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  1. #include <stdio.h>
  2. #include <math.h>
  3.  
  4. // https://e...content-available-to-author-only...a.org/wiki/Fast_inverse_square_root
  5. float Q_rsqrt( float number )
  6. {
  7.   long i;
  8.   float x2, y;
  9.   const float threehalfs = 1.5F;
  10.  
  11.   x2 = number * 0.5F;
  12.   y  = number;
  13.   i  = * ( long * ) &y;                       // evil floating point bit level hacking
  14.   i  = 0x5f3759df - ( i >> 1 );               // what the fuck?
  15.   y  = * ( float * ) &i;
  16.   y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration
  17. //y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed
  18.  
  19.   return y;
  20. }
  21.  
  22. void print(float n) {
  23.   float result_1 = 1 / n;
  24.  
  25.   float x = Q_rsqrt(n);
  26.   float result_2 = x * x;
  27.   printf("%f %f\n", result_1, result_2);
  28. }
  29.  
  30. int main() {
  31.   print(1000.0f);
  32.   print(100.0f);
  33.   print(10.0f);
  34.   print(1.0f);
  35.   print(0.1f);
  36.   print(0.01f);
  37.   return 0;
  38. }
Success #stdin #stdout 0s 4516KB
comments (?)
stdin 
Standard input is empty
stdout 
0.001000 0.000997
0.010000 0.009969
0.100000 0.099658
1.000000 0.996617
10.000000 9.968114
100.000000 99.650749
https://ideone.com/3ZkwP7
language:
C (gcc 8.3)
created:
6 years ago
visibility:
public

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