Re: Fast DES IP implementation

From: Paul Rubin (//phr.cx_at_NOSPAM.invalid)
Date: 12/09/04 

Date: 09 Dec 2004 02:49:47 -0800

"Arnaud Carré" <arnaud.carreNOSPAM@freesurf.fr> writes:
> Hey that's not "trick", that's logic ! Do you think the "xor trick" does not
> work when i == j ?? :-)
>
> Try yourself !!
>
> int i = 0x12345678;
> int j = i;
>
> i ^= j;
> j ^= i;
> i ^= j;
>
> i and j are swapped, whatever the content !! (even if they are equal or
> course !)

You're not understanding the problem. You don't want to swap i and j.
You want to swap a[i] and a[j]. Try like this:

    int a[2] = {0x12345678, 1};

    int i = 0;
    int j = 1;

    a[i] ^= a[j];
    a[j] ^= a[i];
    a[i] ^= a[j];

This works ok, since i != j. Now try with i=j :-(.