1
2/* @(#)e_log10.c 1.3 95/01/18 */
3/*
4 * ====================================================
5 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
6 *
7 * Developed at SunSoft, a Sun Microsystems, Inc. business.
8 * Permission to use, copy, modify, and distribute this
9 * software is freely granted, provided that this notice
10 * is preserved.
11 * ====================================================
12 */
13
14#include <sys/cdefs.h>
15__FBSDID("$FreeBSD$");
16
17/*
18 * Return the base 10 logarithm of x. See k_log.c for details on the algorithm.
19 */
20
21#include "math.h"
22#include "math_private.h"
23#include "k_log.h"
24
25static const double
26two54      =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
27ivln10hi   =  4.34294481878168880939e-01, /* 0x3fdbcb7b, 0x15200000 */
28ivln10lo   =  2.50829467116452752298e-11, /* 0x3dbb9438, 0xca9aadd5 */
29log10_2hi  =  3.01029995663611771306e-01, /* 0x3FD34413, 0x509F6000 */
30log10_2lo  =  3.69423907715893078616e-13; /* 0x3D59FEF3, 0x11F12B36 */
31
32static const double zero   =  0.0;
33
34double
35__ieee754_log10(double x)
36{
37	double f,hi,lo,y,z;
38	int32_t i,k,hx;
39	u_int32_t lx;
40
41	EXTRACT_WORDS(hx,lx,x);
42
43        k=0;
44        if (hx < 0x00100000) {                  /* x < 2**-1022  */
45            if (((hx&0x7fffffff)|lx)==0)
46                return -two54/zero;             /* log(+-0)=-inf */
47            if (hx<0) return (x-x)/zero;        /* log(-#) = NaN */
48            k -= 54; x *= two54; /* subnormal number, scale up x */
49	    GET_HIGH_WORD(hx,x);
50        }
51	if (hx >= 0x7ff00000) return x+x;
52	k += (hx>>20)-1023;
53	hx &= 0x000fffff;
54	i = (hx+0x95f64)&0x100000;
55	SET_HIGH_WORD(x,hx|(i^0x3ff00000));	/* normalize x or x/2 */
56	k += (i>>20);
57	y = (double)k;
58	f = __kernel_log(x);
59	hi = x = x - 1;
60	SET_LOW_WORD(hi,0);
61	lo = x - hi;
62	z = y*log10_2lo + (x+f)*ivln10lo + (lo+f)*ivln10hi + hi*ivln10hi;
63	return  z+y*log10_2hi;
64}
65