code: func AddUint64(h uint64, u uint64) uint64 {
  	h = h + u + constant
  	h = (h<<shift | h>>(64-shift)) ^ u
  	h = ((h<<shift | h>>(64-shift)) ^ constant) + u
  	return h
  }
docs: // AddUint64 adds the hash value of the 8 bytes of u to h.