How does this sort function work?

Question

As part of my job, I'm occasionally called upon to evaluate candidates for programming positions. A code snippet recently passed across my desk and my first thoughts were that I wasn't sure code like this would even compile any more. But compile it does, and it works as well.

Can anyone explain why and how this works? The mandate was to provide a function to sort five integer values.

void order5(arr) int *arr; {
    int i,*a,*b,*c,*d,*e;
    a=arr,b=arr+1,c=arr+2,d=arr+3,e=arr+4;
    L1: if(*a >*b){*a^=*b;*b^=*a;*a^=*b;}
    L2: if(*b >*c){*b^=*c;*c^=*b;*b^=*c;goto L1;}
    L3: if(*c >*d){*c^=*d;*d^=*c;*c^=*d;goto L2;}
        if(*d >*e){*d^=*e;*e^=*d;*d^=*e;goto L3;}
}

Now I can see the disadvantages of this approach (lack of readability and maintainability for anyone born after 1970) but can anyone think of any advantages? I'm hesitant to dismiss it out of hand but, before we decide whether or not to bring this person back in for round 2, I'd like to know if it has any redeeming features beyond job security for the author.

Answer 1

It's a fully unrolled bubble sort with the XOR-swap trick expressed inline. I compiled it with several different options hoping it produced some awesome compact code, but it's really not that impressive. I threw in some __restrict__ keywords so that the compiler would know that none of the *a could alias each other, which does help quite a bit. Overall though, I think the attempted cleverness has gone so far outside the norm that the compiler is really not optimizing the code very well at all.

I think the only advantage here is novelty. It certainly caught your eye! I would have been more impressed with abuses of more modern technology, like sorting with MMX/SSE or the GPU, or using 5 threads which all fight it out to try to insert their elements into the right place. Or perhaps an external merge sort, just in case the 5 element array can't fit in core.

Answer 2

The xor trick just swaps two integers. The goto's are the imitation of the loop. Advantages? None at all except for showing off how obfuscated a code you can write. The parameter after function () is a deprecated feature. And having an array on hand and havong 5 distinct pointers pointing at each elem of the array is just horrible. To sum it up: Yuck! :)