Fast Inverse Square Root (Quake III Arena)

Q_rsqrt.c

float Q_rsqrt( float number )
{
	long i;
	float x2, y;
	const float threehalfs = 1.5F;

	x2 = number * 0.5F;
	y  = number;
	i  = * ( long * ) &y;                       // Store physical representation of bits
	i  = 0x5f3759df - ( i >> 1 );               // Make an initial guess
	y  = * ( float * ) &i;                      // Convert back to float
	y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration of Newton's method
//	y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed

	return y;
}