source: Sophya/trunk/Poubelle/archediab.old/archediab.sources/c/aa_hadec.c@ 2010

#include <stdio.h>
#include <math.h>
#include "aa_hadec.h"
#define GOODATAN2
#define PI M_PI

static void aaha_aux (double lat, double x, double y, double* p, double* q);


/* given latitude (n+, radians), lat, altitude (up+, radians), alt, and
 * azimuth (angle round to the east from north+, radians),
 * return hour angle (radians), ha, and declination (radians), dec.
 */
void aa_hadec (double lat, double alt, double az, double* ha, double* dec)
{
        aaha_aux (lat, az, alt, ha, dec);
}

/* given latitude (n+, radians), lat, hour angle (radians), ha, and declination
 * (radians), dec,
 * return altitude (up+, radians), alt, and
 * azimuth (angle round to the east from north+, radians),
 */
void hadec_aa (double lat, double ha, double dec, double* alt, double* az)
{
        aaha_aux (lat, ha, dec, az, alt);
}

/* the actual formula is the same for both transformation directions so
 * do it here once for each way.
 * N.B. all arguments are in radians.
 */
static void
aaha_aux (double lat, double x, double y, double* p, double* q)
{
        static double lastlat = -1000.;
        static double sinlastlat, coslastlat;
        double sy, cy;
        double sx, cx;
#ifndef GOODATAN2
        double sq, cq;
        double a;
        double cp;
#endif

        /* latitude doesn't change much, so try to reuse the sin and cos evals.
         */
        if (lat != lastlat) {
            sinlastlat = sin (lat);
            coslastlat = cos (lat);
            lastlat = lat;
        }

        sy = sin (y);
        cy = cos (y);
        sx = sin (x);
        cx = cos (x);

/* define GOODATAN2 if atan2 returns full range -PI through +PI.
 */
#ifdef GOODATAN2
        *q = asin ((sy*sinlastlat) + (cy*coslastlat*cx));
        *p = atan2 (-cy*sx, -cy*cx*sinlastlat + sy*coslastlat);
#else
#define EPS     (1e-20)
        sq = (sy*sinlastlat) + (cy*coslastlat*cx);
        *q = asin (sq);
        cq = cos (*q);
        a = coslastlat*cq;
        if (a > -EPS && a < EPS)
            a = a < 0 ? -EPS : EPS; /* avoid / 0 */
        cp = (sy - (sinlastlat*sq))/a;
        if (cp >= 1.0)  /* the /a can be slightly > 1 */
            *p = 0.0;
        else if (cp <= -1.0)
            *p = PI;
        else
            *p = acos ((sy - (sinlastlat*sq))/a);
        if (sx>0) *p = 2.0*PI - *p;
#endif
}