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    return x;}

Chris Lomont