lucene/analysis/common/src/java/org/apache/lucene/analysis/en/PorterStemFilter.java

/*
 * 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.en;

import java.io.IOException;
import org.apache.lucene.analysis.TokenFilter;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;
import org.apache.lucene.analysis.tokenattributes.KeywordAttribute;

/**
 * Transforms the token stream as per the Porter stemming algorithm. Note: the input to the stemming
 * filter must already be in lower case, so you will need to use LowerCaseFilter or
 * LowerCaseTokenizer farther down the Tokenizer chain in order for this to work properly!
 *
 * <p>To use this filter with other analyzers, you'll want to write an Analyzer class that sets up
 * the TokenStream chain as you want it. To use this with LowerCaseTokenizer, for example, you'd
 * write an analyzer like this: <br>
 *
 * <pre class="prettyprint">
 * class MyAnalyzer extends Analyzer {
 *   {@literal @Override}
 *   protected TokenStreamComponents createComponents(String fieldName) {
 *     Tokenizer source = new LowerCaseTokenizer(version, reader);
 *     return new TokenStreamComponents(source, new PorterStemFilter(source));
 *   }
 * }
 * </pre>
 *
 * <p>Note: This filter is aware of the {@link KeywordAttribute}. To prevent certain terms from
 * being passed to the stemmer {@link KeywordAttribute#isKeyword()} should be set to <code>true
 * </code> in a previous {@link TokenStream}.
 *
 * <p>Note: For including the original term as well as the stemmed version, see {@link
 * org.apache.lucene.analysis.miscellaneous.KeywordRepeatFilterFactory}
 */
public final class PorterStemFilter extends TokenFilter {
  private final PorterStemmer stemmer = new PorterStemmer();
  private final CharTermAttribute termAtt = addAttribute(CharTermAttribute.class);
  private final KeywordAttribute keywordAttr = addAttribute(KeywordAttribute.class);

  public PorterStemFilter(TokenStream in) {
    super(in);
  }

  @Override
  public final boolean incrementToken() throws IOException {
    if (!input.incrementToken()) return false;

    if ((!keywordAttr.isKeyword()) && stemmer.stem(termAtt.buffer(), 0, termAtt.length()))
      termAtt.copyBuffer(stemmer.getResultBuffer(), 0, stemmer.getResultLength());
    return true;
  }
}