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I was just experimenting with a 12 player round robin scenario, when I noticed that for the 11th round a pairing was made which had already occurred in the tournament, instead of the last people who haven't played each other being paired. I thought that "no two players play each other twice" is an unbreakable rule in McMahon and Swiss, how come this pairing was made?
Swiss is not compatible with Round-Robin. If you want to play round-robin, you should make a complete pairing for all rounds in advance. You cannot just keep pairing Swiss and hope that it works, at some point you will run into the problem that there no longer exists a pairing where every player gets an opponent they have not met yet.
Generally the only stronger rule in Swiss/McMahon than "no two players play each other twice" is the rule "every player shall have an opponent this round". So some pairing programs will pair the same players again if that is the only way to give everyone a game.
Thanks for the explanation, I didn't know this. Would you mind if I deleted the discussion, because this discussion page isn't probably the right place (I just couldn't find a better place for it). All the info is in your amazing article anyway.
I don't think it is necessary to delete this thread. The information is relevant to the parent article, so if anyone ever has the same question, they'll be more likely to find it like this.
I'm not sure, BTW, whether all pairing programs allow the same players to meet again. Gerlach's MacMahon uses Maximum Weight Perfect Matching with a completely connected graph, but some programs may remove edges from the graph or may use other algorithms that do not allow repeat pairings.