some q3 code

I know this has been looked at before a lot, but I was wondering what a line in this q3 code does. This code is very confusing partly because of that crazy number they invent.

Code:

---

float Q_rsqrt( float number )
{
        long i;
        float x2, y;

        y  = number;
        i  = * ( long * ) &y;                        //what does this do?                       
        i  = 0x5f3759df - ( i >> 1 );             
        y  = * ( float * ) &i;
        y  = y * ( threehalfs - ( x2 * y * y ) );  // 1st iteration

        return y;
}

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What exactly does casting the address of a float do?

Well .. i = * ( long * ) &y
&y - gives the address of y variable
( long * ) &y - is casting that float address to an address of type long
* ( long * ) &y - gets the value using our new pointer of type long

It is a bad way of doing the following

Code:

---

union FloatBits
{
    float Float;
    int    Bits;
};

{
  float f = 123.456;
  FloatBits fb;
  fb.Float = f;
  int i = fb.Bits;
}

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A bit OT, but here's a link to an article on the origin of the function in question: Origin of Quake3's Fast InvSqrt(). Doesn't go too much into detail, but you might find it interesting. It also links to an article on the math behind the function if you're curious. :)

Interesting article.

Quote:

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Originally Posted by TheCPUWizard

It is a bad way of doing the following

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Isn't it also bad to write to one member, and then read from another member, of the same union?

Quote:

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Isn't it also bad to write to one member, and then read from another member, of the same union?

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Not if you understand the implications :D :D

There are many analogies I am thinking of, but each one is sure to embarass/offend someone in this international community. :blush:

ah ok thanks. I guess it just confused me