MVC Pagination helper

PagerHelper.cs

//orignal post - http://joelabrahamsson.com/twitter-style-paging-with-aspnet-mvc-and-jquery/

  public static class PagerHelper
  {
     public static MvcHtmlString BootstrapPager(this HtmlHelper helper, int currentPageIndex, int totalItems,
                                                int pageSize = 10, int numberOfLinks = 5)
      {
          if (totalItems <= 0) return MvcHtmlString.Empty;

          var totalPages = (int)Math.Ceiling(totalItems / (double)pageSize);
          var lastPageNumber = (int)Math.Ceiling((double)currentPageIndex / numberOfLinks) * numberOfLinks;
          var firstPageNumber = lastPageNumber - (numberOfLinks - 1);
          var hasPreviousPage = currentPageIndex > 1;
          var hasNextPage = currentPageIndex < totalPages;

          if (lastPageNumber > totalPages) lastPageNumber = totalPages;

          var ul = new TagBuilderExtension("ul");
          ul.AddCssClass("pagination");
          ul.Append(AddLink(1, currentPageIndex == 1, "disabled", "<<", "First Page"));
          ul.Append(AddLink(currentPageIndex - 1, !hasPreviousPage, "disabled", "<", "Previous Page"));
          for (var i = firstPageNumber; i <= lastPageNumber; i++)
              ul.Append(AddLink(i, i == currentPageIndex, "active", i.ToString(), i.ToString()));

          ul.Append(AddLink(currentPageIndex + 1, !hasNextPage, "disabled", ">", "Next Page"));
          ul.Append(AddLink(totalPages, currentPageIndex == totalPages, "disabled", ">>", "Last Page"));
          return MvcHtmlString.Create(ul.ToString());
      }

      private static TagBuilder AddLink(int index, bool condition,
          string classToAdd, string linkText, string tooltip)
      {
          var li = new TagBuilder("li");
          li.MergeAttribute("title", tooltip);
          if (condition) li.AddCssClass(classToAdd);
          var a = new TagBuilder("a");
          a.MergeAttribute("href", !condition ? "javascript:getpage('" + index + "')" : "javascript:");
          a.SetInnerText(linkText);
          li.InnerHtml = a.ToString();
          return li;
      }
  }

  public class TagBuilderExtension : TagBuilder
  {
      private readonly StringBuilder sb = new StringBuilder();

      public TagBuilderExtension(string tagName) : base(tagName) { }

      public void Append(TagBuilder tb) => sb.Append(tb);

      public override string ToString()
      {
          InnerHtml = sb.ToString();
          return base.ToString();
      }
  }