As you know sometimes microcontrollers struggle with computation intensive functions, and you want to optimize the code to make life easier for them. So you probably remember the famous hack from Quake ||| to compute the inverse square root of x, if not, here is a remainder how it looks:

Code: Select all

```
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 f***?
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;
}
```

I did a little bit of googling and found that one crazy kid found better coefficients for the same hack, and it looks like this:

Code: Select all

```
float inv_sqrt(float x)
{ union { float f; uint32 u; } y = {x};
y.u = 0x5F1FFFF9ul - (y.u >> 1);
return 0.703952253f * y.f * (2.38924456f - x * y.f * y.f);
}
```

Best regards,

Mantas

Edited out swear word as people under 18 could be reading forums..Martin