Multiple-paths search with concurrent thread scheduling for fast AND/OR tree search

This paper proposes a fast AND/OR tree search algorithm using a multiple-paths concurrent search method. Conventional heuristic AND/OR tree search algorithms expand nodes in only a descending order of heuristic evaluation values. However, since the evaluation values are heuristic, a solution node group sometimes includes nodes with lower evaluation values. The tree which has a solution node group including nodes with lower evaluation values requires a long time to be solved by the conventional algorithms. The proposed algorithm allows us to search paths including nodes with lower evaluation values and paths including nodes with higher evaluation values concurrently. For searching various paths concurrently, the proposed algorithm uses pseudo-threads and a pseudo-thread scheduler managed by a user program with low overhead compared with the OS thread management. The pseudo-thread scheduler can weight the amount of search on each path and schedule the pseudo-threads. The proposed algorithm can solve trees which have solutions including nodes with lower evaluation values also quickly. For performance evaluation, the proposed algorithm was applied to a tsume-shogi (Japanese chess problem) solver as a typical AND/OR tree search problem. In tsume-shogi, players can reuse captured pieces. Performance evaluation results on 385 problems show that the proposed algorithm is 1.67 times faster on the average than the previous algorithm df-pn.