/*
 * call-seq:
 *   attribute_at(index)
 *
 * Get the value of attribute at +index+
 */
static VALUE attribute_at(VALUE self, VALUE index)
{
  xmlTextReaderPtr reader;
  Data_Get_Struct(self, xmlTextReader, reader);

  if(index == Qnil) return Qnil;
  index = rb_funcall(index, rb_intern("to_i"), 0);

  xmlChar * value = xmlTextReaderGetAttributeNo(
      reader,
      NUM2INT(index)
  );
  if(value == NULL) return Qnil;

  VALUE MAYBE_UNUSED(enc) = rb_iv_get(self, "@encoding");
  VALUE rb_value = NOKOGIRI_STR_NEW2(value,
      RTEST(enc) ? StringValuePtr(enc) : NULL);
  xmlFree(value);
  return rb_value;
}