lib/libm/src/s_cos.c

   1 /* @(#)s_cos.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  * $NetBSD: s_cos.c,v 1.10 2002/05/26 22:01:54 wiz Exp $
  13  * $DragonFly: src/lib/libm/src/s_cos.c,v 1.1 2005/07/26 21:15:20 joerg Exp $
  14  */
  15
  16 /* cos(x)
  17  * Return cosine function of x.
  18  *
  19  * kernel function:
  20  *      __kernel_sin            ... sine function on [-pi/4,pi/4]
  21  *      __kernel_cos            ... cosine function on [-pi/4,pi/4]
  22  *      __libm_rem_pio2         ... argument reduction routine
  23  *
  24  * Method.
  25  *      Let S,C and T denote the sin, cos and tan respectively on
  26  *      [-PI/4, +PI/4]. Reduce the argument x to y1+y2 = x-k*pi/2
  27  *      in [-pi/4 , +pi/4], and let n = k mod 4.
  28  *      We have
  29  *
  30  *          n        sin(x)      cos(x)        tan(x)
  31  *     ----------------------------------------------------------
  32  *          0          S           C             T
  33  *          1          C          -S            -1/T
  34  *          2         -S          -C             T
  35  *          3         -C           S            -1/T
  36  *     ----------------------------------------------------------
  37  *
  38  * Special cases:
  39  *      Let trig be any of sin, cos, or tan.
  40  *      trig(+-INF)  is NaN, with signals;
  41  *      trig(NaN)    is that NaN;
  42  *
  43  * Accuracy:
  44  *      TRIG(x) returns trig(x) nearly rounded
  45  */
  46
  47 #include <math.h>
  48 #include "math_private.h"
  49
  50 double
  51 cos(double x)
  52 {
  53         double y[2],z=0.0;
  54         int32_t n, ix;
  55
  56     /* High word of x. */
  57         GET_HIGH_WORD(ix,x);
  58
  59     /* |x| ~< pi/4 */
  60         ix &= 0x7fffffff;
  61         if(ix <= 0x3fe921fb) return __kernel_cos(x,z);
  62
  63     /* cos(Inf or NaN) is NaN */
  64         else if (ix>=0x7ff00000) return x-x;
  65
  66     /* argument reduction needed */
  67         else {
  68             n = __libm_rem_pio2(x,y);
  69             switch(n&3) {
  70                 case 0: return  __kernel_cos(y[0],y[1]);
  71                 case 1: return -__kernel_sin(y[0],y[1],1);
  72                 case 2: return -__kernel_cos(y[0],y[1]);
  73                 default:
  74                         return  __kernel_sin(y[0],y[1],1);
  75             }
  76         }
  77 }