func oneBit16(x int16) bool { return x&(x-1) == 0 && x != 0 }
func oneBit32(x int32) bool { return x&(x-1) == 0 && x != 0 }
func oneBit64(x int64) bool { return x&(x-1) == 0 && x != 0 }

// nto returns the number of trailing ones.
func nto(x int64) int64 {
	return int64(ntz64(^x))
}

// logX returns logarithm of n base 2.
// n must be a positive power of 2 (isPowerOfTwoX returns true).
func log8(n int8) int64 {