checking for overflow in C

[Paul Hudson]

This does it, slowly, and is really over cautious. Typedefs and constants
self-explanatory, I hope.

Feel free to criticise, but constructively, please!

Paul Hudson	 MAIL: Monotype ADG, Science Park, Cambridge, CB4 4FQ, UK.
		PHONE: +44 (223) 420018	  EMAIL: paul at moncam.co.uk,
	;"	  FAX: +44 (223) 420911		 ...!ukc!acorn!moncam!paul
 `"";";"        "/dev/null full: please empty the bit bucket"

	Integer a,b;
	unsigned long hi_a, lo_a, hi_b, lo_b;
	Integer result, temp = 0;
	logical minus = FALSE;
	logical overflowed = FALSE;

	a = one->value.integer;
	b = two->value.integer;
	if (a < 0)
	{
	    if (a != MIN_INTEGER) a = (-a);
	    minus = TRUE;
	}
	if (b < 0)
	{
	    if (b != MIN_INTEGER) b = (-b);
	    minus = !minus;
	}

	hi_a  = (unsigned long)a >> 16;
	lo_a  = (unsigned long)a & ( (1<<16) -1);
	hi_b  = (unsigned long)b >> 16;
	lo_b  = (unsigned long)b & ( (1<<16) -1);

	if (hi_a == 0)
	{
	    temp = hi_b * lo_a;
	}
	else if (hi_b == 0)
	{
	    temp = hi_a * lo_b;
	}
	else overflowed = TRUE;
	if (temp >= (1<<15)) overflowed = TRUE;
	result = lo_a * lo_b;
	if (result < 0) overflowed = TRUE;
	result += (temp << 16);
	if (result < 0) overflowed = TRUE;
	result = minus ? -result : result;
-- 
Paul Hudson	 MAIL: Monotype ADG, Science Park, Cambridge, CB4 4FQ, UK.
		PHONE: +44 (223) 420018	  EMAIL: paul at moncam.co.uk,
	;"	  FAX: +44 (223) 420911		 ...!ukc!acorn!moncam!paul
 `"";";"        "/dev/null full: please empty the bit bucket"