Fast (not exact) Inverse Square Root (from Quake source code)

fast_inverse_sqrt.c

float fast_inverse_sqrt(float number)
{
	long i;
	float x2, y;
	const float threehalfs = 1.5F;
 
	x2 = number * 0.5F;
	y  = number;
	i  = *(long*)&y;
	i  = 0x5f3759df - (i >> 1);
	y  = *(float*)&i;
	y  = y * (threehalfs - (x2 * y * y));
 
	return y;
}