/* DEC	*ArcCosineDecimal(pDst,pSrc)
 *
 * ARGUMENT
 *	DEC	*pDst;
 *	DEC	*pSrc;
 *
 * DESCRIPTION
 *	Sets pDst = the	arccosine (in radians) of pSrc,
 *  which always lies between 0	and pi
 *
 * RETURNS
 *	Returns	pointer	to pDst	if successful, otherwise a GM_NULL.
 *
 * POSSIBLE ERROR CODES
 *
 *	GM_NULLPOINTER
 *	GM_IMAG		if | pSrc | > 1
 *
 * AUTHOR
 *  Jared Levy		Aug 7, 1987
 *   Copyright (C) 1987-1990 Greenleaf Software	Inc.  All rights reserved.
 *
 * MODIFICATIONS
 *
 */

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

DEC	*ArcCosineDecimal(pDst,pSrc)
DEC	*pDst;
DEC	*pSrc;
{
	int	isn = 0;
	DEC	*nsrc, dnsrc, *temp1, dtemp1, *temp2, dtemp2;
	DEC	*temp3,	dtemp3,	*temp4,	dtemp4;

	_MacStart(GM_DACOS);

	_MacInVar(pSrc,GM_NULL);
	_MacOutVar(pDst,GM_NULL);

	nsrc = &dnsrc;
	_MacDCopy(nsrc,	pSrc);
	if (_MacIsDecN(nsrc))  {
		isn = 1;
		_MacDChgs(nsrc);
		}

	if ((CompareDecimal(nsrc,&decOne)) == 1)  {
		_MacErr(GM_IMAG);
		_MacRet(GM_NULL);
	}

	if (_MacIsDecZ(pSrc))  {
		_MacDCopy(pDst,&decPiOver2);
		_MacRet(pDst);
	}

	(void) _MulDec80Bit(temp1= &dtemp1, nsrc, nsrc);
	(void) _SubDec80Bit(temp2= &dtemp2, &decOne, temp1);
	       _SqrtDec80Bit(temp3=&dtemp3, temp2);

	if (_MacIsDecZ(temp3))			/* arcsin(1) = pi/2 */
		{_MacDZero(pDst);}
	else  {
		(void) _DivDec80Bit(temp4=&dtemp4, temp3, nsrc);
		_ATanDec80Bit(pDst, temp4);
	}

	if (isn)
		(void) _SubDec80Bit(pDst, &decPi, pDst);

	/* reduce from 80-bit back to 64-bit */
	(void) _Sq5UnsTo4Uns(pDst);


	_MacRet(pDst);
}