POK
/home/jaouen/pok_official/pok/trunk/libpok/libm/e_log2.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 
00018 /* @(#)e_log.c 1.3 95/01/18 */
00019 /*
00020  * ====================================================
00021  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
00022  *
00023  * Developed at SunSoft, a Sun Microsystems, Inc. business.
00024  * Permission to use, copy, modify, and distribute this
00025  * software is freely granted, provided that this notice 
00026  * is preserved.
00027  * ====================================================
00028  */
00029 
00030 #ifdef POK_NEEDS_LIBMATH
00031 
00032 #include "math_private.h"
00033 
00034 static const double
00035 ln2 = 0.6931471805599452862268,
00036 two54   =  1.80143985094819840000e+16,  /* 43500000 00000000 */
00037 Lg1 = 6.666666666666735130e-01,  /* 3FE55555 55555593 */
00038 Lg2 = 3.999999999940941908e-01,  /* 3FD99999 9997FA04 */
00039 Lg3 = 2.857142874366239149e-01,  /* 3FD24924 94229359 */
00040 Lg4 = 2.222219843214978396e-01,  /* 3FCC71C5 1D8E78AF */
00041 Lg5 = 1.818357216161805012e-01,  /* 3FC74664 96CB03DE */
00042 Lg6 = 1.531383769920937332e-01,  /* 3FC39A09 D078C69F */
00043 Lg7 = 1.479819860511658591e-01;  /* 3FC2F112 DF3E5244 */
00044 
00045 static const double zero   =  0.0;
00046 
00047 double
00048 __ieee754_log2(double x)
00049 {
00050         double hfsq,f,s,z,R,w,t1,t2,dk;
00051         int32_t k,hx,i,j;
00052         uint32_t lx;
00053 
00054         EXTRACT_WORDS(hx,lx,x);
00055 
00056         k=0;
00057         if (hx < 0x00100000) {                  /* x < 2**-1022  */
00058             if (((hx&0x7fffffff)|lx)==0) 
00059                 return -two54/zero;             /* log(+-0)=-inf */
00060             if (hx<0) return (x-x)/zero;        /* log(-#) = NaN */
00061             k -= 54; x *= two54; /* subnormal number, scale up x */
00062             GET_HIGH_WORD(hx,x);
00063         } 
00064         if (hx >= 0x7ff00000) return x+x;
00065         k += (hx>>20)-1023;
00066         hx &= 0x000fffff;
00067         i = (hx+0x95f64)&0x100000;
00068         SET_HIGH_WORD(x,hx|(i^0x3ff00000));     /* normalize x or x/2 */
00069         k += (i>>20);
00070         f = x-1.0;
00071         dk = (double)k;
00072         if((0x000fffff&(2+hx))<3) {     /* |f| < 2**-20 */
00073             if (f==zero) 
00074                     return (dk);
00075             R = f*f*(0.5-0.33333333333333333*f);
00076             return (dk-(R-f)/ln2);
00077         }
00078         s = f/(2.0+f); 
00079         z = s*s;
00080         i = hx-0x6147a;
00081         w = z*z;
00082         j = 0x6b851-hx;
00083         t1= w*(Lg2+w*(Lg4+w*Lg6)); 
00084         t2= z*(Lg1+w*(Lg3+w*(Lg5+w*Lg7))); 
00085         i |= j;
00086         R = t2+t1;
00087         if(i>0) {
00088             hfsq=0.5*f*f;
00089             return (dk-(hfsq-s*(hfsq+R)-f)/ln2);
00090         } else
00091                 return (dk-((s*(f-R))-f)/ln2);
00092 }
00093 #endif