Merging Metrics of Special Rules in Chinese Chess Endgame Databases

Perpetual check and perpetual chase are two special rules in the endgame phase of Chinese chess. Two metrics, distance-to-check and distance-to-chase, respectively record the distances measured in plies from the current position to the positions where the two rules apply in endgame databases. In the past, the two metrics are separately recorded at different levels in the databases. However, the endgame strategy is not efficient if the distances of the two metrics are not compared. Moreover, the more the number of levels is, the bigger the database is. In this paper, we modify retrograde analysis, an algorithm to build endgame databases, to merge the two metrics for special rules. The experiments show that the endgame strategy becomes efficient and the sizes of endgame databases are reduced as well.