e_atan2.c revision 1.4 
1/* @(#)e_atan2.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: e_atan2.c,v 1.4 1994/03/03 17:04:07 jtc Exp $";
15#endif
16
17/* __ieee754_atan2(y,x)
18 * Method :
19 *	1. Reduce y to positive by atan2(y,x)=-atan2(-y,x).
20 *	2. Reduce x to positive by (if x and y are unexceptional):
21 *		ARG (x+iy) = arctan(y/x)   	   ... if x > 0,
22 *		ARG (x+iy) = pi - arctan[y/(-x)]   ... if x < 0,
23 *
24 * Special cases:
25 *
26 *	ATAN2((anything), NaN ) is NaN;
27 *	ATAN2(NAN , (anything) ) is NaN;
28 *	ATAN2(+-0, +(anything but NaN)) is +-0  ;
29 *	ATAN2(+-0, -(anything but NaN)) is +-pi ;
30 *	ATAN2(+-(anything but 0 and NaN), 0) is +-pi/2;
31 *	ATAN2(+-(anything but INF and NaN), +INF) is +-0 ;
32 *	ATAN2(+-(anything but INF and NaN), -INF) is +-pi;
33 *	ATAN2(+-INF,+INF ) is +-pi/4 ;
34 *	ATAN2(+-INF,-INF ) is +-3pi/4;
35 *	ATAN2(+-INF, (anything but,0,NaN, and INF)) is +-pi/2;
36 *
37 * Constants:
38 * The hexadecimal values are the intended ones for the following
39 * constants. The decimal values may be used, provided that the
40 * compiler will convert from decimal to binary accurately enough
41 * to produce the hexadecimal values shown.
42 */
43
44#include <math.h>
45#include <machine/endian.h>
46
47#if BYTE_ORDER == LITTLE_ENDIAN
48#define n0	1
49#else
50#define n0	0
51#endif
52
53#ifdef __STDC__
54static const double
55#else
56static double
57#endif
58tiny  = 1.0e-300,
59zero  = 0.0,
60pi_o_4  = 7.8539816339744827900E-01, /* 0x3FE921FB, 0x54442D18 */
61pi_o_2  = 1.5707963267948965580E+00, /* 0x3FF921FB, 0x54442D18 */
62pi      = 3.1415926535897931160E+00, /* 0x400921FB, 0x54442D18 */
63pi_lo   = 1.2246467991473531772E-16; /* 0x3CA1A626, 0x33145C07 */
64
65#ifdef __STDC__
66	double __ieee754_atan2(double y, double x)
67#else
68	double __ieee754_atan2(y,x)
69	double  y,x;
70#endif
71{
72	double z;
73	int k,m,hx,hy,ix,iy;
74	unsigned lx,ly;
75
76	hx = *(n0+(int*)&x); ix = hx&0x7fffffff;
77	lx = *(1-n0+(int*)&x);
78	hy = *(n0+(int*)&y); iy = hy&0x7fffffff;
79	ly = *(1-n0+(int*)&y);
80	if(((ix|((lx|-lx)>>31))>0x7ff00000)||
81	   ((iy|((ly|-ly)>>31))>0x7ff00000))	/* x or y is NaN */
82	   return x+y;
83	if((hx-0x3ff00000|lx)==0) return atan(y);   /* x=1.0 */
84	m = ((hy>>31)&1)|((hx>>30)&2);	/* 2*sign(x)+sign(y) */
85
86    /* when y = 0 */
87	if((iy|ly)==0) {
88	    switch(m) {
89		case 0:
90		case 1: return y; 	/* atan(+-0,+anything)=+-0 */
91		case 2: return  pi+tiny;/* atan(+0,-anything) = pi */
92		case 3: return -pi-tiny;/* atan(-0,-anything) =-pi */
93	    }
94	}
95    /* when x = 0 */
96	if((ix|lx)==0) return (hy<0)?  -pi_o_2-tiny: pi_o_2+tiny;
97
98    /* when x is INF */
99	if(ix==0x7ff00000) {
100	    if(iy==0x7ff00000) {
101		switch(m) {
102		    case 0: return  pi_o_4+tiny;/* atan(+INF,+INF) */
103		    case 1: return -pi_o_4-tiny;/* atan(-INF,+INF) */
104		    case 2: return  3.0*pi_o_4+tiny;/*atan(+INF,-INF)*/
105		    case 3: return -3.0*pi_o_4-tiny;/*atan(-INF,-INF)*/
106		}
107	    } else {
108		switch(m) {
109		    case 0: return  zero  ;	/* atan(+...,+INF) */
110		    case 1: return -zero  ;	/* atan(-...,+INF) */
111		    case 2: return  pi+tiny  ;	/* atan(+...,-INF) */
112		    case 3: return -pi-tiny  ;	/* atan(-...,-INF) */
113		}
114	    }
115	}
116    /* when y is INF */
117	if(iy==0x7ff00000) return (hy<0)? -pi_o_2-tiny: pi_o_2+tiny;
118
119    /* compute y/x */
120	k = (iy-ix)>>20;
121	if(k > 60) z=pi_o_2+0.5*pi_lo; 	/* |y/x| >  2**60 */
122	else if(hx<0&&k<-60) z=0.0; 	/* |y|/x < -2**60 */
123	else z=atan(fabs(y/x));		/* safe to do y/x */
124	switch (m) {
125	    case 0: return       z  ;	/* atan(+,+) */
126	    case 1: *(n0+(int*)&z) ^= 0x80000000;
127		    return       z  ;	/* atan(-,+) */
128	    case 2: return  pi-(z-pi_lo);/* atan(+,-) */
129	    default: /* case 3 */
130	    	    return  (z-pi_lo)-pi;/* atan(-,-) */
131	}
132}
133