github.com/afumu/libc@v0.0.6/musl/src/math/__sin.c (about)

     1  /* origin: FreeBSD /usr/src/lib/msun/src/k_sin.c */
     2  /*
     3   * ====================================================
     4   * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
     5   *
     6   * Developed at SunSoft, 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  /* __sin( x, y, iy)
    13   * kernel sin function on ~[-pi/4, pi/4] (except on -0), pi/4 ~ 0.7854
    14   * Input x is assumed to be bounded by ~pi/4 in magnitude.
    15   * Input y is the tail of x.
    16   * Input iy indicates whether y is 0. (if iy=0, y assume to be 0).
    17   *
    18   * Algorithm
    19   *      1. Since sin(-x) = -sin(x), we need only to consider positive x.
    20   *      2. Callers must return sin(-0) = -0 without calling here since our
    21   *         odd polynomial is not evaluated in a way that preserves -0.
    22   *         Callers may do the optimization sin(x) ~ x for tiny x.
    23   *      3. sin(x) is approximated by a polynomial of degree 13 on
    24   *         [0,pi/4]
    25   *                               3            13
    26   *              sin(x) ~ x + S1*x + ... + S6*x
    27   *         where
    28   *
    29   *      |sin(x)         2     4     6     8     10     12  |     -58
    30   *      |----- - (1+S1*x +S2*x +S3*x +S4*x +S5*x  +S6*x   )| <= 2
    31   *      |  x                                               |
    32   *
    33   *      4. sin(x+y) = sin(x) + sin'(x')*y
    34   *                  ~ sin(x) + (1-x*x/2)*y
    35   *         For better accuracy, let
    36   *                   3      2      2      2      2
    37   *              r = x *(S2+x *(S3+x *(S4+x *(S5+x *S6))))
    38   *         then                   3    2
    39   *              sin(x) = x + (S1*x + (x *(r-y/2)+y))
    40   */
    41  
    42  #include "libm.h"
    43  
    44  static const double
    45  S1  = -1.66666666666666324348e-01, /* 0xBFC55555, 0x55555549 */
    46  S2  =  8.33333333332248946124e-03, /* 0x3F811111, 0x1110F8A6 */
    47  S3  = -1.98412698298579493134e-04, /* 0xBF2A01A0, 0x19C161D5 */
    48  S4  =  2.75573137070700676789e-06, /* 0x3EC71DE3, 0x57B1FE7D */
    49  S5  = -2.50507602534068634195e-08, /* 0xBE5AE5E6, 0x8A2B9CEB */
    50  S6  =  1.58969099521155010221e-10; /* 0x3DE5D93A, 0x5ACFD57C */
    51  
    52  double __sin(double x, double y, int iy)
    53  {
    54  	double_t z,r,v,w;
    55  
    56  	z = x*x;
    57  	w = z*z;
    58  	r = S2 + z*(S3 + z*S4) + z*w*(S5 + z*S6);
    59  	v = z*x;
    60  	if (iy == 0)
    61  		return x + v*(S1 + z*r);
    62  	else
    63  		return x - ((z*(0.5*y - v*r) - y) - v*S1);
    64  }