+  private static $USER_AGENT = "Mozilla/5.0 (Windows NT 6.1; rv:22.0) Gecko/20130405 Firefox/22.0";
+
+  public function __construct($generate_enclosure = FALSE) {
+    $this->generate_enclosure = $generate_enclosure;
+  }
+
+  public static function epoch_to_gmdate($timestamp)
+  {
+    if (!is_numeric($timestamp) || is_nan($timestamp)) {
+      $timestamp = 0;
+    }
+
+    return gmdate('D, d M Y H:i:s', $timestamp) . ' GMT';
+  }
+
+  public static function str_to_gmdate($date)
+  {
+    $timestamp = strtotime($date);
+    if (FALSE === $timestamp) {
+      $timestamp = 0;
+    }
+
+    return Tweeper::epoch_to_gmdate($timestamp);
+  }
+
+  private static function get_contents($url)
+  {
+    $ch = curl_init($url);
+    curl_setopt_array($ch, array(
+      CURLOPT_HEADER => FALSE,
+      CURLOPT_FOLLOWLOCATION => TRUE,  // follow http redirects to get the real URL
+      CURLOPT_RETURNTRANSFER => TRUE,
+      CURLOPT_SSL_VERIFYHOST => FALSE,
+      CURLOPT_SSL_VERIFYPEER => FALSE,
+      CURLOPT_HTTPHEADER => array('Accept-language: en'),
+      CURLOPT_USERAGENT => Tweeper::$USER_AGENT,
+    ));
+    $contents = curl_exec($ch);
+    curl_close($ch);
+
+    return $contents;
+  }
+
+  private static function get_info($url)
+  {
+    $ch = curl_init($url);
+    curl_setopt_array($ch, array(
+      CURLOPT_HEADER => TRUE,
+      CURLOPT_NOBODY => TRUE,
+      CURLOPT_FOLLOWLOCATION => TRUE,  // follow http redirects to get the real URL
+      CURLOPT_RETURNTRANSFER => TRUE,
+      CURLOPT_SSL_VERIFYHOST => FALSE,
+      CURLOPT_SSL_VERIFYPEER => FALSE,
+      CURLOPT_USERAGENT => Tweeper::$USER_AGENT,
+    ));
+    curl_exec($ch);
+    $url_info = curl_getinfo($ch);
+    curl_close($ch);
+
+    return $url_info;
+  }
+
+  public static function generate_enclosure($url)
+  {
+    $supported_content_types = array(
+      "application/ogg",
+      "audio/aac",
+      "audio/mp4",
+      "audio/mpeg",
+      "audio/ogg",
+      "audio/vorbis",
+      "audio/wav",
+      "audio/webm",
+      "audio/x-midi",
+      "image/gif",
+      "image/jpeg",
+      "video/avi",
+      "video/mp4",
+      "video/mpeg",
+      "video/ogg",
+    );
+
+    // The RSS specification says that the enclosure element url must be http.
+    // See http://sourceforge.net/p/feedvalidator/bugs/72/
+    $http_url = preg_replace("/^https/", "http", $url);
+
+    $url_info = Tweeper::get_info($http_url);
+
+    $supported = in_array($url_info['content_type'], $supported_content_types);
+    if (!$supported) {
+      error_log("Unsupported enclosure content type \"" . $url_info['content_type'] . "\" for URL: " . $url_info['url']);
+      return '';
+    }
+
+    $dom = new DomDocument();
+    $enc = $dom->createElement('enclosure');
+    $enc->setAttribute('url', $url_info['url']);
+    $enc->setAttribute('length', $url_info['download_content_length']);
+    $enc->setAttribute('type', $url_info['content_type']);
+
+    $dom->appendChild($enc);
+
+    return $dom->saveXML($enc);
+  }
+
+  /* Mimic the message from libxml.c::php_libxml_ctx_error_level() */
+  private function log_xml_error($error) {
+    $output = "";
+
+    switch ($error->level) {
+    case LIBXML_ERR_WARNING:
+      $output .= "Warning $error->code: ";
+      break;
+    case LIBXML_ERR_ERROR:
+      $output .= "Error $error->code: ";
+      break;
+    case LIBXML_ERR_FATAL:
+      $output .= "Fatal Error $error->code: ";
+      break;
+    }
+
+    $output .= trim($error->message);
+
+    if ($error->file) {
+      $output .= " in $error->file";
+    } else {
+      $output .= " in Entity,";
+    }
+
+    $output .=" line $error->line";
+
+    error_log($output);
+  }
+
+  private function load_stylesheet($host) {
+    $stylesheet = "file://" . __DIR__ . "/rss_converter_" . $host . ".xsl";
+    if (FALSE === file_exists($stylesheet)) {
+      trigger_error("Conversion to RSS not supported for $host ($stylesheet not found)", E_USER_ERROR);
+      return NULL;
+    }