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<?php

namespace WPGMZA;

class Distance
{
    const UNITS_KM      = "km";
    const UNITS_MI      = "mi";
    
    const MILES_PER_KILOMETER = 0.621371;
    const KILOMETERS_PER_MILE = 1.60934;
    
    public static function between($a, $b)
    {
        if(!($a instanceof LatLng))
            throw new \Exception('First argument is not an instance of WPGMZA\LatLng');
        
        if(!($b instanceof LatLng))
            throw new \Exception('Second argument is not an instance of WPGMZA\LatLng');
        
        $lat1 = $a->lat;
        $lon1 = $a->lng;
        $lat2 = $b->lat;
        $lon2 = $b->lng;
        
        $dLat = deg2rad($lat2 - $lat1);
        $dLon = deg2rad($lon2 - $lon1);
        
        $a =
            sin($dLat / 2) * sin($dLat / 2) +
            cos(deg2rad($lat1)) * cos(deg2rad($lat2)) *
            sin($dLon / 2) * sin($dLon / 2);
        
        $c = 2 * atan2(sqrt($a), sqrt(1 - $a));
        $d = 6371 * $c;
        
        return $d;
    }
}