Shahaf Shperberg, Lior Siag, Nathan Sturtevant, Ariel Felner

Position Paper: On the Impact of Direction-Selection in BAE*

Introduction

Position paper: on the impact of direction-selection in bae*. This paper reviews direction-selection policies in BAE* and bidirectional search, analyzing their impact on optimality. Discover a low-overhead solution preventing f > C* node expansion.

Abstract

BAE*, and the independently developed DIBBS, are state-of-the-art bidirectional heuristic search algorithms that exploit heuristic consistency to efficiently prove solution optimality. Historically, BAE* has been studied with various direction-selection policies, determining whether to expand the next state from the forward or backward search. However, some of these policies expand nodes with an f-value exceeding the optimal solution cost, C*, which clearly cannot be part of any optimal solution. In this position paper, we review direction-selection strategies in BAE* and bidirectional search more broadly, analyzing their impact on the behavior of the search. Additionally, we present a low-overhead solution that prevents the expansion of nodes with f > C* across all direction-selection strategies.

Review

This position paper tackles a critical efficiency concern within state-of-the-art bidirectional heuristic search algorithms, particularly BAE* and DIBBS, which are lauded for their ability to efficiently prove solution optimality through heuristic consistency. The authors pinpoint a long-standing issue concerning direction-selection policies in BAE*, where certain strategies inadvertently lead to the expansion of nodes whose f-values exceed the true optimal solution cost, C*. This directly contradicts the goal of optimal search by performing unnecessary work on paths that cannot be part of any optimal solution. The paper thus establishes a clear and relevant problem area deserving of focused attention in the heuristic search community. The core of this work lies in its comprehensive review and analysis of existing direction-selection strategies within BAE* and the wider landscape of bidirectional search. By dissecting the various policies, the paper sheds light on their nuanced impact on search behavior and computational overhead. Crucially, beyond mere analysis, the authors propose a practical and "low-overhead solution" specifically designed to prevent the problematic expansion of f > C* nodes. This proposed enhancement is significant as it claims broad applicability "across all direction-selection strategies," suggesting a fundamental improvement that can universally boost the efficiency of BAE* by eliminating wasted expansions, without compromising the algorithm's ability to prove optimality. Overall, this position paper offers a valuable contribution to the field of optimal pathfinding. By clearly articulating a recognized inefficiency in advanced bidirectional search algorithms and subsequently offering a generalizable, low-overhead solution, it provides both a critical retrospective and a practical way forward. Researchers and practitioners working with BAE* and other bidirectional search variants will find the review insightful for understanding the performance implications of their chosen direction-selection policies, and the proposed solution presents an immediate opportunity for enhancing algorithm efficiency. This paper effectively consolidates existing knowledge while pushing the boundaries for more efficient and robust optimal search methodologies.

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