/*	$Id: distaz.c,v 1.2 2000/12/29 22:10:37 pwessel Exp $	*/
#include "gmt.h"

#define CONSTANTE2 0.9931177
#define EPSILON 0.0001
#define RAYON 6371.0

void distaz(double lat1,double lon1,double lat2,double lon2,double *distrad,double *distdeg,double *distkm,double *az12rad,double *az12deg,double *az21rad,double *az21deg,int syscoord)
/*
  coordonnees geographiques en degres  : syscoord = 0
  coordonnees geocentriques en radians : syscoord = 1
  coordonnees cartesiennes en km       : syscoord = 2
*/

{
    double slat1, clat1, slon1, clon1;
    double slat2, clat2, slon2, clon2;
    double a1, b1, g1, h1;
    double a2, b2, g2, h2;
    double c1, c3, c4, c5, c6;

    if(syscoord == 2) {
        *distkm = sqrt((lon1 - lon2) * (lon1 - lon2) + (lat1 - lat2) * (lat1 - lat2));
        *distrad = *distkm / RAYON;
        *distdeg = *distrad * R2D;
        *az12rad = atan2((lon2 - lon1), (lat2 - lat1));
        *az21rad = atan2((lon1 - lon2), (lat1 - lat2));
    }
    else {
        if(syscoord <= 0) {
            lat1 *= D2R;
            lon1 *= D2R;
            lat2 *= D2R;
            lon2 *= D2R;
            if((M_PI_2 - fabs(lat1)) > EPSILON)
                lat1 = atan(CONSTANTE2 * tan(lat1));
            if((M_PI_2 - fabs(lat2)) > EPSILON)
                lat2 = atan(CONSTANTE2 * tan(lat2));
         }
    sincos (lat1, &slat1, &clat1);
    sincos (lon1, &slon1, &clon1);
    sincos (lat2, &slat2, &clat2);
    sincos (lon2, &slon2, &clon2);
  
         a1 = clat1 * clon1;
         b1 = clat1 * slon1;
         g1 = slat1 * clon1;
         h1 = slat1 * slon1;

         a2 = clat2 * clon2;
         b2 = clat2 * slon2;
         g2 = slat2 * clon2;
         h2 = slat2 * slon2;

         c1 = a1 * a2 + b1 * b2 + slat1 * slat2;
         if(fabs(c1) < 0.94)
             *distrad = acos(c1);
         else if(c1 > 0.)
             *distrad = asin(sqrt((a1 - a2) * (a1 - a2) + (b1 - b2) * (b1 - b2) + (slat1 - slat2) * (slat1 - slat2)) / 2.) * 2.;
         else
             *distrad = acos(sqrt((a1 + a2) * (a1 + a2) + (b1 + b2) * (b1 + b2) + (slat1 + slat2) * (slat1 + slat2)) / 2.) * 2.;
         *distkm = *distrad * RAYON;
         *distdeg = *distrad * R2D;

         c3 = (a2 - slon1) * (a2 - slon1) + (b2 + clon1) * (b2 + clon1) + slat2 * slat2 - 2.;
         c4 = (a2 - g1) * (a2 - g1) + (b2 - h1) * (b2 - h1) + (slat2 + clat1) * (slat2 + clat1) - 2.;
         *az12rad = atan2(c3, c4);

         c5 = (a1 - slon2) * (a1 - slon2) + (b1 + clon2) * (b1 + clon2) + slat1 * slat1 - 2.;
         c6 = (a1 - g2) * (a1 - g2) + (b1 - h2) * (b1 - h2) + (slat1 + clat2) * (slat1 + clat2) - 2.;
         *az21rad = atan2(c5, c6);
    }

    if(*az12rad < 0.)
        *az12rad += M_PI * 2;
    *az12deg = *az12rad * R2D;
    if(*az21rad < 0.)
        *az21rad += M_PI * 2;
    *az21deg = *az21rad * R2D;

    return;
}
