sysdeps/libm-ieee754/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
  13 #if defined(LIBM_SCCS) && !defined(lint)
  14 static char rcsid[] = "$NetBSD: s_cos.c,v 1.7 1995/05/10 20:47:02 jtc Exp $";
  15 #endif
  16
  17 /* cos(x)
  18  * Return cosine function of x.
  19  *
  20  * kernel function:
  21  *      __kernel_sin            ... sine function on [-pi/4,pi/4]
  22  *      __kernel_cos            ... cosine function on [-pi/4,pi/4]
  23  *      __ieee754_rem_pio2      ... argument reduction routine
  24  *
  25  * Method.
  26  *      Let S,C and T denote the sin, cos and tan respectively on
  27  *      [-PI/4, +PI/4]. Reduce the argument x to y1+y2 = x-k*pi/2
  28  *      in [-pi/4 , +pi/4], and let n = k mod 4.
  29  *      We have
  30  *
  31  *          n        sin(x)      cos(x)        tan(x)
  32  *     ----------------------------------------------------------
  33  *          0          S           C             T
  34  *          1          C          -S            -1/T
  35  *          2         -S          -C             T
  36  *          3         -C           S            -1/T
  37  *     ----------------------------------------------------------
  38  *
  39  * Special cases:
  40  *      Let trig be any of sin, cos, or tan.
  41  *      trig(+-INF)  is NaN, with signals;
  42  *      trig(NaN)    is that NaN;
  43  *
  44  * Accuracy:
  45  *      TRIG(x) returns trig(x) nearly rounded
  46  */
  47
  48 #include "math.h"
  49 #include "math_private.h"
  50
  51 #ifdef __STDC__
  52         double __cos(double x)
  53 #else
  54         double __cos(x)
  55         double x;
  56 #endif
  57 {
  58         double y[2],z=0.0;
  59         int32_t n, ix;
  60
  61     /* High word of x. */
  62         GET_HIGH_WORD(ix,x);
  63
  64     /* |x| ~< pi/4 */
  65         ix &= 0x7fffffff;
  66         if(ix <= 0x3fe921fb) return __kernel_cos(x,z);
  67
  68     /* cos(Inf or NaN) is NaN */
  69         else if (ix>=0x7ff00000) return x-x;
  70
  71     /* argument reduction needed */
  72         else {
  73             n = __ieee754_rem_pio2(x,y);
  74             switch(n&3) {
  75                 case 0: return  __kernel_cos(y[0],y[1]);
  76                 case 1: return -__kernel_sin(y[0],y[1],1);
  77                 case 2: return -__kernel_cos(y[0],y[1]);
  78                 default:
  79                         return  __kernel_sin(y[0],y[1],1);
  80             }
  81         }
  82 }
  83 weak_alias (__cos, cos)