// sunpos.c (taken from XEarth)
// kirk johnson
// july 1993
//
// code for calculating the position on the earth's surface for which
// the sun is directly overhead (adapted from _practical astronomy
// with your calculator, third edition_, peter duffett-smith,
// cambridge university press, 1988.)
//
// RCS $Id$
//
// Copyright (C) 1989, 1990, 1993, 1994, 1995 Kirk Lauritz Johnson
//
// Parts of the source code (as marked) are:
//   Copyright (C) 1989, 1990, 1991 by Jim Frost
//   Copyright (C) 1992 by Jamie Zawinski <jwz@lucid.com>
//
// Permission to use, copy, modify and freely distribute xearth for
// non-commercial and not-for-profit purposes is hereby granted
// without fee, provided that both the above copyright notice and this
// permission notice appear in all copies and in supporting
// documentation.
//
// The author makes no representations about the suitability of this
// software for any purpose. It is provided "as is" without express or
// implied warranty.
//
// THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
// INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS,
// IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT
// OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
// NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
//
// $Id$
// (Log is kept at end of this file)


#ifdef HAVE_CONFIG_H
#  include <config.h>
#endif

#include <math.h>
#include <stdio.h>
#include <time.h>

#include <Astro/orbits.hxx>
#include <Include/fg_constants.h>
#include <Main/views.hxx>
#include <Math/fg_geodesy.h>
#include <Math/mat3.h>
#include <Math/polar.h>
#include <Math/vector.h>
#include <Scenery/scenery.h>

#include "fg_time.hxx"
#include "sunpos.hxx"


#undef E


/*
 * the epoch upon which these astronomical calculations are based is
 * 1990 january 0.0, 631065600 seconds since the beginning of the
 * "unix epoch" (00:00:00 GMT, Jan. 1, 1970)
 *
 * given a number of seconds since the start of the unix epoch,
 * DaysSinceEpoch() computes the number of days since the start of the
 * astronomical epoch (1990 january 0.0)
 */

#define EpochStart           (631065600)
#define DaysSinceEpoch(secs) (((secs)-EpochStart)*(1.0/(24*3600)))

/*
 * assuming the apparent orbit of the sun about the earth is circular,
 * the rate at which the orbit progresses is given by RadsPerDay --
 * FG_2PI radians per orbit divided by 365.242191 days per year:
 */

#define RadsPerDay (FG_2PI/365.242191)

/*
 * details of sun's apparent orbit at epoch 1990.0 (after
 * duffett-smith, table 6, section 46)
 *
 * Epsilon_g    (ecliptic longitude at epoch 1990.0) 279.403303 degrees
 * OmegaBar_g   (ecliptic longitude of perigee)      282.768422 degrees
 * Eccentricity (eccentricity of orbit)                0.016713
 */

#define Epsilon_g    (279.403303*(FG_2PI/360))
#define OmegaBar_g   (282.768422*(FG_2PI/360))
#define Eccentricity (0.016713)

/*
 * MeanObliquity gives the mean obliquity of the earth's axis at epoch
 * 1990.0 (computed as 23.440592 degrees according to the method given
 * in duffett-smith, section 27)
 */
#define MeanObliquity (23.440592*(FG_2PI/360))

/* static double solve_keplers_equation(double); */
/* static double sun_ecliptic_longitude(time_t); */
static void   ecliptic_to_equatorial(double, double, double *, double *);
static double julian_date(int, int, int);
static double GST(time_t);

/*
 * solve Kepler's equation via Newton's method
 * (after duffett-smith, section 47)
 */
/*
static double solve_keplers_equation(double M) {
    double E;
    double delta;

    E = M;
    while (1) {
	delta = E - Eccentricity*sin(E) - M;
	if (fabs(delta) <= 1e-10) break;
	E -= delta / (1 - Eccentricity*cos(E));
    }

    return E;
}
*/


/* compute ecliptic longitude of sun (in radians) (after
 * duffett-smith, section 47) */
/*
static double sun_ecliptic_longitude(time_t ssue) {
    // time_t ssue;              //  seconds since unix epoch
    double D, N;
    double M_sun, E;
    double v;

    D = DaysSinceEpoch(ssue);

    N = RadsPerDay * D;
    N = fmod(N, FG_2PI);
    if (N < 0) N += FG_2PI;

    M_sun = N + Epsilon_g - OmegaBar_g;
    if (M_sun < 0) M_sun += FG_2PI;

    E = solve_keplers_equation(M_sun);
    v = 2 * atan(sqrt((1+Eccentricity)/(1-Eccentricity)) * tan(E/2));

    return (v + OmegaBar_g);
}
*/


/* convert from ecliptic to equatorial coordinates (after
 * duffett-smith, section 27) */

static void ecliptic_to_equatorial(double lambda, double beta, 
				   double *alpha, double *delta) {
    /* double  lambda;            ecliptic longitude       */
    /* double  beta;              ecliptic latitude        */
    /* double *alpha;             (return) right ascension */
    /* double *delta;             (return) declination     */

    double sin_e, cos_e;

    sin_e = sin(MeanObliquity);
    cos_e = cos(MeanObliquity);

    *alpha = atan2(sin(lambda)*cos_e - tan(beta)*sin_e, cos(lambda));
    *delta = asin(sin(beta)*cos_e + cos(beta)*sin_e*sin(lambda));
}


/* computing julian dates (assuming gregorian calendar, thus this is
 * only valid for dates of 1582 oct 15 or later) (after duffett-smith,
 * section 4) */

static double julian_date(int y, int m, int d) {
    /* int y;                    year (e.g. 19xx)          */
    /* int m;                    month (jan=1, feb=2, ...) */
    /* int d;                    day of month              */

    int    A, B, C, D;
    double JD;

    /* lazy test to ensure gregorian calendar */
    if (y < 1583) {
	printf("WHOOPS! Julian dates only valid for 1582 oct 15 or later\n");
    }

    if ((m == 1) || (m == 2)) {
	y -= 1;
	m += 12;
    }

    A = y / 100;
    B = 2 - A + (A / 4);
    C = (int)(365.25 * y);
    D = (int)(30.6001 * (m + 1));

    JD = B + C + D + d + 1720994.5;

    return JD;
}


/* compute greenwich mean sidereal time (GST) corresponding to a given
 * number of seconds since the unix epoch (after duffett-smith,
 * section 12) */
static double GST(time_t ssue) {
    /* time_t ssue;           seconds since unix epoch */

    double     JD;
    double     T, T0;
    double     UT;
    struct tm *tm;

    tm = gmtime(&ssue);

    JD = julian_date(tm->tm_year+1900, tm->tm_mon+1, tm->tm_mday);
    T  = (JD - 2451545) / 36525;

    T0 = ((T + 2.5862e-5) * T + 2400.051336) * T + 6.697374558;

    T0 = fmod(T0, 24.0);
    if (T0 < 0) T0 += 24;

    UT = tm->tm_hour + (tm->tm_min + tm->tm_sec / 60.0) / 60.0;

    T0 += UT * 1.002737909;
    T0 = fmod(T0, 24.0);
    if (T0 < 0) T0 += 24;

    return T0;
}


/* given a particular time (expressed in seconds since the unix
 * epoch), compute position on the earth (lat, lon) such that sun is
 * directly overhead.  (lat, lon are reported in radians */

void fgSunPosition(time_t ssue, double *lon, double *lat) {
    /* time_t  ssue;           seconds since unix epoch */
    /* double *lat;            (return) latitude        */
    /* double *lon;            (return) longitude       */

    /* double lambda; */
    double alpha, delta;
    double tmp;

    /* lambda = sun_ecliptic_longitude(ssue); */
    /* ecliptic_to_equatorial(lambda, 0.0, &alpha, &delta); */
    ecliptic_to_equatorial (solarPosition.lonSun, 0.0, &alpha, &delta);

    tmp = alpha - (FG_2PI/24)*GST(ssue);
    if (tmp < -FG_PI) {
	do tmp += FG_2PI;
	while (tmp < -FG_PI);
    } else if (tmp > FG_PI) {
	do tmp -= FG_2PI;
	while (tmp < -FG_PI);
    }

    *lon = tmp;
    *lat = delta;
}


// update the cur_time_params structure with the current sun position
void fgUpdateSunPos( void ) {
    struct fgLIGHT *l;
    struct fgTIME *t;
    struct fgVIEW *v;
    MAT3vec nup, nsun, v0;
    double sun_gd_lat, sl_radius;
    double ntmp;

    l = &cur_light_params;
    t = &cur_time_params;
    v = &current_view;

    printf("  Updating Sun position\n");

    fgSunPosition(t->cur_time, &l->sun_lon, &sun_gd_lat);
    fgSunPosition(t->cur_time, &l->sun_lon, &sun_gd_lat);

    fgGeodToGeoc(sun_gd_lat, 0.0, &sl_radius, &l->sun_gc_lat);

    l->fg_sunpos = fgPolarToCart(l->sun_lon, l->sun_gc_lat, sl_radius);

    printf( "    t->cur_time = %ld\n", t->cur_time);
    printf( "    Sun Geodetic lat = %.5f Geocentric lat = %.5f\n",
	    sun_gd_lat, l->sun_gc_lat);

    // I think this will work better for generating the sun light vector
    l->sun_vec[0] = l->fg_sunpos.x;
    l->sun_vec[1] = l->fg_sunpos.y;
    l->sun_vec[2] = l->fg_sunpos.z;
    MAT3_NORMALIZE_VEC(l->sun_vec, ntmp);
    MAT3_SCALE_VEC(l->sun_vec_inv, l->sun_vec, -1.0);

    // make sure these are directional light sources only
    l->sun_vec[3] = 0.0;
    l->sun_vec_inv[3] = 0.0;

    printf("  l->sun_vec = %.2f %.2f %.2f\n", l->sun_vec[0], l->sun_vec[1],
	   l->sun_vec[2]);

    // calculate the sun's relative angle to local up
    MAT3_COPY_VEC(nup, v->local_up);
    nsun[0] = l->fg_sunpos.x; 
    nsun[1] = l->fg_sunpos.y;
    nsun[2] = l->fg_sunpos.z;
    MAT3_NORMALIZE_VEC(nup, ntmp);
    MAT3_NORMALIZE_VEC(nsun, ntmp);

    l->sun_angle = acos(MAT3_DOT_PRODUCT(nup, nsun));
    printf("  SUN ANGLE relative to current location = %.3f rads.\n", 
	   l->sun_angle);
    
    // calculate vector to sun's position on the earth's surface
    v->to_sun[0] = l->fg_sunpos.x - (v->view_pos.x + scenery.center.x);
    v->to_sun[1] = l->fg_sunpos.y - (v->view_pos.y + scenery.center.y);
    v->to_sun[2] = l->fg_sunpos.z - (v->view_pos.z + scenery.center.z);
    // printf( "Vector to sun = %.2f %.2f %.2f\n",
    //         v->to_sun[0], v->to_sun[1], v->to_sun[2]);

    // make a vector to the current view position
    MAT3_SET_VEC(v0, v->view_pos.x, v->view_pos.y, v->view_pos.z);

    // Given a vector from the view position to the point on the
    // earth's surface the sun is directly over, map into onto the
    // local plane representing "horizontal".
    map_vec_onto_cur_surface_plane(v->local_up, v0, v->to_sun, 
				   v->surface_to_sun);
    MAT3_NORMALIZE_VEC(v->surface_to_sun, ntmp);
    // printf("Surface direction to sun is %.2f %.2f %.2f\n",
    //        v->surface_to_sun[0], v->surface_to_sun[1], v->surface_to_sun[2]);
    // printf("Should be close to zero = %.2f\n", 
    //        MAT3_DOT_PRODUCT(v->local_up, v->surface_to_sun));
}


// $Log$
// Revision 1.3  1998/04/25 20:24:03  curt
// Cleaned up initialization sequence to eliminate interdependencies
// between sun position, lighting, and view position.  This creates a
// valid single pass initialization path.
//
// Revision 1.2  1998/04/24 00:52:31  curt
// Wrapped "#include <config.h>" in "#ifdef HAVE_CONFIG_H"
// Fog color fixes.
// Separated out lighting calcs into their own file.
//
// Revision 1.1  1998/04/22 13:24:07  curt
// C++ - ifiing the code a bit.
// Starting to reorginize some of the lighting calcs to use a table lookup.
//
// Revision 1.27  1998/04/03 22:12:57  curt
// Converting to Gnu autoconf system.
// Centralized time handling differences.
//
// Revision 1.26  1998/02/23 19:08:00  curt
// Incorporated Durk's Astro/ tweaks.  Includes unifying the sun position
// calculation code between sun display, and other FG sections that use this
// for things like lighting.
//
// Revision 1.25  1998/02/09 15:07:53  curt
// Minor tweaks.
//
// Revision 1.24  1998/01/27 00:48:07  curt
// Incorporated Paul Bleisch's <bleisch@chromatic.com> new debug message
// system and commandline/config file processing code.
//
// Revision 1.23  1998/01/19 19:27:21  curt
// Merged in make system changes from Bob Kuehne <rpk@sgi.com>
// This should simplify things tremendously.
//
// Revision 1.22  1998/01/19 18:40:40  curt
// Tons of little changes to clean up the code and to remove fatal errors
// when building with the c++ compiler.
//
// Revision 1.21  1997/12/30 23:10:19  curt
// Calculate lighting parameters here.
//
// Revision 1.20  1997/12/30 22:22:43  curt
// Further integration of event manager.
//
// Revision 1.19  1997/12/30 20:47:59  curt
// Integrated new event manager with subsystem initializations.
//
// Revision 1.18  1997/12/23 04:58:40  curt
// Tweaked the sky coloring a bit to build in structures to allow finer rgb
// control.
//
// Revision 1.17  1997/12/15 23:55:08  curt
// Add xgl wrappers for debugging.
// Generate terrain normals on the fly.
//
// Revision 1.16  1997/12/11 04:43:57  curt
// Fixed sun vector and lighting problems.  I thing the moon is now lit
// correctly.
//
// Revision 1.15  1997/12/10 22:37:55  curt
// Prepended "fg" on the name of all global structures that didn't have it yet.
// i.e. "struct WEATHER {}" became "struct fgWEATHER {}"
//
// Revision 1.14  1997/12/09 04:25:39  curt
// Working on adding a global lighting params structure.
//
// Revision 1.13  1997/11/25 19:25:42  curt
// Changes to integrate Durk's moon/sun code updates + clean up.
//
// Revision 1.12  1997/11/15 18:15:39  curt
// Reverse direction of sun vector, so object normals can be more normal.
//
// Revision 1.11  1997/10/28 21:07:21  curt
// Changed GLUT/ -> Main/
//
// Revision 1.10  1997/09/13 02:00:09  curt
// Mostly working on stars and generating sidereal time for accurate star
// placement.
//
// Revision 1.9  1997/09/05 14:17:31  curt
// More tweaking with stars.
//
// Revision 1.8  1997/09/05 01:36:04  curt
// Working on getting stars right.
//
// Revision 1.7  1997/09/04 02:17:40  curt
// Shufflin' stuff.
//
// Revision 1.6  1997/08/27 03:30:37  curt
// Changed naming scheme of basic shared structures.
//
// Revision 1.5  1997/08/22 21:34:41  curt
// Doing a bit of reorganizing and house cleaning.
//
// Revision 1.4  1997/08/19 23:55:09  curt
// Worked on better simulating real lighting.
//
// Revision 1.3  1997/08/13 20:23:49  curt
// The interface to sunpos now updates a global structure rather than returning
// current sun position.
//
// Revision 1.2  1997/08/06 00:24:32  curt
// Working on correct real time sun lighting.
//
// Revision 1.1  1997/08/01 15:27:56  curt
// Initial revision.
//