source: trunk/sys/libm/s_tan.c @ 1

Last change on this file since 1 was 1, checked in by alain, 5 years ago

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[1]1
2/* @(#)s_tan.c 5.1 93/09/24 */
3/*
4 * ====================================================
5 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
6 *
7 * Developed at SunPro, 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/* tan(x)
15 * Return tangent function of x.
16 *
17 * kernel function:
18 *      __kernel_tan            ... tangent function on [-pi/4,pi/4]
19 *      __ieee754_rem_pio2      ... argument reduction routine
20 *
21 * Method.
22 *      Let S,C and T denote the sin, cos and tan respectively on
23 *      [-PI/4, +PI/4]. Reduce the argument x to y1+y2 = x-k*pi/2
24 *      in [-pi/4 , +pi/4], and let n = k mod 4.
25 *      We have
26 *
27 *          n        sin(x)      cos(x)        tan(x)
28 *     ----------------------------------------------------------
29 *          0          S           C             T
30 *          1          C          -S            -1/T
31 *          2         -S          -C             T
32 *          3         -C           S            -1/T
33 *     ----------------------------------------------------------
34 *
35 * Special cases:
36 *      Let trig be any of sin, cos, or tan.
37 *      trig(+-INF)  is NaN, with signals;
38 *      trig(NaN)    is that NaN;
39 *
40 * Accuracy:
41 *      TRIG(x) returns trig(x) nearly rounded
42 */
43
44#include <libm/fdlibm.h>
45
46#ifdef __STDC__
47static const double one=1.0;
48#else
49static double one=1.0;
50#endif
51
52#ifdef __STDC__
53        double tan(double x)
54#else
55        double tan(x)
56        double x;
57#endif
58{
59        double y[2],z=0.0;
60        int n, ix;
61
62    /* High word of x. */
63        ix = *( (((*(int*)&one)>>29)^1) + (int*)&x);
64
65    /* |x| ~< pi/4 */
66        ix &= 0x7fffffff;
67        if(ix <= 0x3fe921fb) return __kernel_tan(x,z,1);
68
69    /* tan(Inf or NaN) is NaN */
70        else if (ix>=0x7ff00000) return x-x;            /* NaN */
71
72    /* argument reduction needed */
73        else {
74            n = __ieee754_rem_pio2(x,y);
75            return __kernel_tan(y[0],y[1],1-((n&1)<<1)); /*   1 -- n even
76                                                        -1 -- n odd */
77        }
78}
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