SUPER-SOMA — Solving Tactical Exchanges in Shogi without Tree Searching

Abstract

A key feature of programs that play games such as Chess and Shogi is the ability to evaluate the outcome of threatened tactical moves. In Chess this is usually solved using a combination of tactical and capture search. This works well as exchanges rapidly simplify and a solution can usually be quickly found. In Shogi (Japanese Chess) the problem is not so simple as captured pieces are immediately available for tactical drops and so tactical threats do not quickly simplify. Since the number of tactical threats in Shogi also tends to be much larger than in Chess, then this makes solving threats using tactical and capture search much more difficult. In the Shogi-playing program SHOTEST I have taken a different approach to this and created a tactical exchange evaluator which can statically do the work of a tactical search. This approach has its ancestry in the well-known and simple SOMA algorithm used to determine single square exchanges. However the algorithm SUPER-SOMA described in this paper can also deal with multi-square captures, pins, ties, discovered attacks, promotions, defensive play, mate threats, mate ties and even positional moves.