fast_align is a simple, fast, unsupervised word aligner.
If you use this software, please cite:
- Chris Dyer, Victor Chahuneau, and Noah A. Smith. (2013). A Simple, Fast, and Effective Reparameterization of IBM Model 2. In Proc. of NAACL.
The source code in this repository is provided under the terms of the Apache License, Version 2.0.
A variant of fast_align is included in the cdec translation system. It uses the same model and produces identical alignments, but it has a few extra features for online alignment with pre-built models.
Input to fast_align must be tokenized and aligned into parallel sentences. Each line is a source language sentence and its target language translation, separated by a triple pipe symbol with leading and trailing white space (|||). An example 3-sentence German–English parallel corpus is:
doch jetzt ist der Held gefallen . ||| but now the hero has fallen .
neue Modelle werden erprobt . ||| new models are being tested .
doch fehlen uns neue Ressourcen . ||| but we lack new resources .
Building fast_align requires only a C++ compiler; this can be done by typing make at the command line prompt. Run fast_align to see a list of command line options.
fast_align generates asymmetric alignments (i.e., by treating either the left or right language in the parallel corpus as primary language being modeled, slightly different alignments will be generated). The usually recommended way to generate source–target (left language–right language) alignments is:
./fast_align -i text.fr-en -d -o -v > forward.align
The usually recommended way to generate target–source alignments is to just add the -r (“reverse”) option:
./fast_align -i text.fr-en -d -o -v -r > reverse.align
Using other tools, the generated forward and reverse alignments can be symmetrized into a (often higher quality) single alignment using intersection or union operations, as well as using a variety of more specialized heuristic criteria.
fast_align produces outputs in the widely-used i-j “Pharaoh format,” where a pair i-j indicates that the ith word (zero-indexed) of the left language (by convention, the source language) is aligned to the jth word of the right sentence (by convention, the target language). For example, a good alignment of the above German–English corpus would be:
0-0 1-1 2-4 3-2 4-3 5-5 6-6
0-0 1-1 2-2 2-3 3-4 4-5
0-0 1-2 2-1 3-3 4-4 5-5
The development of this software was sponsored in part by the U.S. Army Research Laboratory and the U.S. Army Research Office under contract/grant number W911NF-10-1-0533.