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| 2 | /* ====================================================================
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| 3 | * The Apache Software License, Version 1.1
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| 4 | *
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| 5 | * Copyright (c) 2001 The Apache Software Foundation. All rights
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| 6 | * reserved.
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| 7 | *
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| 8 | * Redistribution and use in source and binary forms, with or without
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| 9 | * modification, are permitted provided that the following conditions
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| 10 | * are met:
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| 11 | *
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| 12 | * 1. Redistributions of source code must retain the above copyright
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| 13 | * notice, this list of conditions and the following disclaimer.
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| 14 | *
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| 15 | * 2. Redistributions in binary form must reproduce the above copyright
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| 16 | * notice, this list of conditions and the following disclaimer in
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| 17 | * the documentation and/or other materials provided with the
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| 18 | * distribution.
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| 19 | *
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| 20 | * 3. The end-user documentation included with the redistribution,
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| 21 | * if any, must include the following acknowledgment:
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| 22 | * "This product includes software developed by the
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| 23 | * Apache Software Foundation (http://www.apache.org/)."
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| 24 | * Alternately, this acknowledgment may appear in the software itself,
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| 25 | * if and wherever such third-party acknowledgments normally appear.
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| 26 | *
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| 27 | * 4. The names "Apache" and "Apache Software Foundation" and
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| 28 | * "Apache Lucene" must not be used to endorse or promote products
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| 29 | * derived from this software without prior written permission. For
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| 30 | * written permission, please contact [email protected].
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| 31 | *
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| 32 | * 5. Products derived from this software may not be called "Apache",
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| 33 | * "Apache Lucene", nor may "Apache" appear in their name, without
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| 34 | * prior written permission of the Apache Software Foundation.
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| 35 | *
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| 36 | * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
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| 37 | * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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| 38 | * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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| 39 | * DISCLAIMED. IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR
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| 40 | * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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| 41 | * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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| 42 | * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
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| 43 | * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
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| 44 | * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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| 45 | * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
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| 46 | * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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| 47 | * SUCH DAMAGE.
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| 48 | * ====================================================================
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| 49 | *
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| 50 | * This software consists of voluntary contributions made by many
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| 51 | * individuals on behalf of the Apache Software Foundation. For more
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| 52 | * information on the Apache Software Foundation, please see
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| 53 | * <http://www.apache.org/>.
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| 54 | */
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| 55 |
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| 56 | /**
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| 57 | * A stemmer for German words. The algorithm is based on the report
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| 58 | * "A Fast and Simple Stemming Algorithm for German Words" by Jörg
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| 59 | * Caumanns ([email protected]).
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| 60 | *
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| 61 | * Changed stem() from protected to public.
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| 62 | * Changed coding for umlaute to unicode.
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| 63 | *
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| 64 | * @author Gerhard Schwarz
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| 65 | * @version $Id: GermanStemmer.java 8815 2004-12-15 01:13:55Z mdewsnip $
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| 66 | */
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| 67 | public class GermanStemmer extends Stemmer
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| 68 | {
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| 69 | /**
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| 70 | * Buffer for the terms while stemming them.
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| 71 | */
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| 72 | private StringBuffer sb = new StringBuffer();
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| 73 |
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| 74 | /**
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| 75 | * Indicates if a term is handled as a noun.
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| 76 | */
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| 77 | private boolean uppercase = false;
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| 78 |
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| 79 | /**
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| 80 | * Amount of characters that are removed with <tt>substitute()</tt> while stemming.
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| 81 | */
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| 82 | private int substCount = 0;
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| 83 |
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| 84 | /**
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| 85 | * Stemms the given term to an unique <tt>discriminator</tt>.
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| 86 | *
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| 87 | * @param term The term that should be stemmed.
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| 88 | * @return Discriminator for <tt>term</tt>
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| 89 | */
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| 90 | public String stem( String term )
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| 91 | {
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| 92 | // Mark a possible noun.
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| 93 | uppercase = Character.isUpperCase( term.charAt( 0 ) );
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| 94 | // Use lowercase for medium stemming.
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| 95 | term = term.toLowerCase();
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| 96 | if ( !isStemmable( term ) )
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| 97 | return term;
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| 98 | // Reset the StringBuffer.
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| 99 | sb.delete( 0, sb.length() );
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| 100 | sb.insert( 0, term );
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| 101 | // Stemming starts here...
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| 102 | substitute( sb );
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| 103 | strip( sb );
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| 104 | optimize( sb );
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| 105 | resubstitute( sb );
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| 106 | removeParticleDenotion( sb );
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| 107 | return sb.toString();
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| 108 | }
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| 109 |
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| 110 | /**
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| 111 | * Checks if a term could be stemmed.
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| 112 | *
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| 113 | * @return true if, and only if, the given term consists in letters.
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| 114 | */
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| 115 | private boolean isStemmable( String term )
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| 116 | {
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| 117 | for ( int c = 0; c < term.length(); c++ ) {
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| 118 | if ( !Character.isLetter( term.charAt( c ) ) ) return false;
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| 119 | }
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| 120 | return true;
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| 121 | }
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| 122 |
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| 123 | /**
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| 124 | * suffix stripping (stemming) on the current term. The stripping is reduced
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| 125 | * to the seven "base" suffixes "e", "s", "n", "t", "em", "er" and * "nd",
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| 126 | * from which all regular suffixes are build of. The simplification causes
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| 127 | * some overstemming, and way more irregular stems, but still provides unique.
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| 128 | * discriminators in the most of those cases.
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| 129 | * The algorithm is context free, except of the length restrictions.
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| 130 | */
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| 131 | private void strip( StringBuffer buffer )
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| 132 | {
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| 133 | boolean doMore = true;
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| 134 | while ( doMore && buffer.length() > 3 ) {
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| 135 | if ( ( buffer.length() + substCount > 5 ) &&
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| 136 | buffer.substring( buffer.length() - 2, buffer.length() ).equals( "nd" ) )
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| 137 | {
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| 138 | buffer.delete( buffer.length() - 2, buffer.length() );
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| 139 | }
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| 140 | else if ( ( buffer.length() + substCount > 4 ) &&
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| 141 | buffer.substring( buffer.length() - 2, buffer.length() ).equals( "em" ) ) {
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| 142 | buffer.delete( buffer.length() - 2, buffer.length() );
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| 143 | }
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| 144 | else if ( ( buffer.length() + substCount > 4 ) &&
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| 145 | buffer.substring( buffer.length() - 2, buffer.length() ).equals( "er" ) ) {
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| 146 | buffer.delete( buffer.length() - 2, buffer.length() );
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| 147 | }
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| 148 | else if ( buffer.charAt( buffer.length() - 1 ) == 'e' ) {
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| 149 | buffer.deleteCharAt( buffer.length() - 1 );
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| 150 | }
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| 151 | else if ( buffer.charAt( buffer.length() - 1 ) == 's' ) {
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| 152 | buffer.deleteCharAt( buffer.length() - 1 );
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| 153 | }
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| 154 | else if ( buffer.charAt( buffer.length() - 1 ) == 'n' ) {
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| 155 | buffer.deleteCharAt( buffer.length() - 1 );
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| 156 | }
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| 157 | // "t" occurs only as suffix of verbs.
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| 158 | else if ( buffer.charAt( buffer.length() - 1 ) == 't' && !uppercase ) {
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| 159 | buffer.deleteCharAt( buffer.length() - 1 );
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| 160 | }
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| 161 | else {
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| 162 | doMore = false;
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| 163 | }
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| 164 | }
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| 165 | }
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| 166 |
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| 167 | /**
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| 168 | * Does some optimizations on the term. This optimisations are
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| 169 | * contextual.
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| 170 | */
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| 171 | private void optimize( StringBuffer buffer )
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| 172 | {
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| 173 | // Additional step for female plurals of professions and inhabitants.
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| 174 | if ( buffer.length() > 5 && buffer.substring( buffer.length() - 5, buffer.length() ).equals( "erin*" ) ) {
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| 175 | buffer.deleteCharAt( buffer.length() -1 );
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| 176 | strip( buffer );
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| 177 | }
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| 178 | // Additional step for irregular plural nouns like "Matrizen -> Matrix".
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| 179 | if ( buffer.charAt( buffer.length() - 1 ) == ( 'z' ) ) {
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| 180 | buffer.setCharAt( buffer.length() - 1, 'x' );
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| 181 | }
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| 182 | }
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| 183 |
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| 184 | /**
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| 185 | * Removes a particle denotion ("ge") from a term.
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| 186 | */
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| 187 | private void removeParticleDenotion( StringBuffer buffer )
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| 188 | {
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| 189 | if ( buffer.length() > 4 ) {
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| 190 | for ( int c = 0; c < buffer.length() - 3; c++ ) {
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| 191 | if ( buffer.substring( c, c + 4 ).equals( "gege" ) ) {
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| 192 | buffer.delete( c, c + 2 );
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| 193 | return;
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| 194 | }
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| 195 | }
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| 196 | }
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| 197 | }
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| 198 |
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| 199 | /**
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| 200 | * Do some substitutions for the term to reduce overstemming:
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| 201 | *
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| 202 | * - Substitute Umlauts with their corresponding vowel: äöü -> aou,
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| 203 | * "ß" is substituted by "ss"
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| 204 | * - Substitute a second char of a pair of equal characters with
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| 205 | * an asterisk: ?? -> ?*
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| 206 | * - Substitute some common character combinations with a token:
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| 207 | * sch/ch/ei/ie/ig/st -> $/§/%/&/#/!
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| 208 | */
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| 209 | private void substitute( StringBuffer buffer )
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| 210 | {
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| 211 | substCount = 0;
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| 212 | for ( int c = 0; c < buffer.length(); c++ ) {
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| 213 | // Replace the second char of a pair of the equal characters with an asterisk
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| 214 | if ( c > 0 && buffer.charAt( c ) == buffer.charAt ( c - 1 ) ) {
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| 215 | buffer.setCharAt( c, '*' );
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| 216 | }
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| 217 | // Substitute Umlauts.
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| 218 | else if ( buffer.charAt( c ) == '\u00E4' ) {
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| 219 | buffer.setCharAt( c, 'a' );
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| 220 | }
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| 221 | else if ( buffer.charAt( c ) == '\u00F6' ) {
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| 222 | buffer.setCharAt( c, 'o' );
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| 223 | }
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| 224 | else if ( buffer.charAt( c ) == '\u00FC' ) {
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| 225 | buffer.setCharAt( c, 'u' );
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| 226 | }
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| 227 | // Take care that at least one character is left left side from the current one
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| 228 | if ( c < buffer.length() - 1 ) {
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| 229 | if ( buffer.charAt( c ) == '\u00DF' ) {
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| 230 | buffer.setCharAt( c, 's' );
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| 231 | buffer.insert( c + 1, 's' );
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| 232 | substCount++;
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| 233 | }
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| 234 | // Masking several common character combinations with an token
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| 235 | else if ( ( c < buffer.length() - 2 ) && buffer.charAt( c ) == 's' &&
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| 236 | buffer.charAt( c + 1 ) == 'c' && buffer.charAt( c + 2 ) == 'h' )
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| 237 | {
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| 238 | buffer.setCharAt( c, '$' );
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| 239 | buffer.delete( c + 1, c + 3 );
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| 240 | substCount =+ 2;
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| 241 | }
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| 242 | else if ( buffer.charAt( c ) == 'c' && buffer.charAt( c + 1 ) == 'h' ) {
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| 243 | buffer.setCharAt( c, '§' );
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| 244 | buffer.deleteCharAt( c + 1 );
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| 245 | substCount++;
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| 246 | }
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| 247 | else if ( buffer.charAt( c ) == 'e' && buffer.charAt( c + 1 ) == 'i' ) {
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| 248 | buffer.setCharAt( c, '%' );
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| 249 | buffer.deleteCharAt( c + 1 );
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| 250 | substCount++;
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| 251 | }
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| 252 | else if ( buffer.charAt( c ) == 'i' && buffer.charAt( c + 1 ) == 'e' ) {
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| 253 | buffer.setCharAt( c, '&' );
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| 254 | buffer.deleteCharAt( c + 1 );
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| 255 | substCount++;
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| 256 | }
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| 257 | else if ( buffer.charAt( c ) == 'i' && buffer.charAt( c + 1 ) == 'g' ) {
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| 258 | buffer.setCharAt( c, '#' );
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| 259 | buffer.deleteCharAt( c + 1 );
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| 260 | substCount++;
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| 261 | }
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| 262 | else if ( buffer.charAt( c ) == 's' && buffer.charAt( c + 1 ) == 't' ) {
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| 263 | buffer.setCharAt( c, '!' );
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| 264 | buffer.deleteCharAt( c + 1 );
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| 265 | substCount++;
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| 266 | }
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| 267 | }
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| 268 | }
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| 269 | }
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| 270 |
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| 271 | /**
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| 272 | * Undoes the changes made by substitute(). That are character pairs and
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| 273 | * character combinations. Umlauts will remain as their corresponding vowel,
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| 274 | * as "ß" remains as "ss".
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| 275 | */
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| 276 | private void resubstitute( StringBuffer buffer )
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| 277 | {
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| 278 | for ( int c = 0; c < buffer.length(); c++ ) {
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| 279 | if ( buffer.charAt( c ) == '*' ) {
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| 280 | char x = buffer.charAt( c - 1 );
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| 281 | buffer.setCharAt( c, x );
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| 282 | }
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| 283 | else if ( buffer.charAt( c ) == '$' ) {
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| 284 | buffer.setCharAt( c, 's' );
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| 285 | buffer.insert( c + 1, new char[]{'c', 'h'}, 0, 2 );
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| 286 | }
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| 287 | else if ( buffer.charAt( c ) == '§' ) {
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| 288 | buffer.setCharAt( c, 'c' );
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| 289 | buffer.insert( c + 1, 'h' );
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| 290 | }
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| 291 | else if ( buffer.charAt( c ) == '%' ) {
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| 292 | buffer.setCharAt( c, 'e' );
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| 293 | buffer.insert( c + 1, 'i' );
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| 294 | }
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| 295 | else if ( buffer.charAt( c ) == '&' ) {
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| 296 | buffer.setCharAt( c, 'i' );
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| 297 | buffer.insert( c + 1, 'e' );
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| 298 | }
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| 299 | else if ( buffer.charAt( c ) == '#' ) {
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| 300 | buffer.setCharAt( c, 'i' );
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| 301 | buffer.insert( c + 1, 'g' );
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| 302 | }
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| 303 | else if ( buffer.charAt( c ) == '!' ) {
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| 304 | buffer.setCharAt( c, 's' );
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| 305 | buffer.insert( c + 1, 't' );
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| 306 | }
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| 307 | }
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| 308 | }
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| 309 | }
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