s_scalbn.c revision 1.3 
1/* @(#)s_scalbn.c 5.1 93/09/24 */
2/*
3 * ====================================================
4 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
5 *
6 * Developed at SunPro, a Sun Microsystems, Inc. business.
7 * Permission to use, copy, modify, and distribute this
8 * software is freely granted, provided that this notice
9 * is preserved.
10 * ====================================================
11 */
12
13#ifndef lint
14static char rcsid[] = "$Id: s_scalbn.c,v 1.3 1994/02/18 02:26:54 jtc Exp $";
15#endif
16
17/*
18 * scalbn (double x, int n)
19 * scalbn(x,n) returns x* 2**n  computed by  exponent
20 * manipulation rather than by actually performing an
21 * exponentiation or a multiplication.
22 */
23
24#include <math.h>
25
26#ifdef __STDC__
27static const double
28#else
29static double
30#endif
31two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
32twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
33huge   = 1.0e+300,
34tiny   = 1.0e-300;
35
36#ifdef __STDC__
37	double scalbn (double x, int n)
38#else
39	double scalbn (x,n)
40	double x; int n;
41#endif
42{
43	int  k,n0,hx,lx;
44	n0 = ((*(int*)&two54)>>30)^1;		/* high word index */
45	hx = *(n0+(int*)&x);
46	lx = *(1-n0+(int*)&x);
47        k = (hx&0x7ff00000)>>20;		/* extract exponent */
48        if (k==0) {				/* 0 or subnormal x */
49            if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
50	    x *= two54;
51	    hx = *(n0+(int*)&x);
52	    k = ((hx&0x7ff00000)>>20) - 54;
53            if (n< -50000) return tiny*x; 	/*underflow*/
54	    }
55        if (k==0x7ff) return x+x;		/* NaN or Inf */
56        k = k+n;
57        if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
58        if (k > 0) 				/* normal result */
59	    {*(n0+(int*)&x) = (hx&0x800fffff)|(k<<20); return x;}
60        if (k <= -54)
61            if (n > 50000) 	/* in case integer overflow in n+k */
62		return huge*copysign(huge,x);	/*overflow*/
63	    else return tiny*copysign(tiny,x); 	/*underflow*/
64        k += 54;				/* subnormal result */
65        *(n0+(int*)&x) = (hx&0x800fffff)|(k<<20);
66        return x*twom54;
67}
68