public function addUrlsToCrawlerQueue(array $urls) {
        // TODO: remove this debug message
        if ($this->getCrawlerDebugEnabled()) {
            foreach ($urls as $url) {
                Mage::helper('turpentine/debug')->log(
                    'Adding URL to queue: %s', $url );
            }
        }
        $oldQueue = $this->_readUrlQueue();
        $newQueue = array_unique(array_merge($oldQueue, $urls));
        $this->_writeUrlQueue($newQueue);
        $diff = count($newQueue) - count($oldQueue);
        return $diff;
    }