gis/distance.c

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#include <math.h>
#include <grass/gis.h>
#include <grass/glocale.h>

static double min4(double, double, double, double);
static double min2(double, double);


static int projection = 0;
static double factor = 1.0;


int G_begin_distance_calculations(void)
{
    double a, e2;

    factor = 1.0;
    switch (projection = G_projection()) {
    case PROJECTION_LL:
        G_get_ellipsoid_parameters(&a, &e2);
        G_begin_geodesic_distance(a, e2);
        return 2;
    default:
        factor = G_database_units_to_meters_factor();
        if (factor <= 0.0) {
            factor = 1.0;       /* assume meter grid */
            return 0;
        }
        return 1;
    }
}


double G_distance(double e1, double n1, double e2, double n2)
{
    if (projection == PROJECTION_LL)
        return G_geodesic_distance(e1, n1, e2, n2);
    else
        return factor * hypot(e1 - e2, n1 - n2);
}


double
G_distance_between_line_segments(double ax1, double ay1,
                                 double ax2, double ay2,
                                 double bx1, double by1,
                                 double bx2, double by2)
{
    double ra, rb;
    double x, y;

    /* if the segments intersect, then the distance is zero */
    if (G_intersect_line_segments(ax1, ay1, ax2, ay2,
                                  bx1, by1, bx2, by2, &ra, &rb, &x, &y) > 0)
        return 0.0;
    return
        min4(G_distance_point_to_line_segment(ax1, ay1, bx1, by1, bx2, by2),
             G_distance_point_to_line_segment(ax2, ay2, bx1, by1, bx2, by2),
             G_distance_point_to_line_segment(bx1, by1, ax1, ay1, ax2, ay2),
             G_distance_point_to_line_segment(bx2, by2, ax1, ay1, ax2, ay2)
        );
}


double G_distance_point_to_line_segment(double xp, double yp,   /* the point */
                                        double x1, double y1, double x2,
                                        double y2)
{                               /* the line segment */
    double dx, dy;
    double x, y;
    double xq, yq, ra, rb;
    int t;

    /* define the perpendicular to the segment thru the point */
    dx = x1 - x2;
    dy = y1 - y2;

    if (dx == 0.0 && dy == 0.0)
        return G_distance(x1, y1, xp, yp);

    if (fabs(dy) > fabs(dx)) {
        xq = xp + dy;
        yq = (dx / dy) * (xp - xq) + yp;
    }
    else {
        yq = yp + dx;
        xq = (dy / dx) * (yp - yq) + xp;
    }

    /* find the intersection of the perpendicular with the segment */
    switch (t =
            G_intersect_line_segments(xp, yp, xq, yq, x1, y1, x2, y2, &ra,
                                      &rb, &x, &y)) {
    case 0:
    case 1:
        break;
    default:
        /* parallel/colinear cases shouldn't occur with perpendicular lines */
        G_warning(_("G_distance_point_to_line_segment: shouldn't happen: "
                    "code=%d P=(%f,%f) S=(%f,%f)(%f,%f)"),
                  t, xp, yp, x1, y1, x2, y2);
        return -1.0;
    }

    /* if x,y lies on the segment, then the distance is from to x,y */
    if (rb >= 0 && rb <= 1.0)
        return G_distance(x, y, xp, yp);

    /* otherwise the distance is the short of the distances to the endpoints
     * of the segment
     */
    return min2(G_distance(x1, y1, xp, yp), G_distance(x2, y2, xp, yp));
}

static double min4(double a, double b, double c, double d)
{
    return min2(min2(a, b), min2(c, d));
}

static double min2(double a, double b)
{
    return a < b ? a : b;
}