Constraint-based lexical selection module

Lexical transfer

This is the output of lt-proc -b on an ambiguous bilingual dictionary.

[74306] ^El<det><def><f><sg>/The<det><def><f><sg>$ 
^estació<n><f><sg>/season<n><sg>/station<n><sg>$ ^més<preadv>/more<preadv>$ 
^plujós<adj><f><sg>/rainy<adj><sint><f><sg>$ 
^ser<vbser><pri><p3><sg>/be<vbser><pri><p3><sg>$ 
^el<det><def><f><sg>/the<det><def><f><sg>$ 
^tardor<n><f><sg>/autumn<n><sg>/fall<n><sg>$^,<cm>/,<cm>$ 
^i<cnjcoo>/and<cnjcoo>$ ^el<det><def><f><sg>/the<det><def><f><sg>$ 
^més<preadv>/more<preadv>$ ^sec<adj><f><sg>/dry<adj><sint><f><sg>$ 
^el<det><def><m><sg>/the<det><def><m><sg>$ 
^estiu<n><m><sg>/summer<n><sg>$^.<sent>/.<sent>$

I.e.

L'estació més plujós és el tardor, i la més sec el estiu

Goes to:

The season/station more rainy is the autumn/fall, and the more dry the summer.

The module requires VM for transfer, or using apertium-transfer -b in order to work.

Compilation

Check out the code from

$ svn co https://apertium.svn.sourceforge.net/svnroot/apertium/trunk/apertium-lex-tools

you can make it using:

$ ./autogen.sh
$ make

Usage

Make a simple rule file,

<rules>
  <rule>
    <match lemma="criminal" tags="adj"/>
    <match lemma="court" tags="n.*"><select lemma="juzgado" tags="n.*"/></match>
  </rule>
</rules>

Then compile it:

$ ./apertium-lrx-comp rules.xml rules.fst
1
Written 1 rules, 3 patterns.

The input is the output of lt-proc -b,

$ echo "^There<adv>/Allí<adv>$ ^be<vbser><pri><p3><sg>/ser<vbser><pri><p3><sg>$ ^a<det><ind><sg>/uno<det><ind><GD><sg>$ 
^criminal<adj>/criminal<adj><mf>/delictivo<adj>$ 
^court<n><sg>/corte<n><f><sg>/cancha<n><f><sg>/juzgado<n><m><sg>/tribunal<n><m><sg>$^.<sent>/.<sent>$" | ./apertium-lrx-proc -t rules2.fst 
1:SELECT:1 juzgado<n><m><sg>
^There<adv>/Allí<adv>$ ^be<vbser><pri><p3><sg>/ser<vbser><pri><p3><sg>$ ^a<det><ind><sg>/uno<det><ind><GD><sg>$ 
^criminal<adj>/criminal<adj><mf>/delictivo<adj>$ ^court<n><sg>/juzgado<n><m><sg>$^.<sent>/.<sent>$

Rule format

A rule is made up of an ordered list of:

• Matches
• Operations (select, remove)

<rule>  
  <match lemma="el"/>  
  <match lemma="dona" tags="n.*">    
    <select lemma="wife"/> 
  </match>  
  <match lemma="de"/>
</rule>

<pre>
<rule>  
  <match lemma="estació" tags="n.*">    
    <select lemma="season"/> 
  </match>  
  <match lemma="més"/>
  <match lemma="plujós"/>
</rule>

<rule>  
  <match lemma="guanyador"/>
  <match lemma="de"/>
  <match/>
  <match lemma="prova" tags="n.*">    
    <select lemma="event"/> 
  </match>  
</rule>

Rule application process

Optimal application

We're interested in the longest match, but not left to right, so what we do is make an automata of the rule contexts (one rule is one transducer, then we compose them), and we read through them, each state is an LU, It needs to be non-deterministic, and you keep a log of alive paths/states, but also their "weight" (how many transitions have been made) -- the longest for each of the ambiguous words is the winner when we get to the end of the sentence.

Writing and generating rules

Writing

Main article: How to get started with lexical selection rules

A good way to start writing lexical selection rules is to take a corpus, and search for the problem word, you can then look at how the word should be translated, and the contexts it appears in.

Generating

Parallel corpus

Main article: Generating lexical-selection rules from a parallel corpus

Monolingual corpora

Compiled

The general structure is as follows:

LSSRECORD = id, len, weight;

<ALPHABET>
<NUM_TRANSDUCERS>
<TRANSDUCER>
<TRANSDUCER>
<TRANSDUCER>
...
"main"
<TRANSDUCER>
<LSRRECORD>
<LSRRECORD>
<LSRRECORD>

Todo and bugs

• xml compiler
• compile rule operation patterns, as well as matching patterns
• make rules with gaps work
• optimal coverage
• fix bug with processing multiple sentences
• instead of having regex OR, insert separate paths/states.
• optimise the bestPath function (don't use strings to store the paths)
• autotoolsise build
• add option to compiler to spit out ATT transducers
• fix bug with outputting an extra '\n' at the end
• edit transfer.cc to allow input from lt-proc -b
• make sure that -b works with -n too.
• testing
• null flush
• add option to processor to spit out ATT transducers
• profiling and speed up
  • why do the regex transducers have to be minimised ?
  • retrieve vector of strings corresponding to paths, instead of a single string corresponding to all of the paths
  • stop using string processing to retrieve rule numbers
  • retrieve vector of vectors of words, not string of words from lttoolbox
  • why does the performance drop substantially with more rules ?
  • add a pattern -> first letter map so we don't have to call recognise() with every transition (didn't work so well)
  • each state with >10 out transitions could have a char-transition list
• there is a problem with the regex recognition code: see bug1 in testing.
• there is a problem with two defaults next to each other; bug2 in testing.
• default to case insensitive ? (perhaps case insensitive for lower case, case sensitive for uppercase) -- see bug4 in testing/.

Rendimiento

• 2011-12-12: 10,000 words / 97 seconds = 103 words/sec (71290 words, 14.84 sec = 4803 words/sec)
• 2011-12-19: 10,000 words / 4 seconds = 2,035 words/sec (71290 words, 8 secs = 8911 words/sec)

Preparedness of language pairs

Pair	LR (L)	LR (L→R)	Fertility	Rules
apertium-is-en	18,563	22,220	1.19	115
apertium-es-fr
apertium-eu-es	16,946	18,550	1.09	250
apertium-eu-en
apertium-br-fr	20,489	20,770	1.01	256
apertium-mk-en	8,568	10,624	1.24	81
apertium-es-pt
apertium-es-it
apertium-es-ro
apertium-en-es	267,469	268,522	1.003	334
apertium-en-ca

See also

• How to get started with lexical selection rules
• SVN Module: apertium-lex-tools

References

• Tyers, F. M., Sánchez-Martínez, F., Forcada, M. L. (2012) "Flexible finite-state lexical selection for rule-based machine translation". Proceedings of the 17th Annual Conference of the European Association of Machine Translation, EAMT12