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));
    }

}