/*

* (c) copyright 1988 by the Vrije Universiteit, Amsterdam, The Netherlands.

* See the copyright notice in the ACK home directory, in the file "Copyright".

*

* Author: Ceriel J.H. Jacobs

*/

/* $Header$ */

#include <math.h>

#include <float.h>

#include <errno.h>

#include "localmath.h"

double

tan(double x)

{

/* Algorithm and coefficients from:

"Software manual for the elementary functions"

by W.J. Cody and W. Waite, Prentice-Hall, 1980

*/

int negative = x < 0;

int invert = 0;

double y;

static double p[] = {

1.0,

-0.13338350006421960681e+0,

0.34248878235890589960e-2,

-0.17861707342254426711e-4

};

static double q[] = {

1.0,

-0.46671683339755294240e+0,

0.25663832289440112864e-1,

-0.31181531907010027307e-3,

0.49819433993786512270e-6

};

if (__IsNan(x)) {

errno = EDOM;

return x;

}

if (negative) x = -x;

/* ??? avoid loss of significance, error if x is too large ??? */

y = x * M_2_PI + 0.5;

if (y >= DBL_MAX/M_PI_2) return 0.0;

/* Use extended precision to calculate reduced argument.

Here we used 12 bits of the mantissa for a1.

Also split x in integer part x1 and fraction part x2.

*/

#define A1 1.57080078125

#define A2 -4.454455103380768678308e-6

{

double x1, x2;

modf(y, &y);

if (modf(0.5*y, &x1)) invert = 1;

x2 = modf(x, &x1);

x = x1 - y * A1;

x += x2;

x -= y * A2;

#undef A1

#undef A2

}

/* ??? avoid underflow ??? */

y = x * x;

x += x * y * POLYNOM2(y, p+1);

y = POLYNOM4(y, q);

if (negative) x = -x;

return invert ? -y/x : x/y;

}