earthdistance.c

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/* contrib/earthdistance/earthdistance.c */

#include "postgres.h"

#include <math.h>

#include "utils/geo_decls.h"    /* for Point */

#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif


PG_MODULE_MAGIC;

/* Earth's radius is in statute miles. */
static const double EARTH_RADIUS = 3958.747716;
static const double TWO_PI = 2.0 * M_PI;


/******************************************************
 *
 * degtorad - convert degrees to radians
 *
 * arg: double, angle in degrees
 *
 * returns: double, same angle in radians
 ******************************************************/

static double
degtorad(double degrees)
{
    return (degrees / 360.0) * TWO_PI;
}

/******************************************************
 *
 * geo_distance_internal - distance between points
 *
 * args:
 *   a pair of points - for each point,
 *     x-coordinate is longitude in degrees west of Greenwich
 *     y-coordinate is latitude in degrees above equator
 *
 * returns: double
 *   distance between the points in miles on earth's surface
 ******************************************************/

static double
geo_distance_internal(Point *pt1, Point *pt2)
{
    double      long1,
                lat1,
                long2,
                lat2;
    double      longdiff;
    double      sino;

    /* convert degrees to radians */

    long1 = degtorad(pt1->x);
    lat1 = degtorad(pt1->y);

    long2 = degtorad(pt2->x);
    lat2 = degtorad(pt2->y);

    /* compute difference in longitudes - want < 180 degrees */
    longdiff = fabs(long1 - long2);
    if (longdiff > M_PI)
        longdiff = TWO_PI - longdiff;

    sino = sqrt(sin(fabs(lat1 - lat2) / 2.) * sin(fabs(lat1 - lat2) / 2.) +
            cos(lat1) * cos(lat2) * sin(longdiff / 2.) * sin(longdiff / 2.));
    if (sino > 1.)
        sino = 1.;

    return 2. * EARTH_RADIUS * asin(sino);
}


/******************************************************
 *
 * geo_distance - distance between points
 *
 * args:
 *   a pair of points - for each point,
 *     x-coordinate is longitude in degrees west of Greenwich
 *     y-coordinate is latitude in degrees above equator
 *
 * returns: float8
 *   distance between the points in miles on earth's surface
 *
 * If float8 is passed-by-value, the oldstyle version-0 calling convention
 * is unportable, so we use version-1.  However, if it's passed-by-reference,
 * continue to use oldstyle.  This is just because we'd like earthdistance
 * to serve as a canary for any unintentional breakage of version-0 functions
 * with float8 results.
 ******************************************************/

#ifdef USE_FLOAT8_BYVAL

Datum       geo_distance(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(geo_distance);

Datum
geo_distance(PG_FUNCTION_ARGS)
{
    Point      *pt1 = PG_GETARG_POINT_P(0);
    Point      *pt2 = PG_GETARG_POINT_P(1);
    float8      result;

    result = geo_distance_internal(pt1, pt2);
    PG_RETURN_FLOAT8(result);
}
#else                           /* !USE_FLOAT8_BYVAL */

double     *geo_distance(Point *pt1, Point *pt2);

double *
geo_distance(Point *pt1, Point *pt2)
{
    double     *resultp = palloc(sizeof(double));

    *resultp = geo_distance_internal(pt1, pt2);
    return resultp;
}

#endif   /* USE_FLOAT8_BYVAL */