67 lines
1.8 KiB
C
Executable File
67 lines
1.8 KiB
C
Executable File
/* logf.c: Computes the natural log of a 32 bit float as outlined in [1].
|
|
|
|
Copyright (C) 2001, 2002 Jesus Calvino-Fraga, jesusc@ieee.org
|
|
|
|
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 */
|
|
|
|
#include "math.h"
|
|
#include "errno.h"
|
|
|
|
/*Constants 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) _FLOAT_FUNC_REENTRANT
|
|
{
|
|
float Rz;
|
|
float f, z, w, znum, zden, xn;
|
|
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);
|
|
}
|