public function expand($shortcode)
 {
     // remove the 1 we added when we created it
     $index = ShortUrl::decode($shortcode) - 1;
     if ($index == -1) {
         return NULL;
     }
     $urls = $this->storage->getYAML('urls');
     $url_data =& $urls[$index];
     $url_data['clicks']++;
     $this->storage->putYAML('urls', $urls);
     return $url_data['url'];
 }