class CssCollectionGrouper

class CssCollectionGrouper implements AssetCollectionGrouperInterface {
    
    /**
     * {@inheritdoc}
     *
     * Puts multiple items into the same group if they are groupable and if they
     * are for the same 'media' and 'browsers'. Items of the 'file' type are
     * groupable if their 'preprocess' flag is TRUE, and items of the 'external'
     * type are never groupable.
     *
     * Also ensures that the process of grouping items does not change their
     * relative order. This requirement may result in multiple groups for the same
     * type, media, and browsers, if needed to accommodate other items in between.
     */
    public function group(array $css_assets) {
        $groups = [];
        // If a group can contain multiple items, we track the information that must
        // be the same for each item in the group, so that when we iterate the next
        // item, we can determine if it can be put into the current group, or if a
        // new group needs to be made for it.
        $current_group_keys = NULL;
        // When creating a new group, we pre-increment $i, so by initializing it to
        // -1, the first group will have index 0.
        $i = -1;
        foreach ($css_assets as $item) {
            // The browsers for which the CSS item needs to be loaded is part of the
            // information that determines when a new group is needed, but the order
            // of keys in the array doesn't matter, and we don't want a new group if
            // all that's different is that order.
            ksort($item['browsers']);
            // If the item can be grouped with other items, set $group_keys to an
            // array of information that must be the same for all items in its group.
            // If the item can't be grouped with other items, set $group_keys to
            // FALSE. We put items into a group that can be aggregated together:
            // whether they will be aggregated is up to the _drupal_css_aggregate()
            // function or an
            // override of that function specified in hook_css_alter(), but regardless
            // of the details of that function, a group represents items that can be
            // aggregated. Since a group may be rendered with a single HTML tag, all
            // items in the group must share the same information that would need to
            // be part of that HTML tag.
            switch ($item['type']) {
                case 'file':
                    // Group file items if their 'preprocess' flag is TRUE.
                    // Help ensure maximum reuse of aggregate files by only grouping
                    // together items that share the same 'group' value.
                    $group_keys = $item['preprocess'] ? [
                        $item['type'],
                        $item['group'],
                        $item['media'],
                        $item['browsers'],
                    ] : FALSE;
                    break;
                case 'external':
                    // Do not group external items.
                    $group_keys = FALSE;
                    break;
            }
            // If the group keys don't match the most recent group we're working with,
            // then a new group must be made.
            if ($group_keys !== $current_group_keys) {
                $i++;
                // Initialize the new group with the same properties as the first item
                // being placed into it. The item's 'data', 'weight' and 'basename'
                // properties are unique to the item and should not be carried over to
                // the group.
                $groups[$i] = $item;
                unset($groups[$i]['data'], $groups[$i]['weight'], $groups[$i]['basename']);
                $groups[$i]['items'] = [];
                $current_group_keys = $group_keys ? $group_keys : NULL;
            }
            // Add the item to the current group.
            $groups[$i]['items'][] = $item;
        }
        return $groups;
    }

}