class JsCollectionGrouper

class JsCollectionGrouper implements AssetCollectionGrouperInterface {
    
    /**
     * {@inheritdoc}
     *
     * Puts multiple items into the same group if they are groupable. Items of
     * the 'file' type are groupable if their 'preprocess' flag is TRUE. Items of
     * the 'external' type are not 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, if needed to accommodate other items in between.
     */
    public function group(array $js_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;
        $index = -1;
        foreach ($js_assets as $item) {
            switch ($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'],
                    ] : 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) {
                $index++;
                // Initialize the new group with the same properties as the first item
                // being placed into it. The item's 'data' and 'weight' properties are
                // unique to the item and should not be carried over to the group.
                $groups[$index] = $item;
                unset($groups[$index]['data'], $groups[$index]['weight']);
                $groups[$index]['items'] = [];
                $current_group_keys = $group_keys ? $group_keys : NULL;
            }
            // Add the item to the current group.
            $groups[$index]['items'][] = $item;
        }
        return $groups;
    }

}