Reverse Square Root
float inverse_sqrt(float x) { long i; float x2, y; const float threehalfs = 1.5F;
x2 = x * 0.5F;
y = x;
i = (long )&y;
i = 0x5f3759df - (i >> 1);
y = (float )&i;
y = y (threehalfs - (x2 y * y));
y = y (threehalfs - (x2 y * y));
return y;
}