FST Technology Overview

Presently the Giella infrastructure supports three fst technologies in parallel:

• Xerox
• Hfst
• Foma

Each of them have their strengths and weaknesses, summarised as follows:

Xerox

The standard in all FST work.

Strengths:

• fast in lookup
• fast in compiling source code

Weaknesses:

• no support for weights
• closed source
• abandoned by its main developer Lauri Karttunen (he is retired, and his effort to open-source it was blocked by Xerox lawyers)

Source code access

Even though the source code is not released, it is possible to get a license to the source code of the c-fsm library ( documented here) by requesting a license for the XLE page. Information and relevant links can be found at the bottom of the project page.

Hfst

A source-code compatible clone of the Xerox tools, based on OpenFST, but with multiple backends (Foma, SFST, OpenFST).

Strengths:

• replicates 99% of the functionality of the Xerox tools
• is actively maintained
• is almost as fast in lookup as Xerox when using the optimised lookup format
• supports weighted fst's
• open source

Weaknesses:

• is slow in compiling fst's compared to Xerox

Foma

A source-code and command interface compatible clone of the Xerox xfst tool, developed and maintained by Måns Huldén. Is open source.

Strengths:

• fast in lookup
• fast in compiling source code
• actively maintained

Weaknesses:

• no support for twolc
• limited support for weighted fst's

... ie the lack of future for Xerox, the lack of twolc in Foma and the lack of speed in Hfst.

Today's dual strategy

• use the fast xfst for developing and for some applications
• use the slow hfst for all applications demanding open source and/or weighting

The future

... is dependent upon

• whether hfst will become faster
• whether foma will include support for twolc
• whether it will be possible to license some version of c-fst that will produce the command line tools we need

In the short run, we can continue as we do now.