class Location
{
	protected $longitude;
	protected $latitude;
	
	/**
	 * Constructs a new location.
	 * 
	 * @param float $lat
	 * @param float $lon
	 * @throws InvalidArgumentException
	 */
	public function __construct($lat, $lon)
	{
		$this->setLatitude($lat);
		$this->setLongitude($lon);
	}
	
	public function bearingTo(Location $location)
	{
		$result = $this->distanceAndBearing($location);
		return $result['bearing'];
	}
	
	/**
	 * Returns the approximate distance in meters between this location and the given location.
	 * 
	 * @returns float
	 */
	public function distanceTo(Location $location)
	{
		$result = $this->distanceAndBearing($location);
		return $result['distance'];
	}
	
	/**
	 * Returns the latitude for this location.
	 * 
	 * @return float
	 */
	public function getLatitude()
	{
		return $this->latitude;
	}

	/**
	 * Returns the longitude for this location.
	 * 
	 * @return float
	 */	
	public function getLongitude()
	{
		return $this->longitude;
	}
	
	/**
	 * Sets the latitude for this location.
	 * 
	 * @param float $latitude
	 * @throws InvalidArgumentException
	 */
	public function setLatitude($latitude)
	{
		if (!is_numeric($latitude))
		{
			throw new InvalidArgumentException("Argument must be numeric.");
		}
		$this->latitude = (float) $latitude;
	}
	
	/**
	 * Sets the longitude for this location.
	 * 
	 * @param float $longitude
	 * @throws InvalidArgumentException
	 */
	public function setLongitude($longitude)
	{
		if (!is_numeric($longitude))
		{
			throw new InvalidArgumentException("Argument must be numeric.");
		}
		$this->longitude = (float) $longitude;
	}
	
	private function distanceAndBearing(Location $location)
	{
        // Based on http://www.ngs.noaa.gov/PUBS_LIB/inverse.pdf
        // using the "Inverse Formula" (section 4)
		$lat1 = deg2rad($this->getLatitude());
		$lat2 = deg2rad($location->getLatitude());
		$lon1 = deg2rad($this->getLongitude());
		$lon2 = deg2rad($location->getLongitude());
		
		$major = 6378137.0;
		$minor = 6356752.3142;
		$f = ($major-$minor)/$major;
		
		$aSqMinusBSqOverBSq = (pow($major, 2) - pow($minor, 2))/($minor*$minor);
		$L = $lon2 - $lon1;
		$U1 = atan((1.0 - $f) * tan($lat1));
		$U2 = atan((1.0 - $f) * tan($lat2));
		
		$cosU1 = cos($U1);
		$cosU2 = cos($U2);
		$sinU1 = sin($U1);
		$sinU2 = sin($U2);
		$sinU1sinU2 = $sinU1 * $sinU2;
		$cosU1cosU2 = $cosU1 * $cosU2;
		
		$A = 0.0;
		$cos2SM = 0.0;
		$sinSigma = 0.0;
		$cosSigma = 0.0;
		$deltaSigma = 0.0;
		$sinLambda = 0.0;
		$cosLambda = 0.0;
		$sigma = 0.0;
		$lambda = $L;
		
		for ($i = 0; $i < 20; $i++)
		{
			$lambdaOrig = $lambda;
			$cosLambda = cos($lambda);
			$sinLambda = sin($lambda);
			$t1 = $cosU1 * $sinLambda;
			$t2 = $cosU1 * $sinU2 - $sinU1 * $cosU2 * $cosLambda;
			$sinSqSigma = pow($t1, 2) + pow($t2, 2);
			$sinSigma = sqrt($sinSqSigma);
			$sinAlpha = ($sinSigma == 0) ? 0.0 : $cosU1cosU2 * $sinLambda / $sinSigma;
			$cosSqAlpha = 1.0 - pow($sinAlpha, 2);
			$uSq = $cosSqAlpha * $aSqMinusBSqOverBSq;

			$A = 1 + ($uSq / 16384.0) *	(4096.0 + $uSq * (-768 + $uSq * (320.0 - 175.0 * $uSq)));
			$B = ($uSq / 1024.0) * (256.0 + $uSq * (-128.0 + $uSq * (74.0 - 47.0 * $uSq)));
			$C = ($f / 16.0) * $cosSqAlpha * (4.0 + $f * (4.0 - 3.0 * $cosSqAlpha));

			$cosSigma = $sinU1sinU2 + $cosU1cosU2 * $cosLambda;
			$cos2SM = ($cosSqAlpha == 0) ? 0.0 : $cosSigma - 2.0 * $sinU1sinU2 / $cosSqAlpha;
			
			$sigma = atan2($sinSigma, $cosSigma);
			$cos2SMSq = pow($cos2SM, 2);
			$deltaSigma = $B * $sinSigma * ($cos2SM + ($B / 4.0) * ($cosSigma * (-1.0 + 2.0 * $cos2SMSq) - ($B / 6.0) * $cos2SM * (-3.0 + 4.0 * pow($sinSigma, 2)) * (-3.0 + 4.0 * $cos2SMSq)));
			
			$lambda = $L + (1.0 - $C) * $f * $sinAlpha * ($sigma + $C * $sinSigma * ($cos2SM + $C * $cosSigma * (-1.0 + 2.0 * $cos2SM * $cos2SM)));
			$delta = ($lambda - $lambdaOrig) / $lambda;
			if (abs($delta) < 1.0e-12) break;
		}
		$distance = (float) ($minor * $A * ($sigma - $deltaSigma));
		$bearing = (float) rad2deg(atan2($cosU2 * $sinLambda, $cosU1 * $sinU2 - $sinU1 * $cosU2 * $cosLambda));
		
		return array(
			'distance' => $distance,
			'bearing' => $bearing
		);
	}
}