Add class to group

Question

I am learning drupal and currently working on Views module.I am learning grouping in views.

I have created two groups using views(set format as Unformatted list).I have modified bit of code in theme.inc of views module which is located in modules > views > theme to add class to it.

So HTML output is like:

I want to add number like 1,2,3 etc(depending upon the group created) just beside bottom-box-data-.(Suppose here I have created 2 groups so output should be like bottom-box-data-1 bottom-box-data-2).

Code of theme.ince is :

    /**
     * Display a single views grouping.
     */
    function theme_views_view_grouping($vars) {
      $view = $vars['view'];
      $title = $vars['title'];
      $content = $vars['content'];

      $output = '<div class="view-grouping bottom-box-data-">';
      $output .= '<div class="view-grouping-header">' . $title . '</div>';
      $output .= '<div class="view-grouping-content">' . $content . '</div>' ;
      $output .= '</div>';

      return $output;
    }

I know, we need to show some effort then and then some one can help here.But I have no idea from where to start to achieve this.If anyone can help me out, I ll really appreciate it.

Thanks and let me know if more information is required or any one has doubt related to this question.

I came across Drupal SO question, but no one answered to that question yet.

EDITED :

Thanks J. Reynolds for your time and valuable suggestion.

So now I have above function in my template.php and I try to achieve my desired result with grouping levels.

  var_dump($vars);
  $grouping = $vars['grouping_level'];//which is 0 for both grouping.

But grouping level always giving 0 for all group hierarchy.