I still don't understand why the original program was not working. It is nasty etc., but it seems it SHOULD work.

"It is nasty etc" "I still don't understand"

Your question answers itself, how in hades can you "understand" something that is "nasty"?

Often, when asked to debug something written by others, I have, instead of debugging, just started rewriting in a clear form. When doing that, in more than half the cases, no debugghing has been neded, the rewrite has made the error stand out.

In case you are interested, when "rewriting in a clear form" I save the original .obj and for every 100 or so lines compile the rewrite and compare to make sure nothing has changed. During that process things that "should work, but are convoluted" are rewritten "as is" but commeted for change in phase 2.

Erik