Recursive Best-First Search with Bounded Overhead

There are two major paradigms for linear-space heuristic search: iterative deepening (IDA*) and recursive best-first search (RBFS). While the node regeneration overhead of IDA* is easily characterized in terms of the heuristic branching factor, the overhead of RBFS depends on how widely the promising nodes are separated in the search tree, and is harder to anticipate. In this paper, we present two simple techniques for improving the performance of RBFS while maintaining its advantages over IDA*. While these techniques work well in practice, they do not provide any theoretical bounds on the amount of regeneration overhead. To this end, we introduce RBFSCR, the first method for provably bounding the regeneration overhead of RBFS. We show empirically that this improves its performance in several domains, both for optimal and suboptimal search, and also yields a better linear-space anytime heuristic search. RBFSCR is the first linear space best-first search robust enough to solve a variety of domains with varying operator costs.