/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.lucene.analysis.de;
import java.util.Locale;
// This file is encoded in UTF-8
A stemmer for German words.
The algorithm is based on the report
"A Fast and Simple Stemming Algorithm for German Words" by Jörg
Caumanns (joerg.caumanns at isst.fhg.de).
/**
* A stemmer for German words.
* <p>
* The algorithm is based on the report
* "A Fast and Simple Stemming Algorithm for German Words" by Jörg
* Caumanns (joerg.caumanns at isst.fhg.de).
* </p>
*/
public class GermanStemmer
{
Buffer for the terms while stemming them.
/**
* Buffer for the terms while stemming them.
*/
private StringBuilder sb = new StringBuilder();
Amount of characters that are removed with substitute() while stemming.
/**
* Amount of characters that are removed with <tt>substitute()</tt> while stemming.
*/
private int substCount = 0;
private static final Locale locale = new Locale("de", "DE");
Stemms the given term to an unique discriminator.
Params: - term – The term that should be stemmed.
Returns: Discriminator for term
/**
* Stemms the given term to an unique <tt>discriminator</tt>.
*
* @param term The term that should be stemmed.
* @return Discriminator for <tt>term</tt>
*/
protected String stem( String term )
{
// Use lowercase for medium stemming.
term = term.toLowerCase(locale);
if ( !isStemmable( term ) )
return term;
// Reset the StringBuilder.
sb.delete( 0, sb.length() );
sb.insert( 0, term );
// Stemming starts here...
substitute( sb );
strip( sb );
optimize( sb );
resubstitute( sb );
removeParticleDenotion( sb );
return sb.toString();
}
Checks if a term could be stemmed.
Returns: true if, and only if, the given term consists in letters.
/**
* Checks if a term could be stemmed.
*
* @return true if, and only if, the given term consists in letters.
*/
private boolean isStemmable( String term )
{
for ( int c = 0; c < term.length(); c++ ) {
if ( !Character.isLetter( term.charAt( c ) ) )
return false;
}
return true;
}
suffix stripping (stemming) on the current term. The stripping is reduced
to the seven "base" suffixes "e", "s", "n", "t", "em", "er" and * "nd",
from which all regular suffixes are build of. The simplification causes
some overstemming, and way more irregular stems, but still provides unique.
discriminators in the most of those cases.
The algorithm is context free, except of the length restrictions.
/**
* suffix stripping (stemming) on the current term. The stripping is reduced
* to the seven "base" suffixes "e", "s", "n", "t", "em", "er" and * "nd",
* from which all regular suffixes are build of. The simplification causes
* some overstemming, and way more irregular stems, but still provides unique.
* discriminators in the most of those cases.
* The algorithm is context free, except of the length restrictions.
*/
private void strip( StringBuilder buffer )
{
boolean doMore = true;
while ( doMore && buffer.length() > 3 ) {
if ( ( buffer.length() + substCount > 5 ) &&
buffer.substring( buffer.length() - 2, buffer.length() ).equals( "nd" ) )
{
buffer.delete( buffer.length() - 2, buffer.length() );
}
else if ( ( buffer.length() + substCount > 4 ) &&
buffer.substring( buffer.length() - 2, buffer.length() ).equals( "em" ) ) {
buffer.delete( buffer.length() - 2, buffer.length() );
}
else if ( ( buffer.length() + substCount > 4 ) &&
buffer.substring( buffer.length() - 2, buffer.length() ).equals( "er" ) ) {
buffer.delete( buffer.length() - 2, buffer.length() );
}
else if ( buffer.charAt( buffer.length() - 1 ) == 'e' ) {
buffer.deleteCharAt( buffer.length() - 1 );
}
else if ( buffer.charAt( buffer.length() - 1 ) == 's' ) {
buffer.deleteCharAt( buffer.length() - 1 );
}
else if ( buffer.charAt( buffer.length() - 1 ) == 'n' ) {
buffer.deleteCharAt( buffer.length() - 1 );
}
// "t" occurs only as suffix of verbs.
else if ( buffer.charAt( buffer.length() - 1 ) == 't' ) {
buffer.deleteCharAt( buffer.length() - 1 );
}
else {
doMore = false;
}
}
}
Does some optimizations on the term. This optimisations are
contextual.
/**
* Does some optimizations on the term. This optimisations are
* contextual.
*/
private void optimize( StringBuilder buffer )
{
// Additional step for female plurals of professions and inhabitants.
if ( buffer.length() > 5 && buffer.substring( buffer.length() - 5, buffer.length() ).equals( "erin*" ) ) {
buffer.deleteCharAt( buffer.length() -1 );
strip( buffer );
}
// Additional step for irregular plural nouns like "Matrizen -> Matrix".
// NOTE: this length constraint is probably not a great value, it's just to prevent AIOOBE on empty terms
if ( buffer.length() > 0 && buffer.charAt( buffer.length() - 1 ) == ( 'z' ) ) {
buffer.setCharAt( buffer.length() - 1, 'x' );
}
}
Removes a particle denotion ("ge") from a term.
/**
* Removes a particle denotion ("ge") from a term.
*/
private void removeParticleDenotion( StringBuilder buffer )
{
if ( buffer.length() > 4 ) {
for ( int c = 0; c < buffer.length() - 3; c++ ) {
if ( buffer.substring( c, c + 4 ).equals( "gege" ) ) {
buffer.delete( c, c + 2 );
return;
}
}
}
}
Do some substitutions for the term to reduce overstemming: - Substitute Umlauts with their corresponding vowel:äöü -> aou, "ß" is substituted by "ss" - Substitute a second char of a pair of equal characters with an asterisk: ?? -> ?* - Substitute some common character combinations with a token: sch/ch/ei/ie/ig/st -> $/§/%/&/#/! /**
* Do some substitutions for the term to reduce overstemming:
*
* - Substitute Umlauts with their corresponding vowel:{@code äöü -> aou},
* "ß" is substituted by "ss"
* - Substitute a second char of a pair of equal characters with
* an asterisk: {@code ?? -> ?*}
* - Substitute some common character combinations with a token:
* {@code sch/ch/ei/ie/ig/st -> $/§/%/&/#/!}
*/
private void substitute( StringBuilder buffer )
{
substCount = 0;
for ( int c = 0; c < buffer.length(); c++ ) {
// Replace the second char of a pair of the equal characters with an asterisk
if ( c > 0 && buffer.charAt( c ) == buffer.charAt ( c - 1 ) ) {
buffer.setCharAt( c, '*' );
}
// Substitute Umlauts.
else if ( buffer.charAt( c ) == 'ä' ) {
buffer.setCharAt( c, 'a' );
}
else if ( buffer.charAt( c ) == 'ö' ) {
buffer.setCharAt( c, 'o' );
}
else if ( buffer.charAt( c ) == 'ü' ) {
buffer.setCharAt( c, 'u' );
}
// Fix bug so that 'ß' at the end of a word is replaced.
else if ( buffer.charAt( c ) == 'ß' ) {
buffer.setCharAt( c, 's' );
buffer.insert( c + 1, 's' );
substCount++;
}
// Take care that at least one character is left left side from the current one
if ( c < buffer.length() - 1 ) {
// Masking several common character combinations with an token
if ( ( c < buffer.length() - 2 ) && buffer.charAt( c ) == 's' &&
buffer.charAt( c + 1 ) == 'c' && buffer.charAt( c + 2 ) == 'h' )
{
buffer.setCharAt( c, '$' );
buffer.delete( c + 1, c + 3 );
substCount += 2;
}
else if ( buffer.charAt( c ) == 'c' && buffer.charAt( c + 1 ) == 'h' ) {
buffer.setCharAt( c, '§' );
buffer.deleteCharAt( c + 1 );
substCount++;
}
else if ( buffer.charAt( c ) == 'e' && buffer.charAt( c + 1 ) == 'i' ) {
buffer.setCharAt( c, '%' );
buffer.deleteCharAt( c + 1 );
substCount++;
}
else if ( buffer.charAt( c ) == 'i' && buffer.charAt( c + 1 ) == 'e' ) {
buffer.setCharAt( c, '&' );
buffer.deleteCharAt( c + 1 );
substCount++;
}
else if ( buffer.charAt( c ) == 'i' && buffer.charAt( c + 1 ) == 'g' ) {
buffer.setCharAt( c, '#' );
buffer.deleteCharAt( c + 1 );
substCount++;
}
else if ( buffer.charAt( c ) == 's' && buffer.charAt( c + 1 ) == 't' ) {
buffer.setCharAt( c, '!' );
buffer.deleteCharAt( c + 1 );
substCount++;
}
}
}
}
Undoes the changes made by substitute(). That are character pairs and
character combinations. Umlauts will remain as their corresponding vowel,
as "ß" remains as "ss".
/**
* Undoes the changes made by substitute(). That are character pairs and
* character combinations. Umlauts will remain as their corresponding vowel,
* as "ß" remains as "ss".
*/
private void resubstitute( StringBuilder buffer )
{
for ( int c = 0; c < buffer.length(); c++ ) {
if ( buffer.charAt( c ) == '*' ) {
char x = buffer.charAt( c - 1 );
buffer.setCharAt( c, x );
}
else if ( buffer.charAt( c ) == '$' ) {
buffer.setCharAt( c, 's' );
buffer.insert( c + 1, new char[]{'c', 'h'}, 0, 2 );
}
else if ( buffer.charAt( c ) == '§' ) {
buffer.setCharAt( c, 'c' );
buffer.insert( c + 1, 'h' );
}
else if ( buffer.charAt( c ) == '%' ) {
buffer.setCharAt( c, 'e' );
buffer.insert( c + 1, 'i' );
}
else if ( buffer.charAt( c ) == '&' ) {
buffer.setCharAt( c, 'i' );
buffer.insert( c + 1, 'e' );
}
else if ( buffer.charAt( c ) == '#' ) {
buffer.setCharAt( c, 'i' );
buffer.insert( c + 1, 'g' );
}
else if ( buffer.charAt( c ) == '!' ) {
buffer.setCharAt( c, 's' );
buffer.insert( c + 1, 't' );
}
}
}
}