Strength Adjustment and Assessment for MCTS-Based Programs [Research Frontier]

This paper proposes an approach to strength adjustment and assessment for Monte-Carlo tree search based game-playing programs. We modify an existing softmax policy with a strength index to choose moves. The most important modification is a mechanism which filters low-quality moves by excluding those that have a lower simulation count than a pre-defined threshold ratio of the maximum simulation count. Through theoretical analysis, we show that the adjusted policy is guaranteed to choose moves exceeding a lower bound in strength by using a threshold ratio. Experimental results show that the strength index is highly correlated to the empirical strength. With an index value between ?2, we can cover a strength range of about 800 Elo ratings. The strength adjustment and assessment methods were also tested in real-world scenarios with human players, ranging from professionals (strongest) to kyu rank amateurs (weakest). For amateur levels, we tested our mechanism on two popular Go online platforms - Fox Weiqi and Tygem. The result shows that our method can adjust program strength to different ranks stably. In terms of strength assessment, we proposed a new dynamic strength adjustment method, then used it to evaluate human professionals, predicting reliably their playing strengths within 15 games. Lastly, we collected survey responses asking players about strength perception, entertainment, and general comments for different aspects of analysis. To our best knowledge, this result is state-ofthe- art in terms of the range of strengths in Elo rating while maintaining a controllable relationship between the strength and a strength index.