Lingua::Stem::Snowball(3) Perl interface to Snowball stemmers.

SYNOPSIS


my @words = qw( horse hooves );
# OO interface:
my $stemmer = Lingua::Stem::Snowball->new( lang => 'en' );
$stemmer->stem_in_place( \@words ); # qw( hors hoov )
# Functional interface:
my @stems = stem( 'en', \@words );

DESCRIPTION

Stemming reduces related words to a common root form --- for instance, ``horse'', ``horses'', and ``horsing'' all become ``hors''. Most commonly, stemming is deployed as part of a search application, allowing searches for a given term to match documents which contain other forms of that term.

This module is very similar to Lingua::Stem --- however, Lingua::Stem is pure Perl, while Lingua::Stem::Snowball is an XS module which provides a Perl interface to the C version of the Snowball stemmers. (<http://snowball.tartarus.org>).

Supported Languages

The following stemmers are available (as of Lingua::Stem::Snowball 0.95): 

    |-----------------------------------------------------------|
    | Language   | ISO code | default encoding | also available |
    |-----------------------------------------------------------|
    | Danish     | da       | ISO-8859-1       | UTF-8          | 
    | Dutch      | nl       | ISO-8859-1       | UTF-8          | 
    | English    | en       | ISO-8859-1       | UTF-8          |
    | Finnish    | fi       | ISO-8859-1       | UTF-8          | 
    | French     | fr       | ISO-8859-1       | UTF-8          |
    | German     | de       | ISO-8859-1       | UTF-8          | 
    | Hungarian  | hu       | ISO-8859-1       | UTF-8          | 
    | Italian    | it       | ISO-8859-1       | UTF-8          | 
    | Norwegian  | no       | ISO-8859-1       | UTF-8          | 
    | Portuguese | pt       | ISO-8859-1       | UTF-8          | 
    | Romanian   | ro       | ISO-8859-2       | UTF-8          | 
    | Russian    | ru       | KOI8-R           | UTF-8          | 
    | Spanish    | es       | ISO-8859-1       | UTF-8          | 
    | Swedish    | sv       | ISO-8859-1       | UTF-8          | 
    | Turkish    | tr       | UTF-8            |                | 
    |-----------------------------------------------------------|

Benchmarks

Here is a comparison of Lingua::Stem::Snowball and Lingua::Stem, using The Works of Edgar Allen Poe, volumes 1-5 (via Project Gutenberg) as source material. It was produced on a 3.2GHz Pentium 4 running FreeBSD 5.3 and Perl 5.8.7. (The benchmarking script is included in this distribution: devel/benchmark_stemmers.plx.) 

    |--------------------------------------------------------------------|
    | total words: 454285 | unique words: 22748                          |
    |--------------------------------------------------------------------|
    | module                        | config        | avg secs | rate    |
    |--------------------------------------------------------------------|
    | Lingua::Stem 0.81             | no cache      | 2.029    | 223881  |
    | Lingua::Stem 0.81             | cache level 2 | 1.280    | 355025  |
    | Lingua::Stem::Snowball 0.94   | stem          | 1.426    | 318636  |
    | Lingua::Stem::Snowball 0.94   | stem_in_place | 0.641    | 708495  |
    |--------------------------------------------------------------------|

METHODS / FUNCTIONS

new

    my $stemmer = Lingua::Stem::Snowball->new(
        lang     => 'es', 
        encoding => 'UTF-8',
    );
    die $@ if $@;

Create a Lingua::Stem::Snowball object. new() accepts the following hash style parameters:

  • lang: An ISO code taken from the table of supported languages, above.
  • encoding: A supported character encoding.

Be careful with the values you supply to new(). If "lang" is invalid, Lingua::Stem::Snowball does not throw an exception, but instead sets $@. Also, if you supply an invalid combination of values for "lang" and "encoding", Lingua::Stem::Snowball will not warn you, but the behavior will change: stem() will always return undef, and stem_in_place() will be a no-op.

stem

    @stemmed = $stemmer->stem( WORDS, [IS_STEMMED] );
    @stemmed = stem( ISO_CODE, WORDS, [LOCALE], [IS_STEMMED] );

Return lowercased and stemmed output. WORDS may be either an array of words or a single scalar word.

In a scalar context, stem() returns the first item in the array of stems: 

    $stem       = $stemmer->stem($word);
    $first_stem = $stemmer->stem(\@words); # probably wrong

LOCALE has no effect; it is only there as a placeholder for backwards compatibility (see Changes). IS_STEMMED must be a reference to a scalar; if it is supplied, it will be set to 1 if the output differs from the input in some way, 0 otherwise.

stem_in_place

    $stemmer->stem_in_place(\@words);

This is a high-performance, streamlined version of stem() (in fact, stem() calls stem_in_place() internally). It has no return value, instead modifying each item in an existing array of words. The words must already be in lower case.

lang

    my $lang = $stemmer->lang;
    $stemmer->lang($iso_language_code);

Accessor/mutator for the lang parameter. If there is no stemmer for the supplied ISO code, the language is not changed (but $@ is set).

encoding

    my $encoding = $stemmer->encoding;
    $stemmer->encoding($encoding);

Accessor/mutator for the encoding parameter.

stemmers

    my @iso_codes = stemmers();
    my @iso_codes = $stemmer->stemmers();

Returns a list of all valid language codes.

REQUESTS & BUGS

Please report any requests, suggestions or bugs via the RT bug-tracking system at http://rt.cpan.org/ or email to [email protected].

http://rt.cpan.org/NoAuth/Bugs.html?Dist=Lingua-Stem-Snowball is the RT queue for Lingua::Stem::Snowball. Please check to see if your bug has already been reported.

AUTHORS

Lingua::Stem::Snowball was originally developed to provide access to stemming algorithms for the OpenFTS (full text search engine) project (<http://openfts.sourceforge.net>), by Oleg Bartunov, <oleg at sai dot msu dot su> and Teodor Sigaev, <teodor at stack dot net>.

Currently maintained by Marvin Humphrey <marvin at rectangular dot com>. Previously maintained by Fabien Potencier <fabpot at cpan dot org>.

COPYRIGHT AND LICENSE

Perl bindings copyright 2004-2008 by Marvin Humphrey, Fabien Potencier, Oleg Bartunov and Teodor Sigaev.

This software may be freely copied and distributed under the same terms and conditions as Perl.

Snowball files and stemmers are covered by the BSD license.

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