code: protected function retrieveItemIndex()
      {
          $this->itemIndex = new QtiTestCompilerIndex();
          try {
              $directories = $this->getCompilationDirectory();
              $data = $directories['private']->read(taoQtiTest_models_classes_QtiTestService::TEST_COMPILED_INDEX);
              if ($data) {
                  $this->itemIndex->unserialize($data);
              }
          } catch(\Exception $e) {
              \common_Logger::d('Ignoring file not found exception for Items Index');
          }
      }
docs: Retrieves the index of compiled items.