IWEC20021 Threat Stacks to Guide Pruning and Search Extensions in Shogi Reijer Grimbergen Department of Information Science Saga University, Japan

IWEC20022 Presentation Overview The problem of time constraints Pruning, search extensions and the horizon effect Pruning and extensions in shogi Threat stacks Using threat stacks for search extensions Using threat stacks for pruning Dealing with horizon moves Preliminary results Conclusions and future work

IWEC20023 How to deal with premature termination of the search? Iterative deepening Ensures that there is a completed search to a certain nominal search depth Search extensions Needed to get a reliable result at each iteration Examples: Quiescence search, Singular extensions Pruning Used to speed up the search and free time for search extensions Examples: Futility pruning, ProbCut The Problem of Time Constraints Horizon problem Push threats over the search horizon

IWEC20024 Pruning and Extensions in Shogi Shogi and chess Shogi is a two-player complete information game similar to chess Differences with chess Drop rule Promotion rule

IWEC20025 Pruning and Extensions in Shogi Extensions in shogi Evaluation function is a combination of material and king danger Need to check both captures and king attack for a position to be quiescent Extended promotion possibilities lead to a sudden change in evaluation value more often Problem in shogi The set of non-quiescent positions is much larger than in chess

IWEC20026 Pruning and Extensions in Shogi Pruning in shogi Futility pruning in shogi It is more difficult to judge inability to recover from a bad position ProbCut in shogi It is more difficult to the establish the likelihood that a deep search can be replaced by a shallow search

IWEC20027 Pruning and Extensions in Shogi The horizon problem in shogi Drop moves make the horizon problem much worse in shogi than in chess In strong shogi programs threat analysis is used for pruning, extensions and detection of horizon moves Gekizashi’s probability classification YSS’s move classification Shotest’s Super-SOMA algorithm No general framework for the use of threats in the search Threat Stacks

IWEC20028 Threat Stacks The method Two stacks to store the threats of both players After each move, the threats by each player are pushed on the stacks When a move is undone, the threat stacks are popped Threat entry Threat identification number Up to three squares for the pieces involved in the threat A value indicating the strength of the threat Weak threats and strong threats Ignore weak threats in case of a strong threat Depend on the phase of the game

IWEC20029 Threat Stacks Threat categories Check Threat to gain material Promotion threat Discovered attack Pins King threats

IWEC Threats Stacks and Extensions Extend the search to empty the threat stacks Procedure a.Determine the strongest threat by each side b.If the side to move has a stronger threat than the opponent, execute the threat c.If the opponent has a stronger threat than the side to move, generate moves to defend against the threat In case of weak threats, this procedure is only followed in the opening and middle game

IWEC Threats Stacks and Pruning Pruning decisions Prune sacrifices one move before the nominal search depth Prune if the number of threats is larger than the remaining search depth In case of a threat, prune moves that neither Execute a threat Resolve a threat Introduce a threat Pruning only done in positions with a strong threat

IWEC Threats and the Horizon Problem Horizon moves are dealt with in two steps 1.Detect a potential horizon move 2.Analyze threat stacks at the nominal search depth to see if the original threat is still there This is why threat stacks are used

IWEC Results Tactical shogi problems VersionTotalSolvedTime used Threat stacks (42%)02:14:12 Quiescence search29897 (33%)02:34:45 Set of 298 tactical shogi problems from Shukan Shogi 30 seconds on a 1.2 GHz Pentium III Comparison of threat stacks and quiescence search

IWEC Results Material vs. Attack/defense 115 tactical problems involving material VersionTotalSolvedTime used Threat stacks11593 (81%)00:50:31 Quiescence search11599 (86%)01:00: tactical problems involving attack and defense VersionTotalSolvedTime used Threat stacks17589 (51%)01:18:27 Quiescence search17557 (33%)01:38:12

IWEC Conclusions and Future Work Conclusions Threat stacks can be used for making decisions about search extensions, pruning and to detect horizon moves Preliminary results indicate that threat stacks may improve the tactical ability of a shogi program Future work Investigate if the tactical improvement results in winning more games Use threat stacks to guide best first search