/* @(#)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.

 * ====================================================

 */

#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

  one = 1.00000000000000000000e+00,     /* 0x3FF00000, 0x00000000 */

    C1 = 4.16666666666666019037e-02,    /* 0x3FA55555, 0x5555554C */

    C2 = -1.38888888888741095749e-03,   /* 0xBF56C16C, 0x16C15177 */

    C3 = 2.48015872894767294178e-05,    /* 0x3EFA01A0, 0x19CB1590 */

    C4 = -2.75573143513906633035e-07,   /* 0xBE927E4F, 0x809C52AD */

    C5 = 2.08757232129817482790e-09,    /* 0x3E21EE9E, 0xBDB4B1C4 */

    C6 = -1.13596475577881948265e-11;   /* 0xBDA8FAE9, 0xBE8838D4 */

#ifdef __STDC__

double attribute_hidden

__kernel_cos(double x, double y)

#else

double attribute_hidden

__kernel_cos(x, y)

     double x, y;

#endif

{

    double a, hz, z, r, qx;

    int32_t ix;

    GET_HIGH_WORD(ix, x);

    ix &= 0x7fffffff;           /* ix = |x|'s high word */

    if (ix < 0x3e400000) {      /* if x < 2**27 */

        if (((int) x) == 0)

            return one;         /* generate inexact */

    }

    z = x * x;

    r = z * (C1 + z * (C2 + z * (C3 + z * (C4 + z * (C5 + z * C6)))));

    if (ix < 0x3FD33333)        /* if |x| < 0.3 */

        return one - (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 = one - qx;

        return a - (hz - (z * r - x * y));

    }

}