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

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1
2/* @(#)s_cos.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/* cos(x)
15 * Return cosine function of x.
16 *
17 * kernel function:
18 *      __kernel_sin            ... sine function on [-pi/4,pi/4]
19 *      __kernel_cos            ... cosine function on [-pi/4,pi/4]
20 *      __ieee754_rem_pio2      ... argument reduction routine
21 *
22 * Method.
23 *      Let S,C and T denote the sin, cos and tan respectively on
24 *      [-PI/4, +PI/4]. Reduce the argument x to y1+y2 = x-k*pi/2
25 *      in [-pi/4 , +pi/4], and let n = k mod 4.
26 *      We have
27 *
28 *          n        sin(x)      cos(x)        tan(x)
29 *     ----------------------------------------------------------
30 *          0          S           C             T
31 *          1          C          -S            -1/T
32 *          2         -S          -C             T
33 *          3         -C           S            -1/T
34 *     ----------------------------------------------------------
35 *
36 * Special cases:
37 *      Let trig be any of sin, cos, or tan.
38 *      trig(+-INF)  is NaN, with signals;
39 *      trig(NaN)    is that NaN;
40 *
41 * Accuracy:
42 *      TRIG(x) returns trig(x) nearly rounded
43 */
44
45#include <libm/fdlibm.h>
46
47#ifdef __STDC__
48static const double one=1.0;
49#else
50static double one=1.0;
51#endif
52
53#ifdef __STDC__
54        double cos(double x)
55#else
56        double cos(x)
57        double x;
58#endif
59{
60        double y[2],z=0.0;
61        int n, ix;
62
63    /* High word of x. */
64        ix = *( (((*(int*)&one)>>29)^1) + (int*)&x);
65
66    /* |x| ~< pi/4 */
67        ix &= 0x7fffffff;
68        if(ix <= 0x3fe921fb) return __kernel_cos(x,z);
69
70    /* cos(Inf or NaN) is NaN */
71        else if (ix>=0x7ff00000) return x-x;
72
73    /* argument reduction needed */
74        else {
75            n = __ieee754_rem_pio2(x,y);
76            switch(n&3) {
77                case 0: return  __kernel_cos(y[0],y[1]);
78                case 1: return -__kernel_sin(y[0],y[1],1);
79                case 2: return -__kernel_cos(y[0],y[1]);
80                default:
81                        return  __kernel_sin(y[0],y[1],1);
82            }
83        }
84}
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