Hierarchical A* Parsing with Bridge Outside Scores

Adam Pauls, Dan Klein

Abstract: Hierarchical A* (HA*) uses of a hierarchy of coarse grammars to speed up parsing without sacrificing optimality. HA* prioritizes search in refined grammars using Viterbi outside costs computed in coarser grammars. We present Bridge Hierarchical A* (BHA*), a modified Hierarchial A* algorithm which computes a novel outside cost called a bridge outside cost. These bridge costs mix finer outside scores with coarser inside scores, and thus constitute tighter heuristics than entirely coarse scores. We show that BHA* substantially outperforms HA* when the hierarchy contains only very coarse grammars, while achieving comparable performance on more refined hierarchies.