Hierarchical A* Parsing with Bridge Outside Scores

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.