something to explain

Hi! I need to explain this code. It''s taken from glVelocity fast code library. But I don''t understand anything of this. It might do rational sqrt. Thnx for help.

__inline float RSqrt(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;
}
 

Dave007 dave007@volny.cz