* ====================================================
* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
*
* Developed at SunPro, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/
* Method
* 1. Reduce x to positive by atan(x) = -atan(-x).
* 2. According to the integer k=4t+0.25 chopped, t=x, the argument
* is further reduced to one of the following intervals and the
* arctangent of t is evaluated by the corresponding formula:
*
* [0,7/16] atan(x) = t-t^3*(a1+t^2*(a2+...(a10+t^2*a11)...)
* [7/16,11/16] atan(x) = atan(1/2) + atan( (t-0.5)/(1+t/2) )
* [11/16.19/16] atan(x) = atan( 1 ) + atan( (t-1)/(1+t) )
* [19/16,39/16] atan(x) = atan(3/2) + atan( (t-1.5)/(1+1.5t) )
* [39/16,INF] atan(x) = atan(INF) + atan( -1/t )
*
* Constants:
* The hexadecimal values are the intended ones for the following
* constants. The decimal values may be used, provided that the
* compiler will convert from decimal to binary accurately enough
* to produce the hexadecimal values shown.
*/
#include "libm.h"
static const double atanhi[] = {
4.63647609000806093515e-01,
7.85398163397448278999e-01,
9.82793723247329054082e-01,
1.57079632679489655800e+00,
};
static const double atanlo[] = {
2.26987774529616870924e-17,
3.06161699786838301793e-17,
1.39033110312309984516e-17,
6.12323399573676603587e-17,
};
static const double aT[] = {
3.33333333333329318027e-01,
-1.99999999998764832476e-01,
1.42857142725034663711e-01,
-1.11111104054623557880e-01,
9.09088713343650656196e-02,
-7.69187620504482999495e-02,
6.66107313738753120669e-02,
-5.83357013379057348645e-02,
4.97687799461593236017e-02,
-3.65315727442169155270e-02,
1.62858201153657823623e-02,
};
double atan(double x)
{
double_t w,s1,s2,z;
uint32_t ix,sign;
int id;
GET_HIGH_WORD(ix, x);
sign = ix >> 31;
ix &= 0x7fffffff;
if (ix >= 0x44100000) {
if (isnan(x))
return x;
z = atanhi[3] + 0x1p-120f;
return sign ? -z : z;
}
if (ix < 0x3fdc0000) {
if (ix < 0x3e400000) {
if (ix < 0x00100000)
FORCE_EVAL((float)x);
return x;
}
id = -1;
} else {
x = fabs(x);
if (ix < 0x3ff30000) {
if (ix < 0x3fe60000) {
id = 0;
x = (2.0*x-1.0)/(2.0+x);
} else {
id = 1;
x = (x-1.0)/(x+1.0);
}
} else {
if (ix < 0x40038000) {
id = 2;
x = (x-1.5)/(1.0+1.5*x);
} else {
id = 3;
x = -1.0/x;
}
}
}
z = x*x;
w = z*z;
s1 = z*(aT[0]+w*(aT[2]+w*(aT[4]+w*(aT[6]+w*(aT[8]+w*aT[10])))));
s2 = w*(aT[1]+w*(aT[3]+w*(aT[5]+w*(aT[7]+w*aT[9]))));
if (id < 0)
return x - x*(s1+s2);
z = atanhi[id] - (x*(s1+s2) - atanlo[id] - x);
return sign ? -z : z;
}
