Everything I Know About The Fast Inverse Square Root Algorithm

CommunityNews · 30 May 2024 01:12

githublog/2024/5/29/fast-inverse-sqrt.md at main · francisrstokes/githublog.
I’m sick of complex blogging solutions, so markdown files in a git repo it is - francisrstokes/githublog

Read in full here:

github.com

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

# 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*.

```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;                            // 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

  return y;
}

This file has been truncated. show original

This thread was posted by one of our members via one of our news source trackers.