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/*	@(#)r_tan.c	1.0	10/01/82	*/

/*
	SINGLE PRECISION floating point tangent

	sin/cos is used after argument reduction to [0,pi/4] range.
	since x is in this range, tan(x) is in [0,1] range and
	no overflow can occur here.
*/

#include <errno.h>

int	errno;
static double invpi = 1.27323954473516268;  /* 4/pi */

float
r_tan(parg)
float *parg;
{
	double arg;
	fortran float sin(), cos();
	double modf();
	float flmax_();
	double temp, e, x, xsq;
	int sign;
	int flag, i;

	arg = *parg;
	flag = 0;
	sign = 1.;
	if(arg < 0.){		/* tan(-arg) = -tan(arg) */
		arg = -arg;
		sign = -1.;
	}
	arg = arg*invpi;   /*overflow?*/
	x = modf(arg,&e);
	i = e;
	switch(i%4) {
	case 1:			/* 2nd octant: tan(x) = 1/tan(1-x) */
		x = 1. - x;
		flag = 1;
		break;

	case 2:			/* 3rd octant: tan(x) = -1/tan(x) */
		sign = - sign;
		flag = 1;
		break;

	case 3:			/* 4th octant: tan(x) = -tan(1-x) */
		x = 1. - x;
		sign = - sign;
		break;

	case 0:			/* 1st octant */
		break;
	}
	x = x/invpi;

	temp = sin(x)/cos(x);

	if(flag == 1) {
		if(temp == 0.) {	/* check for singular "point" */
			errno = ERANGE;
			if (sign>0)
				return(flmax_());
			return(-flmax_());
		}
		temp = 1./temp;
	}
	return(sign*temp);
}

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