Inverse Square Root by Greg Walsh

gistfile1.c

float invSqrt(float x)
{
	union {
		float x;
		int i;
	} u;

	float xhalf = 0.5f * x;
	u.x = x;
	u.i = 0x5f3759df - (u.i >> 1);
	u.x = u.x * (1.5f - xhalf * u.x * u.x);
	return u.x;
}