134126Sbostic /*
2*61285Sbostic  * Copyright (c) 1985, 1993
3*61285Sbostic  *	The Regents of the University of California.  All rights reserved.
434126Sbostic  *
542657Sbostic  * %sccs.include.redist.c%
624594Szliu  */
724594Szliu 
824594Szliu #ifndef lint
9*61285Sbostic static char sccsid[] = "@(#)exp__E.c	8.1 (Berkeley) 06/04/93";
1034126Sbostic #endif /* not lint */
1124594Szliu 
1224594Szliu /* exp__E(x,c)
1324594Szliu  * ASSUMPTION: c << x  SO THAT  fl(x+c)=x.
1424594Szliu  * (c is the correction term for x)
1524594Szliu  * exp__E RETURNS
1624594Szliu  *
1724594Szliu  *			 /  exp(x+c) - 1 - x ,  1E-19 < |x| < .3465736
1824594Szliu  *       exp__E(x,c) = 	|
1924594Szliu  *			 \  0 ,  |x| < 1E-19.
2024594Szliu  *
2124594Szliu  * DOUBLE PRECISION (IEEE 53 bits, VAX D FORMAT 56 BITS)
2224594Szliu  * KERNEL FUNCTION OF EXP, EXPM1, POW FUNCTIONS
2324594Szliu  * CODED IN C BY K.C. NG, 1/31/85;
2424594Szliu  * REVISED BY K.C. NG on 3/16/85, 4/16/85.
2524594Szliu  *
2624594Szliu  * Required system supported function:
2724594Szliu  *	copysign(x,y)
2824594Szliu  *
2924594Szliu  * Method:
3024594Szliu  *	1. Rational approximation. Let r=x+c.
3124594Szliu  *	   Based on
3224594Szliu  *                                   2 * sinh(r/2)
3324594Szliu  *                exp(r) - 1 =   ----------------------   ,
3424594Szliu  *                               cosh(r/2) - sinh(r/2)
3524594Szliu  *	   exp__E(r) is computed using
3624594Szliu  *                   x*x            (x/2)*W - ( Q - ( 2*P  + x*P ) )
3724594Szliu  *                   --- + (c + x*[---------------------------------- + c ])
3824594Szliu  *                    2                          1 - W
3924594Szliu  * 	   where  P := p1*x^2 + p2*x^4,
4024594Szliu  *	          Q := q1*x^2 + q2*x^4 (for 56 bits precision, add q3*x^6)
4124594Szliu  *	          W := x/2-(Q-x*P),
4224594Szliu  *
4324594Szliu  *	   (See the listing below for the values of p1,p2,q1,q2,q3. The poly-
4424594Szliu  *	    nomials P and Q may be regarded as the approximations to sinh
4524594Szliu  *	    and cosh :
4624594Szliu  *		sinh(r/2) =  r/2 + r * P  ,  cosh(r/2) =  1 + Q . )
4724594Szliu  *
4824594Szliu  *         The coefficients were obtained by a special Remez algorithm.
4924594Szliu  *
5024594Szliu  * Approximation error:
5124594Szliu  *
5224594Szliu  *   |	exp(x) - 1			   |        2**(-57),  (IEEE double)
5324594Szliu  *   | ------------  -  (exp__E(x,0)+x)/x  |  <=
5424594Szliu  *   |	     x			           |	    2**(-69).  (VAX D)
5524594Szliu  *
5624594Szliu  * Constants:
5724594Szliu  * The hexadecimal values are the intended ones for the following constants.
5824594Szliu  * The decimal values may be used, provided that the compiler will convert
5924594Szliu  * from decimal to binary accurately enough to produce the hexadecimal values
6024594Szliu  * shown.
6124594Szliu  */
6224594Szliu 
6335679Sbostic #include "mathimpl.h"
6424594Szliu 
6535679Sbostic vc(p1, 1.5150724356786683059E-2 ,3abe,3d78,066a,67e1,  -6, .F83ABE67E1066A)
6635679Sbostic vc(p2, 6.3112487873718332688E-5 ,5b42,3984,0173,48cd, -13, .845B4248CD0173)
6735679Sbostic vc(q1, 1.1363478204690669916E-1 ,b95a,3ee8,ec45,44a2,  -3, .E8B95A44A2EC45)
6835679Sbostic vc(q2, 1.2624568129896839182E-3 ,7905,3ba5,f5e7,72e4,  -9, .A5790572E4F5E7)
6935679Sbostic vc(q3, 1.5021856115869022674E-6 ,9eb4,36c9,c395,604a, -19, .C99EB4604AC395)
7035679Sbostic 
7135679Sbostic ic(p1, 1.3887401997267371720E-2,  -7, 1.C70FF8B3CC2CF)
7235679Sbostic ic(p2, 3.3044019718331897649E-5, -15, 1.15317DF4526C4)
7335679Sbostic ic(q1, 1.1110813732786649355E-1,  -4, 1.C719538248597)
7435679Sbostic ic(q2, 9.9176615021572857300E-4, -10, 1.03FC4CB8C98E8)
7535679Sbostic 
7635679Sbostic #ifdef vccast
7735679Sbostic #define       p1    vccast(p1)
7835679Sbostic #define       p2    vccast(p2)
7935679Sbostic #define       q1    vccast(q1)
8035679Sbostic #define       q2    vccast(q2)
8135679Sbostic #define       q3    vccast(q3)
8235679Sbostic #endif
8335679Sbostic 
8457452Sbostic double __exp__E(x,c)
8524594Szliu double x,c;
8624594Szliu {
8735679Sbostic 	const static double zero=0.0, one=1.0, half=1.0/2.0, small=1.0E-19;
8835679Sbostic 	double z,p,q,xp,xh,w;
8924594Szliu 	if(copysign(x,one)>small) {
9024594Szliu            z = x*x  ;
9124594Szliu 	   p = z*( p1 +z* p2 );
9231853Szliu #if defined(vax)||defined(tahoe)
9324594Szliu            q = z*( q1 +z*( q2 +z* q3 ));
9431853Szliu #else	/* defined(vax)||defined(tahoe) */
9524594Szliu            q = z*( q1 +z*  q2 );
9631853Szliu #endif	/* defined(vax)||defined(tahoe) */
9724594Szliu            xp= x*p     ;
9824594Szliu 	   xh= x*half  ;
9924594Szliu            w = xh-(q-xp)  ;
10024594Szliu 	   p = p+p;
10124594Szliu 	   c += x*((xh*w-(q-(p+xp)))/(one-w)+c);
10224594Szliu 	   return(z*half+c);
10324594Szliu 	}
10424594Szliu 	/* end of |x| > small */
10524594Szliu 
10624594Szliu 	else {
10724594Szliu 	    if(x!=zero) one+small;	/* raise the inexact flag */
10824594Szliu 	    return(copysign(zero,x));
10924594Szliu 	}
11024594Szliu }
111