logf.c

sdcc是为51等小型嵌入式cpu设计的c语言编译器支持数种不同类型的cpu

/*  logf.c: Computes the natural log of a 32 bit float as outlined in [1].

    Copyright (C) 2001, 2002  Jesus Calvino-Fraga, [e-mail] 

    This library 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 2.1 of the License, or (at your option) any later version.

    This library 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
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA */

/* [1] William James Cody and W.  M.  Waite.  _Software manual for the
   elementary functions_, Englewood Cliffs, N.J.:Prentice-Hall, 1980. */

/* Version 1.0 - Initial release */

/*
** $Id: logf.c 3654 2005-01-24 10:38:53Z vrokas $
*/

#include <float.h>
#include <math.h>
#include <errno.h>

/*Constans for 24 bits or less (8 decimal digits)*/
#define A0 -0.5527074855E+0
#define B0 -0.6632718214E+1
#define A(w) (A0)
#define B(w) (w+B0)

#define C0  0.70710678118654752440
#define C1  0.693359375 /*355.0/512.0*/
#define C2 -2.121944400546905827679E-4

float logf(const float x) _MATH_REENTRANT
{
    float Rz;
    float f, z, w, znum, zden, xn;
    FS_STATIC int n;

    if (x<=0.0)
    {
        errno=EDOM;
        return 0.0;
    }
    f=frexpf(x, &n);
    znum=f-0.5;
    if (f>C0)
    {
        znum-=0.5;
        zden=(f*0.5)+0.5;
    }
    else
    {
        n--;
        zden=znum*0.5+0.5;
    }
    z=znum/zden;
    w=z*z;

    Rz=z+z*(w*A(w)/B(w));
    xn=n;
    return ((xn*C2+Rz)+xn*C1);
}