Backgammon Programming

Programming

Adjusting to a weaker opponent

From:   Brian Sheppard
Address:   brians@mstone.com
Date:   3 July 1997
Subject:   Re: If a bot could look at your rating
Forum:   rec.games.backgammon
Google:   01bc87b7$06cd6420$3ac032cf@polaris

Don Banks wrote: > I think the main advantage that human experts have > over computer programs is the ability to make a certain > play based on the skill level of the opponent. > In other words, a program like jellyfish plays exactly > the same against everyone, whereas human players can > make tactical adjustments. Jake Jacobs and Walter Trice have written a book showing how humans should handle the cube in an unequal match. See their web page <http://homepage.interaccess.com/~itaewon/> > But, on Fibs, a program can actually *see* how good you > are before the match even starts. Is there anything that > can be gained by knowing the players' ratings? > > Of course. Here's an example. Suppose that: > jellyfish, rated 2000, is playing X, rated 1000, in a > 4 point match. At some point, player X doubles and jf > owns the 2 cube. Now suppose the game turns into a race > and jellyfish is left with two checkers, one on the 2 > point and one on the five point, while the opponent is left > with one checker on his one point. It's jellyfish's roll. > > What to do. Double? There are 19/36 winning sequences for > jf, and this is its last chance to double. > But of course, if jf considers the difference in their > rating, doubling is a very bad move. The stakes are not > equal in this match. Player X stands to win something like > 8 ratings points, jf maybe around 4. > In fact, jf would need to have an overwhelming advantage > to re-double, in order to make this a profitable game > in terms of ratings points. It is possible to build a match equity table that shows your chance of winning a match as a function of the difference in your skill levels. To my knowledge, JF does not use this capability. It is possible that other programs do. It is not hard to do, since these programs have match equity calculators already. Another dimension is checker play. Stronger players should aim for complex, dynamic positions where things are "up in the air." Computer programs are stronger players, so they should strive for complicated play. I do not know whether any programs do this. It is harder to implement than the cube-handling trick above. One approximation could be to choose opening plays on the basis of one's opponent, and after that allow the program to play its natural moves. Another dimension of human play is modifying your behavior based upon observed weaknesses in the opponent. For instance, if you are not sure whether to double or not, you might be swayed by the fact that the opponent incorrectly dropped a similar double. This is hard to implement. > I just think it would be interesting if this calculation > were included in a bot's positional analysis, to see if it > would significantly improve its rating. I think you would be disappointed by the results. It may improve the program's play, or it may not. The problem is that programs are already playing so well, that it is hard to change anything without making them weaker. The program gives up equity every time it plays a non-optimal move. Will it regain that equity by errors of the opponent? Or not? The doubling cube provides us with examples and counterexamples. For instance, in the last-roll situation you give it would be a clear error to double. But please note the reason why it would be an error: the opponent is presumed to make the correct take/drop decision. But in general a weak opponent cannot be presumed to make correct take/drop decisions. What do you do then? Some players (Kit Woolsey?) have argued that being more aggressive with the cube pays dividends because weaker players make two characteristic errors: 1) They often drop takeable doubles, and 2) Even if they correctly take, they are likely to botch the game after that point. Other players (Jacobs and Trice?) argue that the doubling cube should be offerred more sparingly. These players back up their assertions with match equity calculations. But are they assuming an unrealistic percentage of correct take/drop decisions? I don't know what the resolution should be. I think that it is safe to use an opponent-adjusted match equity table for deciding whether to take/drop. It is safe, though unenterprising, to offer doubles strictly on the basis of theoretical equity. My belief is that programs have plenty of room for improvement in their equity judgment. IMO, adjusting the program to "play-the-man" is not the biggest problem at this point. Warm Regards, Brian

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