[feat. req.] Inverse update (undo)?

[cyqsimon]

In some cases the user may wish to retroactively invalidate a game and reverse the effects it had on the ratings of the players involved. For example when:

• a game's result was recorded incorrectly;
• a player was retroactively disqualified;
• the tournament in which the game was played was cancelled;
• etc.

So I think it would be a good idea to provide the inverse of an "update" function, an "undo" if you will.

As far as I know the update algorithm for Elo is invertible, but I'm not sure about other rating systems. Do you think this is something that can be implemented without substantial effort?

[atomflunder]

Hi, thanks for the feature request!

I am by no means a maths expert, but I think since the Elo algorithm uses the previous values to calculate the expected score for the new ratings it would be non-trivial to revert the calculations as far as I know.
As for some of the other algorithms that are not based on Elo I am fairly pessimistic, just off the top of my head the volatility calculations for Glicko-2 will probably be an issue, and the general complexity of the TrueSkill calculations will certainly not be easy.

But if you can find anything please do let me know. I would like to be proven wrong here since I do think this would be a nice addition.

I also run a leaderboard using TrueSkill calculations and the way I revert matches there is to store the ratings and deviations before the game happened in a database to revert back to those values. But this is only an option if both players have not played additional matches, or you would need to re-calculate the games afterwards.
I know this is not the most practical solution.

Again, I do really like the solution and I am open to work on this, if it does prove to be doable. If you want to contribute that would of course be greatly appreciated as well!

[marcospb19]

I also run a leaderboard using TrueSkill calculations and the way I revert matches there is to store the ratings and deviations before the game happened in a database to revert back to those values.

After invalidating one game, do you re-run calculations for all matches that happened after, or do you just subtract the diff?

[atomflunder]

@marcospb19 Yes, you would need to re-run the calculations for every match affected by it, unfortunately. Subtracting would yield inaccurate results

Maybe we should add a sort of "bulk match calculation" function for this scenario 🤔

[marcospb19]

I think I'd rather do a for-loop instead :smiley:.