fast inverse square root explained
Fast Inverse Square Root in C
The fast inverse square root algorithm is a method for approximating ( \frac{1}{\sqrt{x}} ) for a 32-bit floating-point number x in C. It is commonly associated with the Quake III Arena game and is known for its clever use of bit manipulation and the magic constant 0x5f3759df.
The algorithm is as follows:
float Q_rsqrt( float number )
{
long i;
float x2, y;
const float threehalfs = 1.5F;
x2 = number * 0.5F;
y = number;
i = ( long ) &y; // evil floating point bit level hacking
i = 0x5f3759df - ( i >> 1 ); // what the hell?
y = ( float ) &i;
y = y ( threehalfs - ( x2 y * y ) ); // 1st iteration
// y = y ( threehalfs - ( x2 y * y ) ); // 2nd iteration, this can be removed
return y;
}
Note: This algorithm is an example of a fast approximation and may not provide the same level of accuracy as the standard library function.
[[SOURCE #1]]