Three types of forward pruning techniques to apply the alpha beta algorithm to turn-based strategy games

Abstract

Turn-based strategy games are interesting testbeds for developing artificial players because their rules present developers with several challenges. Currently, Monte-Carlo tree search variants are often utilized to address these challenges. However, we consider it worthwhile introducing minimax search variants with pruning techniques because a turn-based strategy is in some points similar to the games of chess and Shogi, in which minimax variants are known to be effective. Thus, we introduced three forward-pruning techniques to enable us to apply alpha beta search (as a minimax search variant) to turn-based strategy games. This type of search involves fixing unit action orders, generating unit actions selectively, and limiting the number of moving units in a search. We applied our proposed pruning methods by implementing an alpha beta-based artificial player in the Turn-based strategy Academic Package (TUBSTAP) open platform of our institute. This player competed against first- and second-rank players in the TUBSTAP AI competition in 2016. Our proposed player won against the other players in five different maps with an average winning ratio exceeding 70%.