Dynamically Shaping the Reordering Search Space of Phrase-Based Statistical Machine Translation

Defining the reordering search space is a crucial issue in phrase-based SMT between distant languages. In fact, the optimal tradeoff between accuracy and complexity of decoding is nowadays reached by harshly limiting the input permutation space. We propose a method to dynamically shape such space and, thus, capture long-range word movements without hurting translation quality nor decoding time. The space defined by loose reordering constraints is dynamically pruned through a binary classifier that predicts whether a given input word should be translated right after another. The integration of this model into a phrase-based decoder improves a strong Arabic-English baseline already including state-of-the-art early distortion cost (Moore and Quirk, 2007) and hierarchical phrase orientation models (Galley and Manning, 2008). Significant improvements in the reordering of verbs are achieved by a system that is notably faster than the baseline, while BLEU and METEOR remain stable, or even increase, at a very high distortion limit.