if n == 0 {
		panic("invalid argument to Uint64N")
	}
	return r.uint64n(n)
}

// uint64n is the no-bounds-checks version of Uint64N.
func (r *Rand) uint64n(n uint64) uint64 {
	if is32bit && uint64(uint32(n)) == n {
		return uint64(r.uint32n(uint32(n)))
	}
	if n&(n-1) == 0 { // n is power of two, can mask
		return r.Uint64() & (n - 1)
	}

	// Suppose we have a uint64 x uniform in the range [0,2⁶⁴)