class CssCollectionOptimizer

class CssCollectionOptimizer implements AssetCollectionOptimizerInterface {
    
    /**
     * A CSS asset grouper.
     *
     * @var \Drupal\Core\Asset\CssCollectionGrouper
     */
    protected $grouper;
    
    /**
     * A CSS asset optimizer.
     *
     * @var \Drupal\Core\Asset\CssOptimizer
     */
    protected $optimizer;
    
    /**
     * An asset dumper.
     *
     * @var \Drupal\Core\Asset\AssetDumper
     */
    protected $dumper;
    
    /**
     * The state key/value store.
     *
     * @var \Drupal\Core\State\StateInterface
     */
    protected $state;
    
    /**
     * The file system service.
     *
     * @var \Drupal\Core\File\FileSystemInterface
     */
    protected $fileSystem;
    
    /**
     * Constructs a CssCollectionOptimizer.
     *
     * @param \Drupal\Core\Asset\AssetCollectionGrouperInterface $grouper
     *   The grouper for CSS assets.
     * @param \Drupal\Core\Asset\AssetOptimizerInterface $optimizer
     *   The optimizer for a single CSS asset.
     * @param \Drupal\Core\Asset\AssetDumperInterface $dumper
     *   The dumper for optimized CSS assets.
     * @param \Drupal\Core\State\StateInterface $state
     *   The state key/value store.
     * @param \Drupal\Core\File\FileSystemInterface $file_system
     *   The file system service.
     */
    public function __construct(AssetCollectionGrouperInterface $grouper, AssetOptimizerInterface $optimizer, AssetDumperInterface $dumper, StateInterface $state, FileSystemInterface $file_system) {
        $this->grouper = $grouper;
        $this->optimizer = $optimizer;
        $this->dumper = $dumper;
        $this->state = $state;
        $this->fileSystem = $file_system;
    }
    
    /**
     * {@inheritdoc}
     *
     * The cache file name is retrieved on a page load via a lookup variable that
     * contains an associative array. The array key is the hash of the file names
     * in $css while the value is the cache file name. The cache file is generated
     * in two cases. First, if there is no file name value for the key, which will
     * happen if a new file name has been added to $css or after the lookup
     * variable is emptied to force a rebuild of the cache. Second, the cache file
     * is generated if it is missing on disk. Old cache files are not deleted
     * immediately when the lookup variable is emptied, but are deleted after a
     * configurable period (@code system.performance.stale_file_threshold @endcode)
     * to ensure that files referenced by a cached page will still be available.
     */
    public function optimize(array $css_assets) {
        // Group the assets.
        $css_groups = $this->grouper
            ->group($css_assets);
        // Now optimize (concatenate + minify) and dump each asset group, unless
        // that was already done, in which case it should appear in
        // drupal_css_cache_files.
        // Drupal contrib can override this default CSS aggregator to keep the same
        // grouping, optimizing and dumping, but change the strategy that is used to
        // determine when the aggregate should be rebuilt (e.g. mtime, HTTPS …).
        $map = $this->state
            ->get('drupal_css_cache_files', []);
        $css_assets = [];
        foreach ($css_groups as $order => $css_group) {
            // We have to return a single asset, not a group of assets. It is now up
            // to one of the pieces of code in the switch statement below to set the
            // 'data' property to the appropriate value.
            $css_assets[$order] = $css_group;
            unset($css_assets[$order]['items']);
            switch ($css_group['type']) {
                case 'file':
                    // No preprocessing, single CSS asset: just use the existing URI.
                    if (!$css_group['preprocess']) {
                        $uri = $css_group['items'][0]['data'];
                        $css_assets[$order]['data'] = $uri;
                    }
                    else {
                        $key = $this->generateHash($css_group);
                        $uri = '';
                        if (isset($map[$key])) {
                            $uri = $map[$key];
                        }
                        if (empty($uri) || !file_exists($uri)) {
                            // Optimize each asset within the group.
                            $data = '';
                            foreach ($css_group['items'] as $css_asset) {
                                $data .= $this->optimizer
                                    ->optimize($css_asset);
                            }
                            // Per the W3C specification at
                            // http://www.w3.org/TR/REC-CSS2/cascade.html#at-import, @import
                            // rules must precede any other style, so we move those to the
                            // top. The regular expression is expressed in NOWDOC since it is
                            // detecting backslashes as well as single and double quotes. It
                            // is difficult to read when represented as a quoted string.
                            $regexp = <<<'REGEXP'
/@import\s*(?:'(?:\\'|.)*'|"(?:\\"|.)*"|url\(\s*(?:\\[\)\'\"]|[^'")])*\s*\)|url\(\s*'(?:\'|.)*'\s*\)|url\(\s*"(?:\"|.)*"\s*\)).*;/iU
REGEXP;
                            preg_match_all($regexp, $data, $matches);
                            $data = preg_replace($regexp, '', $data);
                            $data = implode('', $matches[0]) . $data;
                            // Dump the optimized CSS for this group into an aggregate file.
                            $uri = $this->dumper
                                ->dump($data, 'css');
                            // Set the URI for this group's aggregate file.
                            $css_assets[$order]['data'] = $uri;
                            // Persist the URI for this aggregate file.
                            $map[$key] = $uri;
                            $this->state
                                ->set('drupal_css_cache_files', $map);
                        }
                        else {
                            // Use the persisted URI for the optimized CSS file.
                            $css_assets[$order]['data'] = $uri;
                        }
                        $css_assets[$order]['preprocessed'] = TRUE;
                    }
                    break;
                case 'external':
                    // We don't do any aggregation and hence also no caching for external
                    // CSS assets.
                    $uri = $css_group['items'][0]['data'];
                    $css_assets[$order]['data'] = $uri;
                    break;
            }
        }
        return $css_assets;
    }
    
    /**
     * Generate a hash for a given group of CSS assets.
     *
     * @param array $css_group
     *   A group of CSS assets.
     *
     * @return string
     *   A hash to uniquely identify the given group of CSS assets.
     */
    protected function generateHash(array $css_group) {
        $css_data = [];
        foreach ($css_group['items'] as $css_file) {
            $css_data[] = $css_file['data'];
        }
        return hash('sha256', serialize($css_data));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getAll() {
        return $this->state
            ->get('drupal_css_cache_files');
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteAll() {
        $this->state
            ->delete('drupal_css_cache_files');
        $delete_stale = function ($uri) {
            // Default stale file threshold is 30 days.
            if (REQUEST_TIME - filemtime($uri) > \Drupal::config('system.performance')->get('stale_file_threshold')) {
                $this->fileSystem
                    ->delete($uri);
            }
        };
        if (is_dir('public://css')) {
            $this->fileSystem
                ->scanDirectory('public://css', '/.*/', [
                'callback' => $delete_stale,
            ]);
        }
    }

}