class CssCollectionRenderer

class CssCollectionRenderer extends CoreCssCollectionRenderer implements AssetCollectionRendererInterface {
    
    /**
     * A config object for the advagg configuration.
     *
     * @var \Drupal\Core\Config\Config
     */
    protected $config;
    
    /**
     * {@inheritdoc}
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   A config factory for retrieving required config objects.
     */
    public function __construct(StateInterface $state, ConfigFactoryInterface $config_factory) {
        $this->state = $state;
        $this->config = $config_factory->get('advagg.settings');
    }
    
    /**
     * {@inheritdoc}
     */
    public function render(array $css_assets) {
        $elements = [
            'prefetch' => [],
        ];
        $prefetch = $this->config
            ->get('dns_prefetch');
        // A dummy query-string is added to filenames, to gain control over
        // browser-caching. The string changes on every update or full cache
        // flush, forcing browsers to load a new copy of the files, as the
        // URL changed.
        $query_string = $this->state
            ->get('system.css_js_query_string') ?: '0';
        // Defaults skeleton for elements.
        $link_element_defaults = [
            '#type' => 'html_tag',
            '#tag' => 'link',
            '#attributes' => [
                'rel' => 'stylesheet',
            ],
        ];
        $style_element_defaults = [
            '#type' => 'html_tag',
            '#tag' => 'style',
        ];
        $prefetch_element_defaults = [
            '#type' => 'html_tag',
            '#tag' => 'link',
            '#attributes' => [
                'rel' => 'dns-prefetch',
            ],
        ];
        // For filthy IE hack.
        $current_ie_group_key = NULL;
        $get_ie_group_key = function ($css_asset) {
            return [
                $css_asset['type'],
                $css_asset['preprocess'],
                $css_asset['group'],
                $css_asset['media'],
                $css_asset['browsers'],
            ];
        };
        // Loop through all CSS assets, by key, to allow for the special IE
        // workaround.
        $css_assets_keys = array_keys($css_assets);
        for ($i = 0; $i < count($css_assets_keys); $i++) {
            $css_asset = $css_assets[$css_assets_keys[$i]];
            switch ($css_asset['type']) {
                // For file items, there are three possibilities.
                // - There are up to 31 CSS assets on the page (some of which may be
                //   aggregated). In this case, output a LINK tag for file CSS assets.
                // - There are more than 31 CSS assets on the page, yet we must stay
                //   below IE<10's limit of 31 total CSS inclusion tags, we handle this
                //   in two ways:
                //   - file CSS assets that are not eligible for aggregation (their
                //     'preprocess' flag has been set to FALSE): in this case, output a
                //     LINK tag.
                //   - file CSS assets that can be aggregated (and possibly have been):
                //     in this case, figure out which subsequent file CSS assets share
                //     the same key properties ('group', 'inline', 'browsers' and if
                //     set also 'media') and output this group into as few STYLE tags
                //     as possible (each STYLE tag must have less than 31 @import
                //     statements).
                case 'file':
                    // Prefetch contained domains (ie calls to url()).
                    if ($prefetch && !empty($css_asset['dns_prefetch'])) {
                        foreach ($css_asset['dns_prefetch'] as $domain) {
                            $element = $prefetch_element_defaults;
                            $element['#attributes']['href'] = '//' . $domain;
                            $elements['prefetch'][] = $element;
                            if ($domain == 'fonts.googleapis.com') {
                                // Add fonts.gstatic.com when fonts.googleapis.com is added.
                                $element['#attributes']['href'] = 'https://fonts.gstatic.com';
                                array_unshift($elements, $element);
                            }
                        }
                    }
                    // The dummy query string needs to be added to the URL to control
                    // browser-caching.
                    $query_string_separator = strpos($css_asset['data'], '?') !== FALSE ? '&' : '?';
                    // As long as the current page will not run into IE's limit for CSS
                    // assets: output a LINK tag for a file CSS asset.
                    if (count($css_assets) <= 31) {
                        $element = $link_element_defaults;
                        $element['#attributes']['href'] = file_url_transform_relative(file_create_url($css_asset['data'])) . $query_string_separator . $query_string;
                        $element['#browsers'] = $css_asset['browsers'];
                        if (isset($css_asset['media'])) {
                            $element['#attributes']['media'] = $css_asset['media'];
                        }
                        $element['#inline'] = !empty($css_asset['inline']) ? TRUE : FALSE;
                        $elements[] = $element;
                    }
                    else {
                        // The file CSS asset is ineligible for aggregation: output it in a
                        // LINK tag.
                        if (!$css_asset['preprocess']) {
                            $element = $link_element_defaults;
                            $element['#attributes']['href'] = file_url_transform_relative(file_create_url($css_asset['data'])) . $query_string_separator . $query_string;
                            $element['#attributes']['media'] = $css_asset['media'];
                            $element['#browsers'] = $css_asset['browsers'];
                            $element['#inline'] = !empty($css_asset['inline']) ? TRUE : FALSE;
                            $elements[] = $element;
                        }
                        else {
                            $import = [];
                            // Start with the current CSS asset, iterate over subsequent CSS
                            // assets and find which ones have the same 'type', 'group',
                            // 'preprocess', 'inline', 'browsers' properties and depending on
                            // settings, also 'media'.
                            $j = $i;
                            $next_css_asset = $css_asset;
                            $current_ie_group_key = $get_ie_group_key($css_asset);
                            do {
                                // The dummy query string needs to be added to the URL to
                                // control browser-caching. IE7 does not support a media type on
                                // the @import statement, so we instead specify the media for
                                // the group on the STYLE tag.
                                $import[] = '@import url("' . Html::escape(file_url_transform_relative(file_create_url($next_css_asset['data'])) . '?' . $query_string) . '");';
                                // Move the outer for loop skip the next item, since we
                                // processed it here.
                                $i = $j;
                                // Retrieve next CSS asset, unless there is none: then break.
                                if ($j + 1 < count($css_assets_keys)) {
                                    $j++;
                                    $next_css_asset = $css_assets[$css_assets_keys[$j]];
                                }
                                else {
                                    break;
                                }
                            } while ($get_ie_group_key($next_css_asset) == $current_ie_group_key);
                            // In addition to IE's limit of 31 total CSS inclusion tags, it
                            // also has a limit of 31 @import statements per STYLE tag.
                            while (!empty($import)) {
                                $import_batch = array_slice($import, 0, 31);
                                $import = array_slice($import, 31);
                                $element = $style_element_defaults;
                                // This simplifies the JavaScript regex, allowing each line
                                // (separated by \n) to be treated as a completely different
                                // string. This means that we can use ^ and $ on one line at a
                                // time, and not worry about style tags since they'll never
                                // match the regex.
                                $element['#value'] = "\n" . implode("\n", $import_batch) . "\n";
                                if (isset($css_asset['media'])) {
                                    $element['#attributes']['media'] = $css_asset['media'];
                                }
                                $element['#browsers'] = $css_asset['browsers'];
                                $elements[] = $element;
                            }
                        }
                    }
                    break;
                // Output a LINK tag for an external CSS asset. The asset's 'data'
                // property contains the full URL.
                case 'external':
                    $element = $link_element_defaults;
                    $element['#attributes']['href'] = $css_asset['data'];
                    $element['#attributes']['media'] = $css_asset['media'];
                    $element['#browsers'] = $css_asset['browsers'];
                    $elements[] = $element;
                    if ($prefetch) {
                        $element = $prefetch_element_defaults;
                        $element['#attributes']['href'] = '//' . parse_url($css_asset['data'], PHP_URL_HOST);
                        $elements['prefetch'][] = $element;
                        if ($element['#attributes']['href'] == 'fonts.googleapis.com') {
                            // Add fonts.gstatic.com when fonts.googleapis.com is added.
                            $element['#attributes']['href'] = 'https://fonts.gstatic.com';
                            $elements['prefetch'][] = $element;
                        }
                    }
                    break;
                default:
                    throw new \Exception('Invalid CSS asset type.');
            }
        }
        if (empty($elements['prefetch'])) {
            unset($elements['prefetch']);
        }
        return $elements;
    }

}