Improving the Efficiency of Parsing with Discontinuous Constituents

We discuss a generalization of Earley’s algorithm to grammars licensing discontinuous constituents of the kind proposed by the socalled linearization approaches in Head-Driven Phrase Structure Grammar. We show how to replace the standard indexing on the string position by bitmasks that act as constraints over possible coverage bitvectors. This improves efficiency of edge access and reduces the number of edges by constraining prediction to those grammar rules which are compatible with known word order properties. The resulting parsing algorithm does not have to process the righthand side categories in the order in which they cover the string, and so a head-driven strategy can be obtained simply by reordering the righthand side categories of the rules. The resulting strategy generalizes head-driven parsing in that it also permits the ordering of non-head categories.