func xor64(x uint64) uint64 {
	x ^= x << 13
	x ^= x >> 7
	x ^= x << 17
	return x
}
func xor64(x uint64) uint64 {
	x ^= x << 13
	x ^= x >> 7
	x ^= x << 17
	return x
}
// xor64 generates the next value of a pseudo-random sequence given a current
// state x.