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/*
        floating point tangent

        A series is used after range reduction.
        Coefficients are #4285 from Hart & Cheney. (19.74D)
 */

#include <u.h>
#include <libc.h>

static double p0         = -0.1306820264754825668269611177e+5;
static double p1          = 0.1055970901714953193602353981e+4;
static double p2         = -0.1550685653483266376941705728e+2;
static double p3          = 0.3422554387241003435328470489e-1;
static double p4          = 0.3386638642677172096076369e-4;
static double q0         = -0.1663895238947119001851464661e+5;
static double q1          = 0.4765751362916483698926655581e+4;
static double q2         = -0.1555033164031709966900124574e+3;

double
tan(double arg)
{
        double temp, e, x, xsq;
        int flag, sign, i;

        flag = 0;
        sign = 0;
        if(arg < 0){
                arg = -arg;
                sign++;
        }
        arg = 2*arg/PIO2;   /* overflow? */
        x = modf(arg, &e);
        i = e;
        switch(i%4) {
        case 1:
                x = 1 - x;
                flag = 1;
                break;

        case 2:
                sign = !sign;
                flag = 1;
                break;

        case 3:
                x = 1 - x;
                sign = !sign;
                break;

        case 0:
                break;
        }

        xsq = x*x;
        temp = ((((p4*xsq+p3)*xsq+p2)*xsq+p1)*xsq+p0)*x;
        temp = temp/(((xsq+q2)*xsq+q1)*xsq+q0);

        if(flag) {
                if(temp == 0)
                        return NaN();
                temp = 1/temp;
        }
        if(sign)
                temp = -temp;
        return temp;
}