code: func (xxh xxHash) Sum(b []byte) []byte {
  	h64 := xxh.Sum64()
  	return append(b, byte(h64), byte(h64>>8), byte(h64>>16), byte(h64>>24), byte(h64>>32), byte(h64>>40), byte(h64>>48), byte(h64>>56))
  }
docs: // Sum appends the current hash to b and returns the resulting slice.
  // It does not change the underlying hash state.