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1 
2 /* @(#)s_cos.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 /* ieee_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        ieee_sin(x)      ieee_cos(x)        ieee_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 "fdlibm.h"
46 
47 #ifdef __STDC__
ieee_cos(double x)48 	double ieee_cos(double x)
49 #else
50 	double ieee_cos(x)
51 	double x;
52 #endif
53 {
54 	double y[2],z=0.0;
55 	int n, ix;
56 
57     /* High word of x. */
58 	ix = __HI(x);
59 
60     /* |x| ~< pi/4 */
61 	ix &= 0x7fffffff;
62 	if(ix <= 0x3fe921fb) return __kernel_cos(x,z);
63 
64     /* ieee_cos(Inf or NaN) is NaN */
65 	else if (ix>=0x7ff00000) return x-x;
66 
67     /* argument reduction needed */
68 	else {
69 	    n = __ieee754_rem_pio2(x,y);
70 	    switch(n&3) {
71 		case 0: return  __kernel_cos(y[0],y[1]);
72 		case 1: return -__kernel_sin(y[0],y[1],1);
73 		case 2: return -__kernel_cos(y[0],y[1]);
74 		default:
75 		        return  __kernel_sin(y[0],y[1],1);
76 	    }
77 	}
78 }
79