L820 rb_funcall(Qnil, id_format, 4, rb_str_new2("NODE%i -> NODE%i;\n"),
^ 4 -> 3
L821 ULONG2NUM((unsigned long) n), ULONG2NUM((unsigned long) n1)));
here has 2 %i parameter, so the 3rd parameter value should be 3.
rb_funcall(Qnil, id_format, 3, rb_str_new2("NODE%i -> NODE%i;\n"),
^