快速平方根倒数算法

unsafe static float InvSqrt(float x)

{

    float xhalf = 0.5f * x;

    int i = *(int*)&x;

    i = 0x5f3759df - (i >> 1);

    x = *(float*)&i;

    x = x * (1.5f - xhalf * x * x);

    return x;

}