C++的快速平方根算法

SquareRootFloat 函数计算sqrt 的快速算法，出自Carmack的QUAKE3，比库函数快得多

 

/* ================
     SquareRootFloat
     ================
*/
#include <iostream>
using namespace std;
float SquareRootFloat( float number)    
{        
         long i;        
         float x, y;        
         const float f = 1.5F;        
         x = number * 0.5F;        
         y    = number;        
         i    = * ( long * ) &y;        
         i    = 0x5f3759df - ( i >> 1 ); //魔术数        
         y    = * ( float * ) &i;        

         y    = y * ( f - ( x * y * y ) );         //迭代1    1/sqrt(number)    
         y    = y * ( f - ( x * y * y ) );         //迭代2    1/sqrt(number)    
         //y    = y * ( f - ( x * y * y ) );        //迭代3    1/sqrt(number)，如需要更高的精度请迭代多次    

         return number * y;
}

main()
{
     float b = 9852.0;
     //cout<<SquareRootFloat(b)<<endl;        
     float c = SquareRootFloat(b);        
     float e = c * c ;
    cout<<e<<endl;        
    system( "pause");
}