offset))
  	util.ByteOrder.PutUint64(buf[16:], uint64(timestamp))
  	util.ByteOrder.PutUint32(buf[24:], uint32(msgSize))
  	crc := crc32.Checksum(buf[:msgIndexRecSize-crcSize], ms.fstore.crcTable)
  	util.ByteOrder.PutUint32(buf[msgIndexRecSize-crcSize:], crc)
  }
docs: // addIndex adds a message index record in the given buffer