s_atan.c revision 8870 
1/* @(#)s_atan.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[] = "$Id: s_atan.c,v 1.1.1.1 1994/08/19 09:39:45 jkh Exp $";
15#endif
16
17/* atan(x)
18 * Method
19 *   1. Reduce x to positive by atan(x) = -atan(-x).
20 *   2. According to the integer k=4t+0.25 chopped, t=x, the argument
21 *      is further reduced to one of the following intervals and the
22 *      arctangent of t is evaluated by the corresponding formula:
23 *
24 *      [0,7/16]      atan(x) = t-t^3*(a1+t^2*(a2+...(a10+t^2*a11)...)
25 *      [7/16,11/16]  atan(x) = atan(1/2) + atan( (t-0.5)/(1+t/2) )
26 *      [11/16.19/16] atan(x) = atan( 1 ) + atan( (t-1)/(1+t) )
27 *      [19/16,39/16] atan(x) = atan(3/2) + atan( (t-1.5)/(1+1.5t) )
28 *      [39/16,INF]   atan(x) = atan(INF) + atan( -1/t )
29 *
30 * Constants:
31 * The hexadecimal values are the intended ones for the following
32 * constants. The decimal values may be used, provided that the
33 * compiler will convert from decimal to binary accurately enough
34 * to produce the hexadecimal values shown.
35 */
36
37#include "math.h"
38#include "math_private.h"
39
40#ifdef __STDC__
41static const double atanhi[] = {
42#else
43static double atanhi[] = {
44#endif
45  4.63647609000806093515e-01, /* atan(0.5)hi 0x3FDDAC67, 0x0561BB4F */
46  7.85398163397448278999e-01, /* atan(1.0)hi 0x3FE921FB, 0x54442D18 */
47  9.82793723247329054082e-01, /* atan(1.5)hi 0x3FEF730B, 0xD281F69B */
48  1.57079632679489655800e+00, /* atan(inf)hi 0x3FF921FB, 0x54442D18 */
49};
50
51#ifdef __STDC__
52static const double atanlo[] = {
53#else
54static double atanlo[] = {
55#endif
56  2.26987774529616870924e-17, /* atan(0.5)lo 0x3C7A2B7F, 0x222F65E2 */
57  3.06161699786838301793e-17, /* atan(1.0)lo 0x3C81A626, 0x33145C07 */
58  1.39033110312309984516e-17, /* atan(1.5)lo 0x3C700788, 0x7AF0CBBD */
59  6.12323399573676603587e-17, /* atan(inf)lo 0x3C91A626, 0x33145C07 */
60};
61
62#ifdef __STDC__
63static const double aT[] = {
64#else
65static double aT[] = {
66#endif
67  3.33333333333329318027e-01, /* 0x3FD55555, 0x5555550D */
68 -1.99999999998764832476e-01, /* 0xBFC99999, 0x9998EBC4 */
69  1.42857142725034663711e-01, /* 0x3FC24924, 0x920083FF */
70 -1.11111104054623557880e-01, /* 0xBFBC71C6, 0xFE231671 */
71  9.09088713343650656196e-02, /* 0x3FB745CD, 0xC54C206E */
72 -7.69187620504482999495e-02, /* 0xBFB3B0F2, 0xAF749A6D */
73  6.66107313738753120669e-02, /* 0x3FB10D66, 0xA0D03D51 */
74 -5.83357013379057348645e-02, /* 0xBFADDE2D, 0x52DEFD9A */
75  4.97687799461593236017e-02, /* 0x3FA97B4B, 0x24760DEB */
76 -3.65315727442169155270e-02, /* 0xBFA2B444, 0x2C6A6C2F */
77  1.62858201153657823623e-02, /* 0x3F90AD3A, 0xE322DA11 */
78};
79
80#ifdef __STDC__
81	static const double
82#else
83	static double
84#endif
85one   = 1.0,
86huge   = 1.0e300;
87
88#ifdef __STDC__
89	double atan(double x)
90#else
91	double atan(x)
92	double x;
93#endif
94{
95	double w,s1,s2,z;
96	int32_t ix,hx,id;
97
98	GET_HIGH_WORD(hx,x);
99	ix = hx&0x7fffffff;
100	if(ix>=0x44100000) {	/* if |x| >= 2^66 */
101	    u_int32_t low;
102	    GET_LOW_WORD(low,x);
103	    if(ix>0x7ff00000||
104		(ix==0x7ff00000&&(low!=0)))
105		return x+x;		/* NaN */
106	    if(hx>0) return  atanhi[3]+atanlo[3];
107	    else     return -atanhi[3]-atanlo[3];
108	} if (ix < 0x3fdc0000) {	/* |x| < 0.4375 */
109	    if (ix < 0x3e200000) {	/* |x| < 2^-29 */
110		if(huge+x>one) return x;	/* raise inexact */
111	    }
112	    id = -1;
113	} else {
114	x = fabs(x);
115	if (ix < 0x3ff30000) {		/* |x| < 1.1875 */
116	    if (ix < 0x3fe60000) {	/* 7/16 <=|x|<11/16 */
117		id = 0; x = (2.0*x-one)/(2.0+x);
118	    } else {			/* 11/16<=|x|< 19/16 */
119		id = 1; x  = (x-one)/(x+one);
120	    }
121	} else {
122	    if (ix < 0x40038000) {	/* |x| < 2.4375 */
123		id = 2; x  = (x-1.5)/(one+1.5*x);
124	    } else {			/* 2.4375 <= |x| < 2^66 */
125		id = 3; x  = -1.0/x;
126	    }
127	}}
128    /* end of argument reduction */
129	z = x*x;
130	w = z*z;
131    /* break sum from i=0 to 10 aT[i]z**(i+1) into odd and even poly */
132	s1 = z*(aT[0]+w*(aT[2]+w*(aT[4]+w*(aT[6]+w*(aT[8]+w*aT[10])))));
133	s2 = w*(aT[1]+w*(aT[3]+w*(aT[5]+w*(aT[7]+w*aT[9]))));
134	if (id<0) return x - x*(s1+s2);
135	else {
136	    z = atanhi[id] - ((x*(s1+s2) - atanlo[id]) - x);
137	    return (hx<0)? -z:z;
138	}
139}
140