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---
e_asin.c (3598B)
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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
14 static char rcsid[] = "$FreeBSD: src/lib/msun/src/e_asin.c,v 1.10 2003/07/23 04:53:46 peter Exp $";
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
51 static const double
52 one = 1.00000000000000000000e+00, /* 0x3FF00000, 0x00000000 */
53 huge = 1.000e+300,
54 pio2_hi = 1.57079632679489655800e+00, /* 0x3FF921FB, 0x54442D18 */
55 pio2_lo = 6.12323399573676603587e-17, /* 0x3C91A626, 0x33145C07 */
56 pio4_hi = 7.85398163397448278999e-01, /* 0x3FE921FB, 0x54442D18 */
57 /* coefficient for R(x^2) */
58 pS0 = 1.66666666666666657415e-01, /* 0x3FC55555, 0x55555555 */
59 pS1 = -3.25565818622400915405e-01, /* 0xBFD4D612, 0x03EB6F7D */
60 pS2 = 2.01212532134862925881e-01, /* 0x3FC9C155, 0x0E884455 */
61 pS3 = -4.00555345006794114027e-02, /* 0xBFA48228, 0xB5688F3B */
62 pS4 = 7.91534994289814532176e-04, /* 0x3F49EFE0, 0x7501B288 */
63 pS5 = 3.47933107596021167570e-05, /* 0x3F023DE1, 0x0DFDF709 */
64 qS1 = -2.40339491173441421878e+00, /* 0xC0033A27, 0x1C8A2D4B */
65 qS2 = 2.02094576023350569471e+00, /* 0x40002AE5, 0x9C598AC8 */
66 qS3 = -6.88283971605453293030e-01, /* 0xBFE6066C, 0x1B8D0159 */
67 qS4 = 7.70381505559019352791e-02; /* 0x3FB3B8C5, 0xB12E9282 */
68
69 double
70 __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 }