e_asin.c revision 97407 
1/* @(#)e_asin.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#ifndef lint
14static char rcsid[] = "$FreeBSD: head/lib/msun/src/e_asin.c 97407 2002-05-28 17:03:12Z alfred $";
15#endif
16
17/* __ieee754_asin(x)
18 * Method :
19 *	Since  asin(x) = x + x^3/6 + x^5*3/40 + x^7*15/336 + ...
20 *	we approximate asin(x) on [0,0.5] by
21 *		asin(x) = x + x*x^2*R(x^2)
22 *	where
23 *		R(x^2) is a rational approximation of (asin(x)-x)/x^3
24 *	and its remez error is bounded by
25 *		|(asin(x)-x)/x^3 - R(x^2)| < 2^(-58.75)
26 *
27 *	For x in [0.5,1]
28 *		asin(x) = pi/2-2*asin(sqrt((1-x)/2))
29 *	Let y = (1-x), z = y/2, s := sqrt(z), and pio2_hi+pio2_lo=pi/2;
30 *	then for x>0.98
31 *		asin(x) = pi/2 - 2*(s+s*z*R(z))
32 *			= pio2_hi - (2*(s+s*z*R(z)) - pio2_lo)
33 *	For x<=0.98, let pio4_hi = pio2_hi/2, then
34 *		f = hi part of s;
35 *		c = sqrt(z) - f = (z-f*f)/(s+f) 	...f+c=sqrt(z)
36 *	and
37 *		asin(x) = pi/2 - 2*(s+s*z*R(z))
38 *			= pio4_hi+(pio4-2s)-(2s*z*R(z)-pio2_lo)
39 *			= pio4_hi+(pio4-2f)-(2s*z*R(z)-(pio2_lo+2c))
40 *
41 * Special cases:
42 *	if x is NaN, return x itself;
43 *	if |x|>1, return NaN with invalid signal.
44 *
45 */
46
47
48#include "math.h"
49#include "math_private.h"
50
51static const double
52one =  1.00000000000000000000e+00, /* 0x3FF00000, 0x00000000 */
53huge =  1.000e+300,
54pio2_hi =  1.57079632679489655800e+00, /* 0x3FF921FB, 0x54442D18 */
55pio2_lo =  6.12323399573676603587e-17, /* 0x3C91A626, 0x33145C07 */
56pio4_hi =  7.85398163397448278999e-01, /* 0x3FE921FB, 0x54442D18 */
57	/* coefficient for R(x^2) */
58pS0 =  1.66666666666666657415e-01, /* 0x3FC55555, 0x55555555 */
59pS1 = -3.25565818622400915405e-01, /* 0xBFD4D612, 0x03EB6F7D */
60pS2 =  2.01212532134862925881e-01, /* 0x3FC9C155, 0x0E884455 */
61pS3 = -4.00555345006794114027e-02, /* 0xBFA48228, 0xB5688F3B */
62pS4 =  7.91534994289814532176e-04, /* 0x3F49EFE0, 0x7501B288 */
63pS5 =  3.47933107596021167570e-05, /* 0x3F023DE1, 0x0DFDF709 */
64qS1 = -2.40339491173441421878e+00, /* 0xC0033A27, 0x1C8A2D4B */
65qS2 =  2.02094576023350569471e+00, /* 0x40002AE5, 0x9C598AC8 */
66qS3 = -6.88283971605453293030e-01, /* 0xBFE6066C, 0x1B8D0159 */
67qS4 =  7.70381505559019352791e-02; /* 0x3FB3B8C5, 0xB12E9282 */
68
69double
70__generic___ieee754_asin(double x)
71{
72	double t=0.0,w,p,q,c,r,s;
73	int32_t hx,ix;
74	GET_HIGH_WORD(hx,x);
75	ix = hx&0x7fffffff;
76	if(ix>= 0x3ff00000) {		/* |x|>= 1 */
77	    u_int32_t lx;
78	    GET_LOW_WORD(lx,x);
79	    if(((ix-0x3ff00000)|lx)==0)
80		    /* asin(1)=+-pi/2 with inexact */
81		return x*pio2_hi+x*pio2_lo;
82	    return (x-x)/(x-x);		/* asin(|x|>1) is NaN */
83	} else if (ix<0x3fe00000) {	/* |x|<0.5 */
84	    if(ix<0x3e400000) {		/* if |x| < 2**-27 */
85		if(huge+x>one) return x;/* return x with inexact if x!=0*/
86	    } else
87		t = x*x;
88		p = t*(pS0+t*(pS1+t*(pS2+t*(pS3+t*(pS4+t*pS5)))));
89		q = one+t*(qS1+t*(qS2+t*(qS3+t*qS4)));
90		w = p/q;
91		return x+x*w;
92	}
93	/* 1> |x|>= 0.5 */
94	w = one-fabs(x);
95	t = w*0.5;
96	p = t*(pS0+t*(pS1+t*(pS2+t*(pS3+t*(pS4+t*pS5)))));
97	q = one+t*(qS1+t*(qS2+t*(qS3+t*qS4)));
98	s = __ieee754_sqrt(t);
99	if(ix>=0x3FEF3333) { 	/* if |x| > 0.975 */
100	    w = p/q;
101	    t = pio2_hi-(2.0*(s+s*w)-pio2_lo);
102	} else {
103	    w  = s;
104	    SET_LOW_WORD(w,0);
105	    c  = (t-w*w)/(s+w);
106	    r  = p/q;
107	    p  = 2.0*s*r-(pio2_lo-2.0*c);
108	    q  = pio4_hi-2.0*w;
109	    t  = pio4_hi-(p-q);
110	}
111	if(hx>0) return t; else return -t;
112}
113