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sqrt函数的实现主要有三种方式：

1 二分法

2 牛顿法

3 卡马克方法

float InvSqrt(float x)
{
    float xhalf = 0.5f*x;
    int i = *(int*)&x; // get bits for floating VALUE 
    i = 0x5f3759df-(i>>1); // gives initial guess y0
    x = *(float*)&i; // convert bits BACK to float
    x = x*(1.5f-xhalf*x*x); // Newton step, repeating increases accuracy
    x = x*(1.5f-xhalf*x*x); // Newton step, repeating increases accuracy
    x = x*(1.5f-xhalf*x*x); // Newton step, repeating increases accuracy

    return 1/x;
}

卡马克方法的详细原理可查看维基百科

https://en.wikipedia.org/wiki/Fast_inverse_square_root