Sorting products so out of stock appear at the bottom in X-Cart 4

//find
if (
            stripos($query, XCRangeProductIds::getSqlMINPriceField()) !== false
            && preg_match('/ORDER BY .*\b' . XCRangeProductIds::getSqlMINPriceField('get_alias_name') . '\b/i', $query)
        ) {
            // To avoid sql error related to 'GROUP BY price '
            $substr_fields = "$sql_tbl[products].productid, " . XCRangeProductIds::getSqlMINPriceField();
        } else {
            $substr_fields = "$sql_tbl[products].productid";
        }

//replace
if (
            stripos($query, XCRangeProductIds::getSqlMINPriceField()) !== false
            && preg_match('/ORDER BY .*\b' . XCRangeProductIds::getSqlMINPriceField('get_alias_name') . '\b/i', $query)
        ) {
            // To avoid sql error related to 'GROUP BY price '
            $substr_fields = "$sql_tbl[products].productid, IF($sql_tbl[products].avail=0,0,1) AS instock, " . XCRangeProductIds::getSqlMINPriceField();
            
        } else {
            $substr_fields = "$sql_tbl[products].productid, IF($sql_tbl[products].avail=0,0,1) AS instock";
        }

//find the code below
if (empty($data['is_modify'])) {
  $fields[] = $sql_tbl['products'] . '.*';
} else {
  $fields[] = $sql_tbl['products'] . '.productid';
}

//replace with 
if (empty($data['is_modify'])) {
  $fields[] = $sql_tbl['products'] . '.*';
  //2022-03-08 add field to sort by out of stock last
  $fields[] = 'IF('.$sql_tbl['products'].'.avail=0,0,1) AS instock';
} else {
  $fields[] = $sql_tbl['products'] . '.productid';
}

//find the code
switch ($data['sort_field']) {
  case 'productcode':
    $sort_string = "$sql_tbl[products].productcode $direction";
    break;

  case 'title':
    $sort_string = "$sql_tbl[products_lng_current].product $direction";
    break;

  case 'orderby':
    $sort_string = "$sql_tbl[products_categories].orderby $direction";
    break;

  case 'sales_stats':
      $inner_joins['product_sales_stats'] = array(
        'on' => "$sql_tbl[product_sales_stats].productid = $sql_tbl[products].productid",
        'only_select' => TRUE,
      );

      $sort_string = "$sql_tbl[product_sales_stats].sales_stats $direction";
    break;

  case 'quantity':
    $sort_string = empty($active_modules['Product_Options'])
      ? "$sql_tbl[products].avail $direction"
      : XCVariantsSQL::getVariantField('avail') . " $direction";
    break;

  case 'price':
    if (
      !empty($active_modules['Special_Offers'])
      && isset($search_data['products']['show_special_prices'])
      && $search_data['products']['show_special_prices']
    ) {
      $sort_string = "x_special_price $direction, price $direction";

    } else {

      $sort_string = "price $direction";
    }

    break;

  default:
    $sort_string = "$sql_tbl[products_lng_current].product";
}

//replace with
//find the code
switch ($data['sort_field']) {
  case 'productcode':
    $sort_string = "instock DESC, $sql_tbl[products].productcode $direction";
    break;

  case 'title':
    $sort_string = "instock DESC, $sql_tbl[products_lng_current].product $direction";
    break;

  case 'orderby':
    $sort_string = "instock DESC, $sql_tbl[products_categories].orderby $direction";
    break;

  case 'sales_stats':
      $inner_joins['product_sales_stats'] = array(
        'on' => "$sql_tbl[product_sales_stats].productid = $sql_tbl[products].productid",
        'only_select' => TRUE,
      );
    	$sort_string = "instock DESC, $sql_tbl[product_sales_stats].sales_stats $direction";

    break;

  case 'quantity':
    $sort_string = empty($active_modules['Product_Options'])
      ? "$sql_tbl[products].avail $direction"
      : XCVariantsSQL::getVariantField('avail') . " $direction";
    break;

  case 'price':
    if (
      !empty($active_modules['Special_Offers'])
      && isset($search_data['products']['show_special_prices'])
      && $search_data['products']['show_special_prices']
    ) {
      $sort_string = "x_special_price $direction, price $direction";

    } else {

      $sort_string = "instock DESC, price $direction";
    }

    break;

  default:
    $sort_string = "instock DESC, $sql_tbl[products_lng_current].product";
}