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[unix-history] / usr / src / libm / tan.c

/*
       floating point tangent

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

#include <errno.h>
#include <math.h>

int     errno;
static double invpi       = 1.27323954473516268;
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(arg)
double arg;
{
       double modf();
       double sign, temp, e, x, xsq;
       int flag, i;

       flag = 0;
       sign = 1.;
       if(arg < 0.){
               arg = -arg;
               sign = -1.;
       }
       arg = arg*invpi;   /*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/(((1.0*xsq+q2)*xsq+q1)*xsq+q0);

       if(flag == 1) {
               if(temp == 0.) {
                       errno = ERANGE;
                       if (sign>0)
                               return(HUGE);
                       return(-HUGE);
               }
               temp = 1./temp;
       }
       return(sign*temp);
}