Searching over Metapositions in Kriegspiel

Abstract

Kriegspiel is a chess variant similar to war games, 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 in the statement legal or illegal, and a legal move may prove to be a capture or a check.

This paper describes the rationale of a program to play basic endgames of Kriegspiel, where a player has only the King left. These endings have been theoretically studied with rule-based mechanisms, whereas very little research has been done on a game-tree-based approach.

We show how the branching factor of a game tree can be reduced in order to employ an evaluation function and a search algorithm. We then deal with game situations which are dependent on a stochastic element and we show how we solve them during the tree visit.