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. 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

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: 1. Plausible move generation test. 2. Move ordering test. 3. Search comparison test. 4. Self play experiment.

8. Experimental Results (1) Plausible Move Generation Test

9. Savings and accuracy of PMG-All and PMG-MMA in 100 test games: VersionNot Generated Accuracy (%) Savings (%) PMG-ALL8199.4%23.7% PMG-MMA14498.8%46.5% PMG-MMA: savings much better and only slightly less accuracy

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

11. Experimental Results (3) Search Comparison Test Performance in 298 tactical shogi problems from Shukan Shogi: CatTotal PosFull WidthPMG-ALLPMG-MMA 150172122 2509813 35010 11 450978 5 544 648456 Tot298545564 Only for PMG-MMA there is a significant improvement

12. Experimental Results (4) Self Play Experiment We played 20 game matches between shogi programs that use PMG-MMA, PMG-All and Full-width search: NoVersion123Result 1PMG-MMAX15-5 (75%) 20-0 (100%) 35-5 (87.5%) 2PMG-ALL5-15 (25%) X16-4 (80%) 21-19 (52.5%) 3Full-width0-20 (0%) 4-16 (20%) X4-36 (10.0%) PMG-MMA outplays 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.