code: func decodedLen(src []byte) (blockLen, headerLen int, err error) {
  	v, n := binary.Uvarint(src)
  	if n == 0 {
  		return 0, 0, ErrCorrupt
  	}
  	if uint64(int(v)) != v {
  		return 0, 0, errors.New("snappy: decoded block is too large")
  	}
  	return int(v), n, nil
  }
docs: // decodedLen returns the length of the decoded block and the number of bytes
  // that the length header occupied.