Plausible Move Generation Using Move Merit Analysis in Shogi

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Plausible Move Generation Using Move Merit
Analysis in Shogi

• Reijer Grimbergen (Electrotechnical Laboratory)
• Hitoshi Matsubara (Future University Hakodate)

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Full-width Search and Plausible Move Generation

• Full-width search
• World championship level programs in chess,
  checkers and Othello.
• Plausible Move Generation
• Important in the early days of chess research.
• Good alternative in domains where full-width
  search can not search deep enough
• Large average number of legal moves (e.g. Go,
  Shogi).
• Single agent search problems with extremely long
  solution sequences (e.g. Sokoban).
• Interesting for cognitive science.

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Why Plausible Move Generation in Shogi?

• High average branching factor (80) compared to
  chess (35).
• Average branching factor increases as the game
  progresses.
• Time constraints are strict.

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Legal Moves in Shogi
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Plausible Move Generator Set for Shogi

• PMG-Goal
• Capture Material
• Promote Piece
• PMG-Th
• Check
• Attack king
• Attack material
• Discovered attack
• Threaten promotion
• PMG-DefTh
• Defend checks
• Defend king
• Defend material
• Defend discovered attacks
• Defend against promotion

• PMG-Pim
• Defend pins
• Tie improvement
• Defend undefended pieces
• Defend against exchange
• Cover squares in own camp
• Develop pieces
• PMG-DefPIm
• Pin piece
• Cover squares in enemy camp
• Avoid development

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Move Merit Analysis

• In our approach it is possible that moves are
  generated by more than one PMG.
• Knowledge about which PMGs generated a move is
  used for analyzing the merit of a move.
• Move Merit Analysis Every PMG gives a value to
  the generated move based on the importance of the
  PMG.
• E.g. Winning a rook is more important than
  improving the position.
• Possible performance improvements
• Move ordering is improved, aiding alpha-beta
  search.
• Additional cuts are possible discard all moves
  with a negative MMA value.

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Experimental Results

• We compared the behavior of
• PMG-All Generate all moves from the plausible
  move generator set.
• PMG-MMA Generate all moves with a positive MMA
  value.
• Four experiments
• Plausible move generation test.
• Move ordering test.
• Search comparison test.
• Self play experiment.

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Experimental Results (1)Plausible Move
Generation Test
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Experimental Results (1)Plausible Move
Generation Test

• Savings and accuracy of PMG-All and PMG-MMA in
  100 test games

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Experimental Results (2)Move Ordering Test
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Experimental Results (3)Search Comparison Test

• Performance in 298 tactical shogi problems from
  Shukan Shogi

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Experimental Results (4)Self Play Experiment

• We played 20 game matches between shogi programs
  that use PMG-MMA, PMG-All and Full-width search

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Conclusions

• Plausible move generation deserves further
  investigation.
• Plausible Move Generation with Move Merit
  Analysis gives important improvements of the
  search performance in shogi.
• Move Merit Analysis is vital for our method of
  plausible move generation.