s_tan.cs

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
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 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
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/* tan(x)
 * Return tangent function of x.
 *
 * kernel function:
 *      __kernel_tan            ... tangent function on [-pi/4,pi/4]
 *      __ieee754_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
 */
 
namespace IKVM.Runtime.Util.Java.Lang
{
    static partial class fdlibm
    {
        internal static double tan(double x)
        {
            double z = 0.0;
            int n, ix;
 
            /* High word of x. */
            ix = __HI(x);
 
            /* |x| ~< pi/4 */
            ix &= 0x7fffffff;
            if (ix <= 0x3fe921fb) return __kernel_tan(x, z, 1);
 
            /* tan(Inf or NaN) is NaN */
            else if (ix >= 0x7ff00000) return x - x;            /* NaN */
 
            /* argument reduction needed */
            else
            {
                double y_0_ = 0.0, y_1_ = 0.0;
                n = __ieee754_rem_pio2(x, ref y_0_, ref y_1_);
                return __kernel_tan(y_0_, y_1_, 1 - ((n & 1) << 1)); /*   1 -- n even
                                                        -1 -- n odd */
            }
        }
    }
}