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