Friday, August 22, 2003

Duff's device

Periodically while my mind wanders I alight upon Duff's device, one of the cleverest pieces of C code I have ever seen. (Apparently Stroustrup thinks so as well; he included it in his book). I understand perfectly well what the code does, but somehow it still seems elusive every time I see it.

send(to, from, count)
	register short *to, *from;
	register count;
	{
		register n=(count+7)/8;
		switch(count%8){
		case 0:	do{	*to = *from++;
		case 7:		*to = *from++;
		case 6:		*to = *from++;
		case 5:		*to = *from++;
		case 4:		*to = *from++;
		case 3:		*to = *from++;
		case 2:		*to = *from++;
		case 1:		*to = *from++;
			}while(--n>0);
		}
	}

One of the best interview questions I have seen is a slightly obfuscated version of Duff's device with the variable names replaced and all comments unhelpfully removed. The questioner then asks a job applicant, What does this function do? Why? Familiarity with the literature is a great help here.

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