Plausible Move Generation Using Move Merit Analysis in Shogi

1. Plausible Move Generation Using Move Merit Analysis in Shogi Reijer Grimbergen(Electrotechnical Laboratory) Hitoshi Matsubara (Future University Hakodate)

2. 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.

3. 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.

4. Legal Moves in Shogi

5. 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 Plausible Move Generator Set for Shogi

6. 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.

7. 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.

8. Experimental Results (1)Plausible Move Generation Test

9. PMG-MMA: savings much better and only slightly less accuracy Experimental Results (1)Plausible Move Generation Test • Savings and accuracy of PMG-All and PMG-MMA in 100 test games:

10. Only very few moves are ordered low by MMA Experimental Results (2)Move Ordering Test

11. Only for PMG-MMA there is a significant improvement Experimental Results (3)Search Comparison Test • Performance in 298 tactical shogi problems from Shukan Shogi:

12. PMG-MMA outplays PMG-All and Full-width search Experimental Results (4)Self Play Experiment • We played 20 game matches between shogi programs that use PMG-MMA, PMG-All and Full-width search:

13. 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.