Do all the levels in Nethack get generated at Start or are they generated as they are reached?
Solution 1:
I have done far more experimenting with Wizard mode and save-scumming than I really ought to have.
I can say with reasonable certainty that the Dungeon branches, are determined at game start, because the CTRL-O command(in wizard mode) tells you where they are.
Level maps themselves are different if you first enter them after loading a copied save than the first time you entered them, so I believe that they are generated on first visit.