BS*: An Admissible Bidirectional Staged Heuristic Search Algorithm

Abstract In order to reap the potential advantage of less extensive searching which bidirectional heuristic search algorithms offer, strategies are needed to influence the two search wavefronts to meet such that early termination will occur. The principled search control strategy aims to achieve this without trading running time, but can be found wanting still. An improved algorithm BS∗ is described which expands significantly less nodes on average than any other algorithm in the same class of non-wave-shaping admissible bidirectional algorithms. When pitted against BHPA, the only other heuristically guided member in this class, BS∗'s average search efficiency in time and space is about 30% better. BS∗'s superior performance stems from the use of all opportunities to achieve early termination and the elimination of unfruitful avenues by search reduction operations: nipping, pruning, trimming and screening. Such operations exploit information gathered during search and have several spin-offs: more accurate guidance of search control, early exposure of nonpromising nodes and reduced bookkeeping overheads, all of which further enhance BS∗'s performance. A further noteworthy feature of BS∗ is that it is the first staged search algorithm which preserves admissibility.