/* DEC  *AddDecimal(pDst,pSrc1,pSrc2)
 *
 * ARGUMENT
 *      pDst is a pointer to the destination DEC structure.
 *      pSrc1 is a ptr to the source1 DEC structure.
 *      pSrc2 is a ptr to the source2 DEC structure.
 *
 * DESCRIPTION
 *      Adds the value in pSrc1 DEC to the value in pSrc2 DEC
 *   and puts the result in dest DEC structure.  The sum is calculated
 *   to the maximum possible accuracy.  The routine requires MINID,
 *   the minimum allowed implied decimal.
 *
 * SIDE EFFECTS
 *      On overflow, the dest value is indeterminate.
 *
 * RETURNS
 *      Returns a pointer to the dest structure unless overflow,
 *   when it returns a GM_NULL.
 *
 * POSSIBLE ERROR CODES
 *
 *      GM_NULLPOINTER
 *      GM_OVERFLOW
 *
 * AUTHOR
 *  Andy Anderson   13-JAN-1987  14:14
 *   Copyright (C) 1987-1990 Greenleaf Software Inc.  All rights reserved.
 *
 * MODIFICATIONS
 *
 */

#include <stdio.h>
#include "gmsystem.h"

DEC     *AddDecimal(pDst,pSrc1,pSrc2)
DEC     *pDst;
DEC     *pSrc1,*pSrc2;
{
	DEC     t1, t2, *pts;
	register  int    i,j;
	int     s1, s2;

	_MacStart(GM_DADD);

	_MacInVar(pSrc1,GM_NULL);
	_MacInVar(pSrc2,GM_NULL);
	_MacOutVar(pDst,GM_NULL);

/* if the source id's are different, make them equal with as much accuracy
   as is possible, and protect source fields from change */


/* line up decimal places */
	i = pSrc1->dc.id-pSrc2->dc.id;
	if (i)  {
		pts = &t1;
		_MacDCopy(pts, pSrc1);
		pts = &t2;
		_MacDCopy(pts, pSrc2);

		if (i<0)  {
			pSrc1 = &t1;
			pSrc2 = &t2;
			i = -i;
		}
		else  {
			pSrc2 = &t1;
			pSrc1 = &t2;
		}

		j = _MulUnsArrByPwrOf10Limited(
			pSrc1->dc.sl, i, 4);
		pSrc1->dc.id += j;
		if (j<i)
			_DivUnsArrByPwrOf10(pSrc2->dc.sl, 4, i-j);
	}

/* add the numbers */
	pDst->dc.id = pSrc1->dc.id;
	s1 = pSrc1->dc.attr & 0x0001;
	s2 = pSrc2->dc.attr & 0x0001;

	if (s1 == s2)  {
/* add absolute values if signs are equal */
		_AddUnsArrToUnsArr(pSrc1->dc.sl, pSrc2->dc.sl,
			pDst->dc.sl,5);

	/* if the first addition is an overflow, round down and try again */
		if (pDst->dc.sl[3] >= 32768L) {
			if (pSrc1->dc.id <= GM_MINID)  {
				_MacErr(GM_OVERFLOW);
				_MacRet(GM_NULL);
			}
			_DivUnsArrByUns(pDst->dc.sl, 10, 4);
			pDst->dc.id--;
		}

	/* sign of pDst same as sign of sources */
		pDst->dc.attr = s1;
	}

	else  {
/* subtract larger absolute value from smaller if signs are different */
		if (_CompareUnsArr(pSrc1->dc.sl,
			 pSrc2->dc.sl, 4) >= 0)  {
		/* pSrc1 has larger absolute value */
			 (void) _SubUnsArrFromUnsArr(pSrc1->dc.sl,
				pSrc2->dc.sl,
				pDst->dc.sl,5);
		/* sum has same sign as pSrc1 */
			pDst->dc.attr = s1;
		}

		else  {
		/* pSrc2 has larger absolute value */
		(void)  _SubUnsArrFromUnsArr(pSrc2->dc.sl,
				pSrc1->dc.sl,
				pDst->dc.sl,5);
		/* sum has same sign as pSrc2 */
		pDst->dc.attr = s2;
		}
	}

	_MacRet(pDst);
}