 int PDFiumEngine::GetVisiblePageIndex(FPDF_PAGE page) {
  for (int page_index : visible_pages_) {
     if (pages_[page_index]->GetPage() == page)
       return page_index;
   }
  return -1;
}

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.