protected function normalizePermissions($permissions)
	{
		// remove the type identifier
		$permissions = substr($permissions, 1);

		// map the string rights to the numeric counterparts
		$map = array(
			'-' => '0',
			'r' => '4',
			'w' => '2',
			'x' => '1'
		);
		$permissions = strtr($permissions, $map);

		// split up the permission groups
		$parts = str_split($permissions, 3);

		// convert the groups
		$mapper = function ($part)
		{
			return array_sum(str_split($part));
		};

		// get the sum of the groups
		return array_sum(array_map($mapper, $parts));
	}