class RequestFormatRouteFilter

class RequestFormatRouteFilter implements FilterInterface {

  /**
   * {@inheritdoc}
   */
  public function filter(RouteCollection $collection, Request $request) {

    // Determine the request format.
    $default_format = static::getDefaultFormat($collection);

    // If the request does not specify a format then use the default.
    if (is_null($request
      ->getRequestFormat(NULL))) {
      $format = $default_format;
      $request
        ->setRequestFormat($default_format);
    }
    else {
      $format = $request
        ->getRequestFormat($default_format);
    }
    $routes_with_requirement = [];
    $routes_without_requirement = [];
    $result_collection = new RouteCollection();

    /** @var \Symfony\Component\Routing\Route $route */
    foreach ($collection as $name => $route) {
      if (!$route
        ->hasRequirement('_format')) {
        $routes_without_requirement[$name] = $route;
        continue;
      }
      else {
        $routes_with_requirement[$name] = $route;
      }
    }
    foreach ($routes_with_requirement as $name => $route) {

      // If the route has no _format specification, we move it to the end. If it
      // does, then no match means the route is removed entirely.
      if (($supported_formats = array_filter(explode('|', $route
        ->getRequirement('_format')))) && in_array($format, $supported_formats, TRUE)) {
        $result_collection
          ->add($name, $route);
      }
    }
    foreach ($routes_without_requirement as $name => $route) {
      $result_collection
        ->add($name, $route);
    }
    if (count($result_collection)) {
      return $result_collection;
    }

    // We do not throw a
    // \Symfony\Component\Routing\Exception\ResourceNotFoundException here
    // because we don't want to return a 404 status code, but rather a 406.
    $available_formats = static::getAvailableFormats($collection);
    $not_acceptable = new NotAcceptableHttpException("No route found for the specified format {$format}. Supported formats: " . implode(', ', $available_formats) . '.');
    if ($available_formats) {
      $links = [];
      foreach ($available_formats as $available_format) {
        $url = Url::fromUri($request
          ->getUri(), [
          'query' => [
            '_format' => $available_format,
          ],
        ])
          ->toString(TRUE)
          ->getGeneratedUrl();
        $content_type = $request
          ->getMimeType($available_format);
        $links[] = "<{$url}>; rel=\"alternate\"; type=\"{$content_type}\"";
      }
      $not_acceptable
        ->setHeaders([
        'Link' => implode(', ', $links),
      ]);
    }
    throw $not_acceptable;
  }

  /**
   * Determines the default request format.
   *
   * By default, use 'html' as the default format. But when there's only a
   * single route match, and that route specifies a '_format' requirement
   * listing a single format, then use that as the default format. Also, if
   * there are multiple routes which all require the same single format then
   * use it.
   *
   * @param \Symfony\Component\Routing\RouteCollection $collection
   *   The route collection to filter.
   *
   * @return string
   *   The default format.
   */
  protected static function getDefaultFormat(RouteCollection $collection) {
    $formats = static::getAvailableFormats($collection);

    // The default format is 'html' unless ALL routes require the same format.
    return count($formats) === 1 ? reset($formats) : 'html';
  }

  /**
   * Gets the set of formats across all routes in the collection.
   *
   * @param \Symfony\Component\Routing\RouteCollection $collection
   *   The route collection to filter.
   *
   * @return string[]
   *   All available formats.
   */
  protected static function getAvailableFormats(RouteCollection $collection) {
    $all_formats = array_reduce($collection
      ->all(), function (array $carry, Route $route) {

      // Routes without a '_format' requirement are assumed to require HTML.
      $route_formats = !$route
        ->hasRequirement('_format') ? [
        'html',
      ] : explode('|', $route
        ->getRequirement('_format'));
      return array_merge($carry, $route_formats);
    }, []);
    return array_unique(array_filter($all_formats));
  }

}