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From Bill Janssen <jans...@parc.com>
Subject Re: finding the analyzer for a language...
Date Sat, 25 Sep 2010 12:29:55 GMT
Robert Muir <rcmuir@gmail.com> wrote:

> On Fri, Sep 24, 2010 at 9:58 PM, Bill Janssen <janssen@parc.com> wrote:
> 
> > I thought that since I'm updating UpLib's Lucene code, I should tackle
> > the issue of document languages, as well.  Right now I'm using an
> > off-the-shelf language identifier, textcat, to figure out which language
> > a Web page or PDF is (mainly) written in.  I then want to analyze that
> > document with an appropriate analyzer.  I'd then like to map to the
> > correct Lucene analyzer for that language, falling back to
> > StandardAnalyzer if the installed Lucene library doesn't have an
> > analyzer for that language.
> >
> > It would be *very* handy if Analyzer had a static method
> >
> >  static Analyzer getAnalyzerForLanguage(String rfc_4646_lang_tag);
> >
> 
> I agree (not sure if it should be in Analyzer itself, maybe we could make an
> Analyzer for this)...

Not sure I followed that...  I wanted to be able to retrieve an instance
of an instantiated Analyzer class, the class that's "designed" to work
with that language, if one exists, otherwise null.  And to have you guys
keep that list up-to-date, instead of having to do it myself :-).
Seemed to me that's the standard kind of thing you make a static method
on the top-level class.

> i mean it sounds like what you want, is for it to work in a similar way to
> ResourceBundle's fallback mechanism?

I'm not sure that's appropriate.  I just want to retrieve an Analyzer
for that language, if such a thing exists.  If by "fallback", you mean
that "en-US" should just return EnglishAnalyzer if there's no analyzer
specifically for US usage -- yes, that's fine.  On the other hand, I
don't think there should be a fallback for languages which have no
macrolanguage Analyzer -- it should just return null or throw an
exception.  The programmer can then explicitly decide how do deal with
that response.

> And I agree with your idea of rfc3066/4646, e.g. you might want to specify
> subtags like "word" (SmartChineseAnalyzer) or "ngram" (CJKAnalyzer) for
> chinese somehow?

Yes, good idea.  Might be interesting to see if those kind of subtags
can be registered with IANA, too.

Although, if one is smart enough about Lucene and one's application to
make these kinds of judgement calls, I think one is probably smart
enough to know which class to use without consulting a generic
mechanism.

> Shai Erera brought a similar idea up before, to use Locale, but my concerns
> are it would be limited by javas Locale mechanism... but we can figure this
> out.
> 
> Maybe you want to create a JIRA issue to pursue this idea further? See
> http://wiki.apache.org/lucene-java/HowToContribute
> 
> 
> > Right now I'm consulting a hand-compiled mapping of
> > langtag-to-Lucene-classname to figure out which Analyzer to use.
> > Wearisome, and it will be out-of-date for future releases of Lucenen
> > which will presumably support more languages.
> >
> 
> yes, but it also brings up interesting backwards compatibility challenges.
> Because if we add more analyzers, say EsperantoAnalyzer, if you upgrade
> lucene then suddenly your Esperanto queries are analyzed differently
> (whereas they were dealt with by StandardAnalyzer before).

Yes, presumably the Version would need to be used with this, too.

> But this becomes less of a problem as we work on modularizing lucene, so we
> can remove Version from analyzers,

Oh goody, another API change to cope with in my code.

> and so you can just use an old analyzers
> jar file (such as 4.1) but upgrade your lucene core jar to say version 4.3.
> 
> 
> >
> > Secondly, if I've got an instance of a SnowballAnalyzer, there's no way
> > to look "inside" it, and see what language it's for.  That's a problem
> > on the search side.  My QueryParser is a subclass of
> > MultiFieldQueryParser, and it looks for a "special" FieldQuery on the
> > field "_query_language", i.e., "_query_language:de" to tell the query
> > parser to use a German analyzer on this query.  What I'd like to be able
> > to do is interrogate the current analyzer attached to the query parser
> > instance, and throw an exception if it's not for the specified language.
> > I can do this for non-Snowball analyzers, because of the brittle
> > hand-compiled mapping mentioned above.  But if it's a SnowballAnalyzer,
> > there's no way to tell what the language inside it is.  So it would be
> > nice if SnowballAnalyzer grew a method
> >
> 
> SnowballAnalyzer had more problems. its actually deprecated in
> trunk/branch_3x and instead there is an Analyzer for each language (English,
> Italian, etc), which now has stopwords lists, and sometimes special behavior
> (e.g. Turkish lowercases differently).
> 
> Put more simply, its an implementation detail for ItalianAnalyzer that we
> implement the stemming with SnowballFilter. One day we might change it to
> use a less aggressive stemming algorithm (e.g. ItalianLightStemFilter) by
> default.

Ah, good.  That will suit my purposes nicely.

> I'd really like to see the stopword work finished, so that a
> > SnowballAnalyzer for a particular language has a decent set of
> > stopwords.
> >
> 
> See above, I think this is finished? The remaining work is actually Solr
> integration.

Excellent.  I looked at the JIRA, but some discussions just seem to
peter out, and I'm having a hard time telling what the resolution is.

> In trunk and branch_3x, all the analyzers have their own package, here's
> Italian:
> 
> Source package: contains Analyzer that uses SnowballFilter(Italian) and
> loads Italian snowball stopwords by default. It also includes an
> alternative, less aggressive stemmer.
> http://svn.apache.org/viewvc/lucene/dev/trunk/modules/analysis/common/src/java/org/apache/lucene/analysis/it/
> 
> The snowball stopwords were all added to the resources directory. This is
> where ItalianAnalyzer loads its set of stopwords from:
> http://svn.apache.org/viewvc/lucene/dev/trunk/modules/analysis/common/src/resources/org/apache/lucene/analysis/
> <http://svn.apache.org/viewvc/lucene/dev/trunk/modules/analysis/common/src/resources/org/apache/lucene/analysis/>

I see there's also an explicit EnglishAnalyzer -- never thought it made
sense to call that StandardAnalyzer.  Great work!

Bill

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