Implement "All" Facet to facet block configuration

This came from a ticket I raised with Acquia as I had a use case that need an All facet. Robert Douglass asked me to raise this as a feature request on drupal.org.

Use Case: Allow a user to select an 'All' facet that essentially removes all previous facets applied for that particular type (example of a site implementing this type of funtionality - http://www.tripadvisor.com/Hotels-g293924-Hanoi-Hotels.html).

Notes: I see this being an option in the apachesolr facet block configuration in a similar way to the 'Include a facet for missing' i.e. 'Include a facet for all'. Below is the code that works around this through theming (I've marked start and end comments around what has changed from the original theme_apachesolr_facet_list, there is also a help function at the bottom for array insert but maintaining the indices), however I think this is best incorporated into the core apachesolr module.

/**
* An implementation of hook_theme_registry_alter()
* Swap in our own replacement for theme_apachesolr_facet_list()
*
* @param mixed $theme_registry
*/
function example_theme_registry_alter(&$theme_registry) {
  $theme_registry['apachesolr_facet_list']['function'] = 'theme_example_apachesolr_facet_list'; 
} 

/**
* Override of the facet list theme. Allows the "All" facet to be output.
*
* @param array $items
* @param mixed $display_limit
* @param mixed $delta
*/
function theme_example_apachesolr_facet_list($items, $display_limit = 0, $delta = '') {
  apachesolr_js();   
 
  $items = array_values($items);
 
  // START:
  // PATCH TO OUTPUT THE "ALL" FACET LINK
  $response = apachesolr_static_response_cache();
 
  if ($fields = apachesolr_cck_fields()) {
   
    $query = apachesolr_current_query();
   
    foreach ($fields as $name => $field) {
      if ($field['field_name'] == $delta) {
        $facet_field = apachesolr_index_key($field);

        $total = 0;
        $filter_applied = FALSE;

        foreach ($response->facet_counts->facet_fields->$facet_field as $facet => $count) {
          $total += $count;
         
          if ($query->has_filter($facet_field, $facet)) {
            $filter_applied = TRUE;
          }
        }
       
        $new_query = clone $query;
        $new_query->remove_filter($facet_field);
       
        $options = array();
        $options['query'] = $new_query->get_url_queryvalues();
   
        if ($filter_applied) {
          // A filter is already applied so "All" should be in an unchecked state
          $link = theme('apachesolr_facet_link',
            t('All'),
            $new_query->get_path(),
            $options,
            $total,
            FALSE,
            $response->response->numFound
          );
        }
        else {
          // A filter isn't applied so "All" should be in a checked state
          $link = theme('apachesolr_unclick_link',
            t('All') . ' ('. $total .')',
            $new_query->get_path(),
            $options
          );
        }
        example_array_insert($items, $link, 0);
      }
    }
  }
  // END:
  // PATCH TO OUTPUT THE "ALL" FACET LINK

 
  // If there is a limit and the facet count is over the limit, hide the rest.
  if (($display_limit > 0) && (count($items) > $display_limit)) {
    // Split items array into displayed and hidden.
    $hidden_items = array_splice($items, $display_limit);
    foreach ($hidden_items as $hidden_item) {
      if (!is_array($hidden_item)) {
        $hidden_item = array('data' => $hidden_item);
      }
      $items[] = $hidden_item + array('class' => 'apachesolr-hidden-facet');
    }
  }
 
  $admin_link = '';
  if (user_access('administer search')) {
    $admin_link = l(t('Configure enabled filters'), 'admin/settings/apachesolr/enabled-filters');
  }
 
  return theme('item_list', $items) . $admin_link;
}

/**
* Inserts an element into the array at a specific position. Maintains
* original key indicies.
*
* @param array $array
* @param mixed $insert
* @param mixed $position
*/
function example_array_insert(&$array, $insert, $position){
 
  $length = count($array);
 
  if(!is_numeric($position))
    return FALSE;
   
  if($position == $length){
    $array = array_merge($array, array($insert));
  }
  else{
    $head = array_slice($array, 0, $position);
    $insert = array($insert);
    $tail = array_slice($array, $position);
    $array = array_merge($head, $insert, $tail);
  }
 
  return FALSE;
}