Space and depth-related enhancements of the history-ADS strategy in game playing

In the field of game playing, it is a well-known fact that powerful strategies, such as alpha-beta search, benefit strongly from proper move ordering. A popular metric of achieving this is the so-called “move history”, that is, prioritizing moves that have performed well, earlier in the search. The literature reports a number of techniques, such as the Killer Moves and History heuristics, that employ such a philosophy. Inspired by techniques from the field of Adaptive Data Structures (ADSs), we¹ have previously introduced the History-ADS heuristic, which uses an adaptive list to record moves, and to improve move ordering based on move history. The History-ADS heuristic has been proven to produce substantial gains in tree pruning in a wide variety of cases. However, it made use of a relatively naive application of an unbounded, single adaptive list. In this work, we attempt to refine the History-ADS heuristic, by examining its performance by constraining the length of its adaptive list, and applying multiple ADSs for each level of the tree. Our results show that the vast majority of the savings from the History-ADS heuristic remain even with a very short list, which can be applied to mitigate the drawbacks of an unbound data structure. Although results for multiple ADSs did not outperform single ADSs, we show that they provide some insight into how similar techniques may be applied in the context of the History-ADS heuristic.