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The code of the fast inverse square root implementation from Quake III Arena. https://github.com/id-Software/Quake-III-Arena/blob/dbe4ddb10315479fc00086f08e25d968b4b43c49/code/game/q_math.c#L546-L581
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#if !idppc | |
/* | |
** float q_rsqrt( float number ) | |
*/ | |
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 fuck? | |
y = * ( float * ) &i; | |
y = y * ( threehalfs - ( x2 * y * y ) ); // 1st iteration | |
// y = y * ( threehalfs - ( x2 * y * y ) ); // 2nd iteration, this can be removed | |
#ifndef Q3_VM | |
#ifdef __linux__ | |
assert( !isnan(y) ); // bk010122 - FPE? | |
#endif | |
#endif | |
return y; | |
} | |
float Q_fabs( float f ) { | |
int tmp = * ( int * ) &f; | |
tmp &= 0x7FFFFFFF; | |
return * ( float * ) &tmp; | |
} | |
#endif |
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