earthdistance.c

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/* contrib/earthdistance/earthdistance.c */
 
#include "postgres.h"
 
#include <math.h>
 
#include "utils/geo_decls.h"    /* for Point */
 
/* X/Open (XSI) requires <math.h> to provide M_PI, but core POSIX does not */
#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
 ******************************************************/
 
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);
}