MM: A Bidirectional Search Algorithm that is Guaranteed to Meet in the Middle
Publication Date
8-22-2017
Document Type
Article
Organizational Units
Daniel Felix Ritchie School of Engineering and Computer Science, Computer Science
Keywords
Heuristic search, Bidirectional search
Abstract
Bidirectional search algorithms interleave two separate searches, a normal search forward from the start state, and a search backward from the goal. It is well known that adding a heuristic to unidirectional search dramatically reduces the search effort. By contrast, despite decades of research, bidirectional heuristic search has not yet had a major impact. Additionally, no comprehensive theory was ever devised to understand the nature of bidirectional heuristic search. In this paper we aim to close this gap. We first present MM, a novel bidirectional heuristic search algorithm. Unlike previous bidirectional heuristic search algorithms, MM's forward and backward searches are guaranteed to “meet in the middle”, i.e. never expand a node beyond the solution midpoint. Based on this unique attribute we present a novel framework for comparing MM, A*, and their brute-force variants. We do this by dividing the entire state space into disjoint regions based on their distance from the start and goal. This allows us to perform a comparison of these algorithms on a per region basis and identify conditions favoring each algorithm. Finally, we present experimental results that support our theoretical analysis.
Publication Statement
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Recommended Citation
Holte, Robert C, et al. “MM: A Bidirectional Search Algorithm That Is Guaranteed to Meet in the Middle.” Artificial Intelligence, vol. 252, 2017, pp. 232–266. doi: 10.1016/j.artint.2017.05.004.