glibc/sysdeps/libm-ieee754/k_cos.c

/* @(#)k_cos.c 5.1 93/09/24 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice
 * is preserved.
 * ====================================================
 */
/* Modified by Naohiko Shimizu/Tokai University, Japan 1997/08/25,
   for performance improvement on pipelined processors.
*/

#if defined(LIBM_SCCS) && !defined(lint)
static char rcsid[] = "$NetBSD: k_cos.c,v 1.8 1995/05/10 20:46:22 jtc Exp $";
#endif

/*
 * __kernel_cos( x,  y )
 * kernel cos function on [-pi/4, pi/4], pi/4 ~ 0.785398164
 * Input x is assumed to be bounded by ~pi/4 in magnitude.
 * Input y is the tail of x.
 *
 * Algorithm
 *	1. Since cos(-x) = cos(x), we need only to consider positive x.
 *	2. if x < 2^-27 (hx<0x3e400000 0), return 1 with inexact if x!=0.
 *	3. cos(x) is approximated by a polynomial of degree 14 on
 *	   [0,pi/4]
 *		  	                 4            14
 *	   	cos(x) ~ 1 - x*x/2 + C1*x + ... + C6*x
 *	   where the remez error is
 *
 * 	|              2     4     6     8     10    12     14 |     -58
 * 	|cos(x)-(1-.5*x +C1*x +C2*x +C3*x +C4*x +C5*x  +C6*x  )| <= 2
 * 	|    					               |
 *
 * 	               4     6     8     10    12     14
 *	4. let r = C1*x +C2*x +C3*x +C4*x +C5*x  +C6*x  , then
 *	       cos(x) = 1 - x*x/2 + r
 *	   since cos(x+y) ~ cos(x) - sin(x)*y
 *			  ~ cos(x) - x*y,
 *	   a correction term is necessary in cos(x) and hence
 *		cos(x+y) = 1 - (x*x/2 - (r - x*y))
 *	   For better accuracy when x > 0.3, let qx = |x|/4 with
 *	   the last 32 bits mask off, and if x > 0.78125, let qx = 0.28125.
 *	   Then
 *		cos(x+y) = (1-qx) - ((x*x/2-qx) - (r-x*y)).
 *	   Note that 1-qx and (x*x/2-qx) is EXACT here, and the
 *	   magnitude of the latter is at least a quarter of x*x/2,
 *	   thus, reducing the rounding error in the subtraction.
 */

#include "math.h"
#include "math_private.h"

#ifdef __STDC__
static const double
#else
static double
#endif
C[] = {
  1.00000000000000000000e+00, /* 0x3FF00000, 0x00000000 */
  4.16666666666666019037e-02, /* 0x3FA55555, 0x5555554C */
 -1.38888888888741095749e-03, /* 0xBF56C16C, 0x16C15177 */
  2.48015872894767294178e-05, /* 0x3EFA01A0, 0x19CB1590 */
 -2.75573143513906633035e-07, /* 0xBE927E4F, 0x809C52AD */
  2.08757232129817482790e-09, /* 0x3E21EE9E, 0xBDB4B1C4 */
 -1.13596475577881948265e-11}; /* 0xBDA8FAE9, 0xBE8838D4 */

#ifdef __STDC__
	double __kernel_cos(double x, double y)
#else
	double __kernel_cos(x, y)
	double x,y;
#endif
{
	double a,hz,z,r,qx,r1,r2,r3,z1,z2,z3;
	int32_t ix;
	z  = x*x;
	GET_HIGH_WORD(ix,x);
	ix &= 0x7fffffff;			/* ix = |x|'s high word*/
	if(ix<0x3e400000) {			/* if x < 2**27 */
	    if(((int)x)==0) return C[0];	/* generate inexact */
	}
#ifdef DO_NOT_USE_THIS
	r  = z*(C1+z*(C2+z*(C3+z*(C4+z*(C5+z*C6)))));
#else
	r1=z*C[6];r1=r1+C[5];z1=z*z;
	r2=z*C[4];r2=r2+C[3];z2=z1*z;
	r3=z*C[2];r3=r3+C[1];z3=z2*z1;
	r=z3*r1+z2*r2+z*r3;
#endif
	if(ix < 0x3FD33333) 			/* if |x| < 0.3 */
	    return C[0] - (0.5*z - (z*r - x*y));
	else {
	    if(ix > 0x3fe90000) {		/* x > 0.78125 */
		qx = 0.28125;
	    } else {
	        INSERT_WORDS(qx,ix-0x00200000,0);	/* x/4 */
	    }
	    hz = 0.5*z-qx;
	    a  = C[0]-qx;
	    return a - (hz - (z*r-x*y));
	}
}