/*
 * call-seq:
 *  evaluate(search_path, handler = nil)
 *
 * Evaluate the +search_path+ returning an XML::XPath object.
 */
static VALUE evaluate(int argc, VALUE *argv, VALUE self)
{
  VALUE search_path, xpath_handler;
  VALUE thing = Qnil;
  xmlXPathContextPtr ctx;
  xmlXPathObjectPtr xpath;
  xmlChar *query;

  Data_Get_Struct(self, xmlXPathContext, ctx);

  if(rb_scan_args(argc, argv, "11", &search_path, &xpath_handler) == 1)
    xpath_handler = Qnil;

  query = (xmlChar *)StringValuePtr(search_path);

  if(Qnil != xpath_handler) {
    /* FIXME: not sure if this is the correct place to shove private data. */
    ctx->userData = (void *)xpath_handler;
    xmlXPathRegisterFuncLookup(ctx, lookup, (void *)xpath_handler);
  }

  xmlResetLastError();
  xmlSetStructuredErrorFunc(NULL, xpath_exception_handler);

  /* For some reason, xmlXPathEvalExpression will blow up with a generic error */
  /* when there is a non existent function. */
  xmlSetGenericErrorFunc(NULL, xpath_generic_exception_handler);

  xpath = xmlXPathEvalExpression(query, ctx);
  xmlSetStructuredErrorFunc(NULL, NULL);
  xmlSetGenericErrorFunc(NULL, NULL);

  if(xpath == NULL) {
    VALUE xpath = rb_const_get(mNokogiriXml, rb_intern("XPath"));
    VALUE klass = rb_const_get(xpath, rb_intern("SyntaxError"));

    xmlErrorPtr error = xmlGetLastError();
    rb_exc_raise(Nokogiri_wrap_xml_syntax_error(klass, error));
  }

  assert(ctx->doc);
  assert(DOC_RUBY_OBJECT_TEST(ctx->doc));

  switch(xpath->type) {
    case XPATH_STRING:
      thing = NOKOGIRI_STR_NEW2(xpath->stringval);
      xmlFree(xpath->stringval);
      break;
    case XPATH_NODESET:
      if(NULL == xpath->nodesetval) {
        thing = Nokogiri_wrap_xml_node_set(xmlXPathNodeSetCreate(NULL),
          DOC_RUBY_OBJECT(ctx->doc));
      } else {
        thing = Nokogiri_wrap_xml_node_set(xpath->nodesetval,
            DOC_RUBY_OBJECT(ctx->doc));
      }
      break;
    case XPATH_NUMBER:
      thing = rb_float_new(xpath->floatval);
      break;
    case XPATH_BOOLEAN:
      thing = xpath->boolval == 1 ? Qtrue : Qfalse;
      break;
    default:
      thing = Nokogiri_wrap_xml_node_set(xmlXPathNodeSetCreate(NULL),
        DOC_RUBY_OBJECT(ctx->doc));
  }

  xmlXPathFreeNodeSetList(xpath);

  return thing;
}