81 lines
2.1 KiB
C
81 lines
2.1 KiB
C
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/* @(#)e_log.c 1.3 95/01/18 */
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/*
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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*
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* Developed at SunSoft, a Sun Microsystems, Inc. business.
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* Permission to use, copy, modify, and distribute this
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* software is freely granted, provided that this notice
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* is preserved.
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* ====================================================
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*/
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#include <sys/cdefs.h>
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#if defined(LIBM_SCCS) && !defined(lint)
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__RCSID("$NetBSD: e_log2.c,v 1.1 2005/07/21 12:55:58 christos Exp $");
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#endif
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#include "math.h"
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#include "math_private.h"
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static const double
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ln2 = 0.6931471805599452862268,
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two54 = 1.80143985094819840000e+16, /* 43500000 00000000 */
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Lg1 = 6.666666666666735130e-01, /* 3FE55555 55555593 */
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Lg2 = 3.999999999940941908e-01, /* 3FD99999 9997FA04 */
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Lg3 = 2.857142874366239149e-01, /* 3FD24924 94229359 */
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Lg4 = 2.222219843214978396e-01, /* 3FCC71C5 1D8E78AF */
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Lg5 = 1.818357216161805012e-01, /* 3FC74664 96CB03DE */
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Lg6 = 1.531383769920937332e-01, /* 3FC39A09 D078C69F */
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Lg7 = 1.479819860511658591e-01; /* 3FC2F112 DF3E5244 */
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static const double zero = 0.0;
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double
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__ieee754_log2(double x)
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{
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double hfsq,f,s,z,R,w,t1,t2,dk;
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int32_t k,hx,i,j;
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uint32_t lx;
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EXTRACT_WORDS(hx,lx,x);
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k=0;
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if (hx < 0x00100000) { /* x < 2**-1022 */
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if (((hx&0x7fffffff)|lx)==0)
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return -two54/zero; /* log(+-0)=-inf */
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if (hx<0) return (x-x)/zero; /* log(-#) = NaN */
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k -= 54; x *= two54; /* subnormal number, scale up x */
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GET_HIGH_WORD(hx,x);
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}
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if (hx >= 0x7ff00000) return x+x;
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k += (hx>>20)-1023;
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hx &= 0x000fffff;
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i = (hx+0x95f64)&0x100000;
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SET_HIGH_WORD(x,hx|(i^0x3ff00000)); /* normalize x or x/2 */
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k += (i>>20);
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f = x-1.0;
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dk = (double)k;
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if((0x000fffff&(2+hx))<3) { /* |f| < 2**-20 */
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if (f==zero)
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return (dk);
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R = f*f*(0.5-0.33333333333333333*f);
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return (dk-(R-f)/ln2);
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}
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s = f/(2.0+f);
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z = s*s;
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i = hx-0x6147a;
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w = z*z;
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j = 0x6b851-hx;
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t1= w*(Lg2+w*(Lg4+w*Lg6));
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t2= z*(Lg1+w*(Lg3+w*(Lg5+w*Lg7)));
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i |= j;
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R = t2+t1;
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if(i>0) {
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hfsq=0.5*f*f;
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return (dk-(hfsq-s*(hfsq+R)-f)/ln2);
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} else
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return (dk-((s*(f-R))-f)/ln2);
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}
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