germanstemmer.java

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package org.apache.lucene.analysis.de;

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/**
 * A stemmer for German words. The algorithm is based on the report
 * "A Fast and Simple Stemming Algorithm for German Words" by J鰎g
 * Caumanns ([e-mail]).
 *
 * @author    Gerhard Schwarz
 * @version   $Id: GermanStemmer.java,v 1.4 2002/04/19 19:09:36 otis Exp $
 */

public class GermanStemmer {

    /**
     * Buffer for the terms while stemming them.
     */
    private StringBuffer sb = new StringBuffer();
    /**
     * Indicates if a term is handled as a noun.
     */
    private boolean uppercase = false;
    /**
     * Amount of characters that are removed with <tt>substitute()</tt> while stemming.
     */
    private int substCount = 0;

    public GermanStemmer() {
    }

    /**
     * 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 ) {
	if ( !isStemmable( term ) ) {
	    return term;
	}
	// Mark a possible noun.
	if ( Character.isUpperCase( term.charAt( 0 ) ) ) {
	    uppercase = true;
	}
	else {
	    uppercase = false;
	}
	// Use lowercase for medium stemming.
	term = term.toLowerCase();
	// Reset the StringBuffer.
	sb.delete( 0, sb.length() );
	sb.insert( 0, term );
	sb = substitute( sb );
	// Nouns have only seven possible suffixes.
	if ( uppercase && sb.length() > 3 ) {
	    if ( sb.substring( sb.length() - 3, sb.length() ).equals( "ern" ) ) {
		sb.delete( sb.length() - 3, sb.length() );
	    }
	    else if ( sb.substring( sb.length() - 2, sb.length() ).equals( "en" ) ) {
		sb.delete( sb.length() - 2, sb.length() );
	    }
	    else if ( sb.substring( sb.length() - 2, sb.length() ).equals( "er" ) ) {
		sb.delete( sb.length() - 2, sb.length() );
	    }
	    else if ( sb.substring( sb.length() - 2, sb.length() ).equals( "es" ) ) {
		sb.delete( sb.length() - 2, sb.length() );
	    }
	    else if ( sb.charAt( sb.length() - 1 ) == 'e' ) {
		sb.deleteCharAt( sb.length() - 1 );
	    }
	    else if ( sb.charAt( sb.length() - 1 ) == 'n' ) {
		sb.deleteCharAt( sb.length() - 1 );
	    }
	    else if ( sb.charAt( sb.length() - 1 ) == 's' ) {
		sb.deleteCharAt( sb.length() - 1 );
	    }
	    // Additional step for female plurals of professions and inhabitants.
	    if ( sb.length() > 5 && sb.substring( sb.length() - 3, sb.length() ).equals( "erin*" ) ) {
		sb.deleteCharAt( sb.length() -1 );
	    }
	    // Additional step for irregular plural nouns like "Matrizen -> Matrix".
	    if ( sb.charAt( sb.length() - 1 ) == ( 'z' ) ) {
		sb.setCharAt( sb.length() - 1, 'x' );
	    }
	}
	// Strip the 7 "base" suffixes: "e", "s", "n", "t", "em", "er", "nd" from all
	// other terms. Adjectives, Verbs and Adverbs have a total of 52 different
	// possible suffixes, stripping only the characters from they are build
	// does mostly the same
	else {
	    // Strip base suffixes as long as enough characters remain.
	    boolean doMore = true;
	    while ( sb.length() > 3 && doMore ) {
		if ( ( sb.length() + substCount > 5 ) && sb.substring( sb.length() - 2, sb.length() ).equals( "nd" ) ) {
		    sb.delete( sb.length() - 2, sb.length() );
		}
		else if ( ( sb.length() + substCount > 4 ) && sb.substring( sb.length() - 2, sb.length() ).equals( "er" ) ) {
		    sb.delete( sb.length() - 2, sb.length() );
		}
		else if ( ( sb.length() + substCount > 4 ) && sb.substring( sb.length() - 2, sb.length() ).equals( "em" ) ) {
		    sb.delete( sb.length() - 2, sb.length() );
		}
		else if ( sb.charAt( sb.length() - 1 ) == 't' ) {
		    sb.deleteCharAt( sb.length() - 1 );
		}
		else if ( sb.charAt( sb.length() - 1 ) == 'n' ) {
		    sb.deleteCharAt( sb.length() - 1 );
		}
		else if ( sb.charAt( sb.length() - 1 ) == 's' ) {
		    sb.deleteCharAt( sb.length() - 1 );
		}
		else if ( sb.charAt( sb.length() - 1 ) == 'e' ) {
		    sb.deleteCharAt( sb.length() - 1 );
		}
		else {
		    doMore = false;
		}
	    }
	}
	sb = resubstitute( sb );
	if ( !uppercase ) {
	    sb = removeParticleDenotion( sb );
	}
	return sb.toString();
    }

    /**
     * Removes a particle denotion ("ge") from a term, but only if at least 3
     * characters will remain.
     *
     * @return  The term without particle denotion, if there was one.
     */
    private StringBuffer removeParticleDenotion( StringBuffer buffer ) {
	for ( int c = 0; c < buffer.length(); c++ ) {
	    // Strip from the beginning of the string to the "ge" inclusive
	    if ( c < ( buffer.length() - 4 ) && buffer.charAt( c ) == 'g' && buffer.charAt ( c + 1 ) == 'e' ) {
		buffer.delete( 0, c + 2 );
	    }
	}
	return sb;
    }

    /**
     * Do some substitutions for the term to reduce overstemming:
     *
     * - Substitute Umlauts with their corresponding vowel: 漩