src/libm/k_cos.c
author Philipp Wiesemann <philipp.wiesemann@arcor.de>
Sun, 28 Dec 2014 22:00:24 +0100
changeset 9301 7377a9a3aed6
parent 6044 35448a5ea044
child 11683 48bcba563d9c
permissions -rw-r--r--
Renamed README-emscripten.txt to README-emscripten.md.
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/* @(#)k_cos.c 5.1 93/09/24 */
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/*
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 * ====================================================
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 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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 *
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 * Developed at SunPro, a Sun Microsystems, Inc. business.
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 * Permission to use, copy, modify, and distribute this
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 * software is freely granted, provided that this notice
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 * is preserved.
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 * ====================================================
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 */
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#if defined(LIBM_SCCS) && !defined(lint)
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static const char rcsid[] =
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    "$NetBSD: k_cos.c,v 1.8 1995/05/10 20:46:22 jtc Exp $";
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#endif
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/*
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 * __kernel_cos( x,  y )
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 * kernel cos function on [-pi/4, pi/4], pi/4 ~ 0.785398164
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 * Input x is assumed to be bounded by ~pi/4 in magnitude.
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 * Input y is the tail of x.
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 *
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 * Algorithm
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 *	1. Since cos(-x) = cos(x), we need only to consider positive x.
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 *	2. if x < 2^-27 (hx<0x3e400000 0), return 1 with inexact if x!=0.
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 *	3. cos(x) is approximated by a polynomial of degree 14 on
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 *	   [0,pi/4]
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 *		  	                 4            14
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 *	   	cos(x) ~ 1 - x*x/2 + C1*x + ... + C6*x
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 *	   where the remez error is
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 *
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 * 	|              2     4     6     8     10    12     14 |     -58
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 * 	|cos(x)-(1-.5*x +C1*x +C2*x +C3*x +C4*x +C5*x  +C6*x  )| <= 2
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 * 	|    					               |
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 *
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 * 	               4     6     8     10    12     14
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 *	4. let r = C1*x +C2*x +C3*x +C4*x +C5*x  +C6*x  , then
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 *	       cos(x) = 1 - x*x/2 + r
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 *	   since cos(x+y) ~ cos(x) - sin(x)*y
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 *			  ~ cos(x) - x*y,
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 *	   a correction term is necessary in cos(x) and hence
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 *		cos(x+y) = 1 - (x*x/2 - (r - x*y))
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 *	   For better accuracy when x > 0.3, let qx = |x|/4 with
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 *	   the last 32 bits mask off, and if x > 0.78125, let qx = 0.28125.
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 *	   Then
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 *		cos(x+y) = (1-qx) - ((x*x/2-qx) - (r-x*y)).
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 *	   Note that 1-qx and (x*x/2-qx) is EXACT here, and the
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 *	   magnitude of the latter is at least a quarter of x*x/2,
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 *	   thus, reducing the rounding error in the subtraction.
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 */
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#include "math_libm.h"
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#include "math_private.h"
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#ifdef __STDC__
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static const double
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#else
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static double
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#endif
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  one = 1.00000000000000000000e+00,     /* 0x3FF00000, 0x00000000 */
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    C1 = 4.16666666666666019037e-02,    /* 0x3FA55555, 0x5555554C */
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    C2 = -1.38888888888741095749e-03,   /* 0xBF56C16C, 0x16C15177 */
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    C3 = 2.48015872894767294178e-05,    /* 0x3EFA01A0, 0x19CB1590 */
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    C4 = -2.75573143513906633035e-07,   /* 0xBE927E4F, 0x809C52AD */
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    C5 = 2.08757232129817482790e-09,    /* 0x3E21EE9E, 0xBDB4B1C4 */
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    C6 = -1.13596475577881948265e-11;   /* 0xBDA8FAE9, 0xBE8838D4 */
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#ifdef __STDC__
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double attribute_hidden
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__kernel_cos(double x, double y)
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#else
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double attribute_hidden
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__kernel_cos(x, y)
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     double x, y;
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#endif
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{
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    double a, hz, z, r, qx;
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    int32_t ix;
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    GET_HIGH_WORD(ix, x);
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    ix &= 0x7fffffff;           /* ix = |x|'s high word */
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    if (ix < 0x3e400000) {      /* if x < 2**27 */
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        if (((int) x) == 0)
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            return one;         /* generate inexact */
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    }
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    z = x * x;
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    r = z * (C1 + z * (C2 + z * (C3 + z * (C4 + z * (C5 + z * C6)))));
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    if (ix < 0x3FD33333)        /* if |x| < 0.3 */
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        return one - (0.5 * z - (z * r - x * y));
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    else {
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        if (ix > 0x3fe90000) {  /* x > 0.78125 */
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            qx = 0.28125;
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        } else {
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            INSERT_WORDS(qx, ix - 0x00200000, 0);       /* x/4 */
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        }
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        hz = 0.5 * z - qx;
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        a = one - qx;
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        return a - (hz - (z * r - x * y));
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    }
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}