protected function getNodeClass($type)
    {
        $type = strtolower($type);

        if (!isset($this->nodeMapping[$type])) {
            throw new \RuntimeException(sprintf('The node type "%s" is not registered.', $type));
        }

        $class = $this->nodeMapping[$type];

        if (!class_exists($class)) {
            throw new \RuntimeException(sprintf('The node class "%s" does not exist.', $class));
        }

        return $class;
    }