> Fast inverse square root is genius but it would just be way-too-clever technical debt if Carmack hadn’t written it and others didn’t learn from him.

FISR is a beautiful hack, but it hardly technical debt. This is the original code. Note how the trick has been hidden behind a normal API such as `Q_rsqrt`:

   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


        return y;
  }

This is how you walk the fine line between technical debt and useful hack.