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/*
* IBM Accurate Mathematical Library
* Copyright (c) International Business Machines Corp., 2001
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
/*************************************************************************/
/* MODULE_NAME:slowpow.c */
/* */
/* FUNCTION:slowpow */
/* */
/*FILES NEEDED:mpa.h */
/* mpa.c mpexp.c mplog.c halfulp.c */
/* */
/* Given two IEEE double machine numbers y,x , routine computes the */
/* correctly rounded (to nearest) value of x^y. Result calculated by */
/* multiplication (in halfulp.c) or if result isn't accurate enough */
/* then routine converts x and y into multi-precision doubles and */
/* calls to mpexp routine */
/*************************************************************************/
#include "mpa.h"
void mpexp(mp_no *x, mp_no *y, int p);
void mplog(mp_no *x, mp_no *y, int p);
double ulog(double);
double halfulp(double x,double y);
double slowpow(double x, double y, double z) {
double res,res1;
mp_no mpx, mpy, mpz,mpw,mpp,mpr,mpr1;
static const mp_no eps = {-3,1.0,4.0};
int p;
res = halfulp(x,y); /* halfulp() returns -10 or x^y */
if (res >= 0) return res; /* if result was really computed by halfulp */
/* else, if result was not really computed by halfulp */
p = 10; /* p=precision */
dbl_mp(x,&mpx,p);
dbl_mp(y,&mpy,p);
dbl_mp(z,&mpz,p);
mplog(&mpx, &mpz, p); /* log(x) = z */
mul(&mpy,&mpz,&mpw,p); /* y * z =w */
mpexp(&mpw, &mpp, p); /* e^w =pp */
add(&mpp,&eps,&mpr,p); /* pp+eps =r */
mp_dbl(&mpr, &res, p);
sub(&mpp,&eps,&mpr1,p); /* pp -eps =r1 */
mp_dbl(&mpr1, &res1, p); /* converting into double precision */
if (res == res1) return res;
p = 32; /* if we get here result wasn't calculated exactly, continue */
dbl_mp(x,&mpx,p); /* for more exact calculation */
dbl_mp(y,&mpy,p);
dbl_mp(z,&mpz,p);
mplog(&mpx, &mpz, p); /* log(c)=z */
mul(&mpy,&mpz,&mpw,p); /* y*z =w */
mpexp(&mpw, &mpp, p); /* e^w=pp */
mp_dbl(&mpp, &res, p); /* converting into double precision */
return res;
}
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