A* Parsing: Fast Exact Viterbi Parse Selection

Abstract

A$^*$ PCFG parsing can dramatically reduce the time required to find the exact Viterbi parse by conservatively estimating outside Viterbi probabilities. We discuss various estimates and give efficient algorithms for computing them. On Penn treebank sentences, our most detailed estimate reduces the total number of edges processed to less than 3% of that required by exhaustive parsing, and even a simpler estimate which can be pre-computed in under a minute still reduces the work by a factor of 5. The algorithm extends the classic A* graph search procedure to a certain hypergraph associated with parsing. Unlike best-first and finite-beam methods for achieving this kind of speed-up, the A$^*$ parser is guaranteed to return the most likely parse, not just an approximation. The algorithm is also correct for a wide range of parser control strategies and maintains a worst-case cubic time bound.