flâneur

githublog/2024/5/29/fast-inverse-sqrt.md at main · francisrstokes/githublog

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Everything I Know About The Fast Inverse Square Root Algorithm The fast inverse square root algorithm , made famous (though not invented) by programming legend John Carmack in the Quake 3 source code, computes an inverse square root $\frac{1}{\sqrt{x}}$ with a bewildering handful of lines that interpret and manipulate the raw bits of float. It's wild . 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 = * ( flo

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