/*!
   \file lib/vector/Vlib/tin.c

   \brief Vector library - TIN

   Higher level functions for reading/writing/manipulating vectors.

   (C) 2001-2009 by the GRASS Development Team

   This program is free software under the GNU General Public License
   (>=v2).  Read the file COPYING that comes with GRASS for details.

   \author Radim Blazek
 */

#include <grass/vector.h>

/*!
   \brief Calculates z coordinate for point from TIN

   \param Map pointer to vector map
   \param tx,ty point coordinates
   \param[out] tz z-coordinate of point
   \param[out] angle angle
   \param[out] slope slope

   \return 1 on success,
   \return 0 point is not in area,
   \return -1 area has not 4 points or has island
 */
int
Vect_tin_get_z(struct Map_info *Map,
	       double tx, double ty, double *tz, double *angle, double *slope)
{
    int i, area, n_points;
    struct Plus_head *Plus;
    struct P_area *Area;
    static struct line_pnts *Points;
    static int first_time = 1;
    double *x, *y, *z;
    double vx1, vx2, vy1, vy2, vz1, vz2;
    double a, b, c, d;

    /* TODO angle, slope */

    Plus = &(Map->plus);
    if (first_time == 1) {
	Points = Vect_new_line_struct();
	first_time = 0;
    }

    area = Vect_find_area(Map, tx, ty);
    G_debug(3, "TIN: area = %d", area);
    if (area == 0)
	return 0;

    Area = Plus->Area[area];
    if (Area->n_isles > 0)
	return -1;

    Vect_get_area_points(Map, area, Points);
    n_points = Points->n_points;
    if (n_points != 4)
	return -1;

    x = Points->x;
    y = Points->y;
    z = Points->z;
    for (i = 0; i < 3; i++) {
	G_debug(3, "TIN: %d %f %f %f", i, x[i], y[i], z[i]);
    }

    vx1 = x[1] - x[0];
    vy1 = y[1] - y[0];
    vz1 = z[1] - z[0];
    vx2 = x[2] - x[0];
    vy2 = y[2] - y[0];
    vz2 = z[2] - z[0];

    a = vy1 * vz2 - vy2 * vz1;
    b = vz1 * vx2 - vz2 * vx1;
    c = vx1 * vy2 - vx2 * vy1;
    d = -a * x[0] - b * y[0] - c * z[0];

    /* OK ? */
    *tz = -(d + a * tx + b * ty) / c;
    G_debug(3, "TIN: z = %f", *tz);

    return 1;
}