usr/src/lib/libm/common/tan.c

/*
 * Copyright (c) 1987 Regents of the University of California.
 *
 * Use and reproduction of this software are granted  in  accordance  with
 * the terms and conditions specified in  the  Berkeley  Software  License
 * Agreement (in particular, this entails acknowledgement of the programs'
 * source, and inclusion of this notice) with the additional understanding
 * that  all  recipients  should regard themselves as participants  in  an
 * ongoing  research  project and hence should  feel  obligated  to report
 * their  experiences (good or bad) with these elementary function  codes,
 * using "sendbug 4bsd-bugs@BERKELEY", to the authors.
 */

#ifndef lint
static char sccsid[] = "@(#)tan.c       1.1     1.1 (ucb.elefunt) %G%";
#endif  /* not lint */

#include "trig.h"
double
tan(x)
double x;
{
        double a,z,ss,cc,c;
        int k;

        if(!finite(x))          /* tan(NaN) and tan(INF) must be NaN */
                return x-x;
        x = drem(x,PI);                 /* reduce x into [-PI/2, PI/2] */
        a = copysign(x,one);            /* ... = abs(x) */
        if (a >= PIo4) {
                k = 1;
                x = copysign(PIo2-a,x);
        }
        else {
                k = 0;
                if (a < small) {
                        big+a;
                        return x;
                }
        }
        z = x*x;
        cc = cos__C(z);
        ss = sin__S(z);
        z *= half;                      /* Next get c = cos(x) accurately */
        c = (z >= thresh ? half-((z-half)-cc) : one-(z-cc));
        if (k == 0)
                return x+(x*(z-(cc-ss)))/c;     /* ... sin/cos */
#ifdef national
        else if (x == zero)
                return copysign(fmax,x);        /* no inf on 32k */
#endif  /* national */
        else
                return c/(x+x*ss);              /* ... cos/sin */
}