gonzui

    1: /* e_logf.c -- float version of e_log.c.
    2:  * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
    3:  */
    4: 
    5: /*
    6:  * ====================================================
    7:  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
    8:  *
    9:  * Developed at SunPro, a Sun Microsystems, Inc. business.
   10:  * Permission to use, copy, modify, and distribute this
   11:  * software is freely granted, provided that this notice 
   12:  * is preserved.
   13:  * ====================================================
   14:  */
   15: 
   16: #include "math.h"
   17: #include "math_private.h"
   18: 
   19: static const float
   20: ln2_hi =   6.9313812256e-01,    /* 0x3f317180 */
   21: ln2_lo =   9.0580006145e-06,    /* 0x3717f7d1 */
   22: two25 =    3.355443200e+07,     /* 0x4c000000 */
   23: Lg1 = 6.6666668653e-01, /* 3F2AAAAB */
   24: Lg2 = 4.0000000596e-01, /* 3ECCCCCD */
   25: Lg3 = 2.8571429849e-01, /* 3E924925 */
   26: Lg4 = 2.2222198546e-01, /* 3E638E29 */
   27: Lg5 = 1.8183572590e-01, /* 3E3A3325 */
   28: Lg6 = 1.5313838422e-01, /* 3E1CD04F */
   29: Lg7 = 1.4798198640e-01; /* 3E178897 */
   30: 
   31: static const float zero   =  0.0;
   32: 
   33: float
   34: logf(float x)
   35: {
   36:         float hfsq,f,s,z,R,w,t1,t2,dk;
   37:         int32_t k,ix,i,j;
   38: 
   39:         GET_FLOAT_WORD(ix,x);
   40: 
   41:         k=0;
   42:         if (ix < 0x00800000) {                 /* x < 2**-126  */
   43:             if ((ix&0x7fffffff)==0) 
   44:                 return -two25/zero;           /* log(+-0)=-inf */
   45:             if (ix<0) return (x-x)/zero;       /* log(-#) = NaN */
   46:             k -= 25; x *= two25; /* subnormal number, scale up x */
   47:             GET_FLOAT_WORD(ix,x);
   48:         } 
   49:         if (ix >= 0x7f800000) return x+x;
   50:         k += (ix>>23)-127;
   51:         ix &= 0x007fffff;
   52:         i = (ix+(0x95f64<<3))&0x800000;
   53:         SET_FLOAT_WORD(x,ix|(i^0x3f800000));   /* normalize x or x/2 */
   54:         k += (i>>23);
   55:         f = x-(float)1.0;
   56:         if((0x007fffff&(15+ix))<16) {  /* |f| < 2**-20 */
   57:             if(f==zero) if(k==0) return zero;  else {dk=(float)k;
   58:                                  return dk*ln2_hi+dk*ln2_lo;}
   59:             R = f*f*((float)0.5-(float)0.33333333333333333*f);
   60:             if(k==0) return f-R; else {dk=(float)k;
   61:                     return dk*ln2_hi-((R-dk*ln2_lo)-f);}
   62:         }
   63:         s = f/((float)2.0+f); 
   64:         dk = (float)k;
   65:         z = s*s;
   66:         i = ix-(0x6147a<<3);
   67:         w = z*z;
   68:         j = (0x6b851<<3)-ix;
   69:         t1= w*(Lg2+w*(Lg4+w*Lg6)); 
   70:         t2= z*(Lg1+w*(Lg3+w*(Lg5+w*Lg7))); 
   71:         i |= j;
   72:         R = t2+t1;
   73:         if(i>0) {
   74:             hfsq=(float)0.5*f*f;
   75:             if(k==0) return f-(hfsq-s*(hfsq+R)); else
   76:                      return dk*ln2_hi-((hfsq-(s*(hfsq+R)+dk*ln2_lo))-f);
   77:         } else {
   78:             if(k==0) return f-s*(f-R); else
   79:                      return dk*ln2_hi-((s*(f-R)-dk*ln2_lo)-f);
   80:         }
   81: }