POK
/home/jaouen/pok_official/pok/trunk/libpok/libm/e_log2f.c
00001 /*
00002  *                               POK header
00003  * 
00004  * The following file is a part of the POK project. Any modification should
00005  * made according to the POK licence. You CANNOT use this file or a part of
00006  * this file is this part of a file for your own project
00007  *
00008  * For more information on the POK licence, please see our LICENCE FILE
00009  *
00010  * Please follow the coding guidelines described in doc/CODING_GUIDELINES
00011  *
00012  *                                      Copyright (c) 2007-2009 POK team 
00013  *
00014  * Created by julien on Fri Jan 30 14:41:34 2009 
00015  */
00016 
00017 /* e_logf.c -- float version of e_log.c.
00018  * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
00019  */
00020 
00021 /*
00022  * ====================================================
00023  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
00024  *
00025  * Developed at SunPro, a Sun Microsystems, Inc. business.
00026  * Permission to use, copy, modify, and distribute this
00027  * software is freely granted, provided that this notice
00028  * is preserved.
00029  * ====================================================
00030  */
00031 
00032 #ifdef POK_NEEDS_LIBMATH
00033 
00034 #include "math_private.h"
00035 
00036 static const float
00037 ln2 = 0.6931471805599452862268, 
00038 two25 =    3.355443200e+07,     /* 0x4c000000 */
00039 Lg1 = 6.6666668653e-01, /* 3F2AAAAB */
00040 Lg2 = 4.0000000596e-01, /* 3ECCCCCD */
00041 Lg3 = 2.8571429849e-01, /* 3E924925 */
00042 Lg4 = 2.2222198546e-01, /* 3E638E29 */
00043 Lg5 = 1.8183572590e-01, /* 3E3A3325 */
00044 Lg6 = 1.5313838422e-01, /* 3E1CD04F */
00045 Lg7 = 1.4798198640e-01; /* 3E178897 */
00046 
00047 static const float zero   =  0.0;
00048 
00049 float
00050 __ieee754_log2f(float x)
00051 {
00052         float hfsq,f,s,z,R,w,t1,t2,dk;
00053         int32_t k,ix,i,j;
00054 
00055         GET_FLOAT_WORD(ix,x);
00056 
00057         k=0;
00058         if (ix < 0x00800000) {                  /* x < 2**-126  */
00059             if ((ix&0x7fffffff)==0)
00060                 return -two25/zero;             /* log(+-0)=-inf */
00061             if (ix<0) return (x-x)/zero;        /* log(-#) = NaN */
00062             k -= 25; x *= two25; /* subnormal number, scale up x */
00063             GET_FLOAT_WORD(ix,x);
00064         }
00065         if (ix >= 0x7f800000) return x+x;
00066         k += (ix>>23)-127;
00067         ix &= 0x007fffff;
00068         i = (ix+(0x95f64<<3))&0x800000;
00069         SET_FLOAT_WORD(x,ix|(i^0x3f800000));    /* normalize x or x/2 */
00070         k += (i>>23);
00071         dk = (float)k;
00072         f = x-(float)1.0;
00073         if((0x007fffff&(15+ix))<16) {   /* |f| < 2**-20 */
00074             if (f==zero) 
00075                     return (dk);
00076             R = f*f*((float)0.5-(float)0.33333333333333333*f);
00077             return (dk-(R-f)/ln2);
00078         }
00079         s = f/((float)2.0+f);
00080         z = s*s;
00081         i = ix-(0x6147a<<3);
00082         w = z*z;
00083         j = (0x6b851<<3)-ix;
00084         t1= w*(Lg2+w*(Lg4+w*Lg6));
00085         t2= z*(Lg1+w*(Lg3+w*(Lg5+w*Lg7)));
00086         i |= j;
00087         R = t2+t1;
00088         if(i>0) {
00089             hfsq=(float)0.5*f*f;
00090             return (dk-(hfsq-s*(hfsq+R)-f)/ln2);
00091         } else
00092                 return (dk-((s*(f-R))-f)/ln2);
00093 }
00094 #endif