Oops. I didn't realise it was a common assembly trick, or I wouldn't have posted it.

Still for none assembly people it can be fun to work out how it does it (preferably on paper, and after a couple of pints).

Thanks for pointing it out.

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'I think the problem lies in the fact that your data doesn't fit my program'.

Essentially it does this:

# let's assume some values for the sake of discussion
$a = 'foo'; $b = 'bar';

$a = $a ^ $b;
# $a now contains both variables, $a and $b logical or'ed
# together, so at this point a=foo^bar and b=bar

$b = $a ^ $b;
# logically or-ing them again (since b still contains bar)
# returns the original value of a, so now a=foo^bar
# and b=foo

$a = $a ^ $b;
# then we logical-or the combined value with b, which now
# contains our original a value. this returns the
# original b value, so now a=bar and b=foo
[download]

It's one of those things that you can look at a dozen times, and suddenly your brain just goes 'ah ha!'.

Update: Of course read 'exclusive or (or xor)' everywhere that I said 'logical or', it must be bedtime.

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We're not surrounded, we're in a target-rich environment!

$a = ord('A');  # 01000001
$b = ord('B');  # 01000010
$a = $a ^ $b;   # 01000001 XOR 01000010 = 00000011
$b = $a ^ $b;   # 00000011 XOR 01000010 = 01000001
$a = $a ^ $b;   # 00000011 XOR 01000001 = 01000010
[download]

Or, if you think in terms of truth tables:

original $a	original $b	$a ^ $b (gives new $a)	$a ^ $b (gives new $b)	$a ^ $b (gives new $a)
0	0	0	0	0
0	1	1	0	1
1	0	1	1	0
1	1	0	1	1

As you can see, original $a is the same as new $b and original $b is the same as new $a.

Not much chance of this one being changed - the paperwork would be horrendous (it's part of the flight control of a Tornado).

The person that wrote it clearly didn't do it for speed as they didn't recognise that 2 if's are quicker than an and.

It appears to have been done primarily to avoid introducing a third variable. No ($a, $b) = ($b, $a) functionality available. Even so, I still believe it to be a prime example of 'how not to code' in a high risk, real time environment when it should count as 'quite important' that maintenance staff can clearly follow the code.

We live and learn I suppose.

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'I think the problem lies in the fact that your data doesn't fit my program'.

let $a = a, $b = b
remember a ^ a = 0, a ^ 0 = a, (a^b)^c = a^(b^c), a^b=b^a

                 $a                   $b
            ------------------ -------------------- 
$a=$a^$b        a^b                    b
$b=$a^$b        a^b           (a^b)^b=a^(b^b)=a^0=a
$a=$a^$b     a^b^a=a^a^b=0^b=b         a