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72 lines
3.0 KiB
C
72 lines
3.0 KiB
C
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/*
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* IBM Accurate Mathematical Library
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* written by International Business Machines Corp.
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* Copyright (C) 2001 Free Software Foundation
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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 2.1 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 Lesser 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, see <http://www.gnu.org/licenses/>.
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*/
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/************************************************************************/
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/* */
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/* MODULE_NAME:mplog.c */
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/* */
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/* FUNCTIONS: mplog */
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/* */
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/* FILES NEEDED: endian.h mpa.h mplog.h */
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/* mpexp.c */
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/* */
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/* Multi-Precision logarithm function subroutine (for precision p >= 4, */
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/* 2**(-1024) < x < 2**1024) and x is outside of the interval */
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/* [1-2**(-54),1+2**(-54)]. Upon entry, x should be set to the */
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/* multi-precision value of the input and y should be set into a multi- */
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/* precision value of an approximation of log(x) with relative error */
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/* bound of at most 2**(-52). The routine improves the accuracy of y. */
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/* */
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/************************************************************************/
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#include "endian.h"
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#include "mpa.h"
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void __mpexp(mp_no *, mp_no *, int);
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void __mplog(mp_no *x, mp_no *y, int p) {
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#include "mplog.h"
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int i,m;
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#if 0
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int j,k,m1,m2,n;
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double a,b;
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#endif
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static const int mp[33] = {0,0,0,0,0,1,1,2,2,2,2,3,3,3,3,3,3,3,3,
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4,4,4,4,4,4,4,4,4,4,4,4,4,4};
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mp_no mpone = {0,{0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,
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0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,
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0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0}};
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mp_no mpt1,mpt2;
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/* Choose m and initiate mpone */
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m = mp[p]; mpone.e = 1; mpone.d[0]=mpone.d[1]=ONE;
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/* Perform m newton iterations to solve for y: exp(y)-x=0. */
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/* The iterations formula is: y(n+1)=y(n)+(x*exp(-y(n))-1). */
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__cpy(y,&mpt1,p);
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for (i=0; i<m; i++) {
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mpt1.d[0]=-mpt1.d[0];
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__mpexp(&mpt1,&mpt2,p);
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__mul(x,&mpt2,&mpt1,p);
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__sub(&mpt1,&mpone,&mpt2,p);
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__add(y,&mpt2,&mpt1,p);
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__cpy(&mpt1,y,p);
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}
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return;
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}
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