Journal of Artificial Intelligence Research 14 (2001) 303–358 Submitted 10/00; published 6/01 GIB: Imperfect Information in a Computationally Challenging Game Matthew L. Ginsberg
[email protected] CIRL 1269 University of Oregon Eugene, OR 97405 USA Abstract This paper investigates the problems arising in the construction of a program to play the game of contract bridge. These problems include both the difficulty of solving the game’s perfect information variant, and techniques needed to address the fact that bridge is not, in fact, a perfect information game. Gib, the program being described, involves five separate technical advances: partition search, the practical application of Monte Carlo techniques to realistic problems, a focus on achievable sets to solve problems inherent in the Monte Carlo approach, an extension of alpha-beta pruning from total orders to arbitrary distributive lattices, and the use of squeaky wheel optimization to find approximately optimal solutions to cardplay problems. Gib is currently believed to be of approximately expert caliber, and is currently the strongest computer bridge program in the world. 1. Introduction Of all the classic games of mental skill, only card games and Go have yet to see the ap- pearance of serious computer challengers. In Go, this appears to be because the game is fundamentally one of pattern recognition as opposed to search; the brute-force techniques that have been so successful in the development of chess-playing programs have failed al- most utterly to deal with Go’s huge branching factor. Indeed, the arguably strongest Go program in the world (Handtalk) was beaten by 1-dan Janice Kim (winner of the 1984 Fuji Women’s Championship) in the 1997 AAAI Hall of Champions after Kim had given the program a monumental 25 stone handicap.