Datok - Finite State Tokenizer

Implementation of a finite state automaton for natural language tokenization, based on a finite state transducer generated with Foma.

The library contains sources for a german tokenizer based on KorAP-Tokenizer.

Tokenizing

$ echo "Es war spät, schon ca. <time datetime=\"02:00\">02:00 Uhr</time>. ;-)" | ./datok tokenize -t testdata/tokenizer.matok -
Es
war
spät
,
schon
ca.
<time datetime="02:00">
02:00
Uhr
</time>
.

;-)

The special END OF TRANSMISSION character (\x04) can be used to mark the end of a text.

Caution: When experimenting with STDIN this way, you may need to disable history expansion.

Conventions

The FST generated by Foma must adhere to the following rules, to be converted by Datok:

  • Character accepting arcs need to be translated only to themselves or to ε (the empty symbol).
  • Multi-character symbols are not allowed, except for the @_TOKEN_SYMBOL_@, that denotes the end of a token.
  • ε accepting arcs (transitions not consuming any character) need to be translated to the @_TOKEN_SYMBOL_@.
  • Two consecutive @_TOKEN_SYMBOL_@s mark a sentence end.
  • Flag diacritics are not supported.
  • Final states are ignored. The @_TOKEN_SYMBOL_@ marks the end of a token instead.

A minimal usable tokenizer written in XFST and following the guidelines to tokenizers in Beesley and Karttunen (2003) and Beesley (2004) would look like this:

define TE "@_TOKEN_SYMBOL_@";

define WS [" "|"\u000a"|"\u0009"];

define PUNCT ["."|"?"|"!"];

define Char \[WS|PUNCT];

define Word Char+;

! Compose token ends
define Tokenizer [[Word|PUNCT] @-> ... TE] .o.
! Compose Whitespace ignorance
       [WS+ @-> 0] .o.
! Compose sentence ends
       [[PUNCT+] @-> ... TE \/ TE _ ];

read regex Tokenizer;

Hint: For development it's easier to replace @_TOKEN_SYMBOL_@ with a newline.

Building

To build the tokenizer tool, run

$ go build ./cmd/datok.go

To create a foma file from the example sources, first install Foma, then run in the root directory of this repository

$ cd src && \
  foma -e "source tokenizer.xfst" \
  -e "save stack ../mytokenizer.fst" -q -s && \
  cd ..

This will load and compile tokenizer.xfst and will save the compiled FST as mytokenizer.fst in the root directory.

To generate a Datok FSA (matrix representation) based on this FST, run

$ datok convert -i mytokenizer.fst -o mytokenizer.datok

To generate a Datok FSA (double array representation*) based on this FST, run

$ datok convert -i mytokenizer.fst -o mytokenizer.datok -d

The final datok file can then be used as a model for the tokenizer.

  • This may take quite some time depending on the number of arcs in the FST and is therefore now deprecated.

Technology

Internally the FSA is represented either as a matrix or as a double array.

Both representations mark all non-word-character targets with a leading bit. The transduction is greedy with a single backtracking option to the last ε (aka tokenend) transition.

The double array representation (Aoe 1989) of all transitions in the FST is implemented as an extended DFA following Mizobuchi et al. (2000) and implementation details following Kanda et al. (2018).

License

Datok is published under the Apache 2.0 License.

The german tokenizer shipped is based on work done by the Lucene project (published under the Apache License), David Hall (published under the Apache License), Çağrı Çöltekin (published under the MIT License), and Marc Kupietz (published under the Apache License).

The foma parser is based on foma2js, written by Mans Hulden (published under the Apache License).

Bibliography

Aoe, Jun-ichi (1989): An Efficient Digital Search Algorithm by Using a Double-Array Structure. IEEE Transactions on Software Engineering, 15 (9), pp. 1066-1077.

Beesley, Kenneth R. & Lauri Karttunen (2003): Finite State Morphology. Stanford, CA: CSLI Publications.

Beesley, Kenneth R. (2004): Tokenizing Transducers. https://web.stanford.edu/~laurik/fsmbook/clarifications/tokfst.html

Hulden, Mans (2009): Foma: a finite-state compiler and library. In: Proceedings of the 12th Conference of the European Chapter of the Association for Computational Linguistics, Association for Computational Linguistics, pp. 29-32.

Mizobuchi, Shoji, Toru Sumitomo, Masao Fuketa & Jun-ichi Aoe (2000): An efficient representation for implementing finite state machines based on the double-array. Information Sciences 129, pp. 119-139.

Kanda, Shunsuke, Yuma Fujita, Kazuhiro Morita & Masao Fuketa (2018): Practical rearrangement methods for dynamic double-array dictionaries. Software: Practice and Experience (SPE), 48(1), pp. 65–83.