source: azure_iot_hub/trunk/musl-1.1.18/src/math/tan.c@ 389

/* origin: FreeBSD /usr/src/lib/msun/src/s_tan.c */
/*
 * ====================================================
 * 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.
 * ====================================================
 */
/* tan(x)
 * Return tangent function of x.
 *
 * kernel function:
 *      __tan           ... tangent function on [-pi/4,pi/4]
 *      __rem_pio2      ... argument reduction routine
 *
 * Method.
 *      Let S,C and T denote the sin, cos and tan respectively on
 *      [-PI/4, +PI/4]. Reduce the argument x to y1+y2 = x-k*pi/2
 *      in [-pi/4 , +pi/4], and let n = k mod 4.
 *      We have
 *
 *          n        sin(x)      cos(x)        tan(x)
 *     ----------------------------------------------------------
 *          0          S           C             T
 *          1          C          -S            -1/T
 *          2         -S          -C             T
 *          3         -C           S            -1/T
 *     ----------------------------------------------------------
 *
 * Special cases:
 *      Let trig be any of sin, cos, or tan.
 *      trig(+-INF)  is NaN, with signals;
 *      trig(NaN)    is that NaN;
 *
 * Accuracy:
 *      TRIG(x) returns trig(x) nearly rounded
 */

#include "libm.h"

double tan(double x)
{
        double y[2];
        uint32_t ix;
        unsigned n;

        GET_HIGH_WORD(ix, x);
        ix &= 0x7fffffff;

        /* |x| ~< pi/4 */
        if (ix <= 0x3fe921fb) {
                if (ix < 0x3e400000) { /* |x| < 2**-27 */
                        /* raise inexact if x!=0 and underflow if subnormal */
                        FORCE_EVAL(ix < 0x00100000 ? x/0x1p120f : x+0x1p120f);
                        return x;
                }
                return __tan(x, 0.0, 0);
        }

        /* tan(Inf or NaN) is NaN */
        if (ix >= 0x7ff00000)
                return x - x;

        /* argument reduction */
        n = __rem_pio2(x, y);
        return __tan(y[0], y[1], n&1);
}