Searching over Metapositions in Kriegspiel

Kriegspiel is a Chess variant similar to wargames, in which players have to deal with uncertainty. Kriegspiel increases the difficulty typical of Chess by hiding from each player his opponent’s moves. Although it is a two person game it needs a referee, whose task consists in accepting the legal moves and rejecting the illegal ones, with respect to the real situation. Neither player knows the whole history of moves and each player has to guess the state of the game on the basis of messages received from the referee. A player’s try may result legal or illegal, and a legal move may prove to be a capture or a check. The paper describes the rationale of a program to play basic endgames of Kriegspiel, where a player has left only the King. These endings have been theoretically studied with rule-based mechanisms, whereas few researches exist on a gametree-based approach. We show how the branch of game tree can be reduced in order to employ an evaluation function and a search algorithm. Then we deal with game situations dependent on stochastic element and we show how we resolve them during the tree visit.