RussianAnalyzer.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.ru;

import java.io.IOException;
import java.io.Reader;
import java.nio.charset.StandardCharsets;
import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.CharArraySet;
import org.apache.lucene.analysis.LowerCaseFilter;
import org.apache.lucene.analysis.StopFilter;
import org.apache.lucene.analysis.StopwordAnalyzerBase;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.Tokenizer;
import org.apache.lucene.analysis.WordlistLoader;
import org.apache.lucene.analysis.miscellaneous.SetKeywordMarkerFilter;
import org.apache.lucene.analysis.snowball.SnowballFilter;
import org.apache.lucene.analysis.standard.StandardTokenizer;
import org.apache.lucene.util.IOUtils;

/**
 * {@link Analyzer} for Russian language.
 *
 * <p>Supports an external list of stopwords (words that will not be indexed at all). A default set
 * of stopwords is used unless an alternative list is specified.
 *
 * @since 3.1
 */
public final class RussianAnalyzer extends StopwordAnalyzerBase {

  /** File containing default Russian stopwords. */
  public static final String DEFAULT_STOPWORD_FILE = "russian_stop.txt";

  private static class DefaultSetHolder {
    static final CharArraySet DEFAULT_STOP_SET;

    static {
      try {
        DEFAULT_STOP_SET =
            WordlistLoader.getSnowballWordSet(
                IOUtils.getDecodingReader(
                    IOUtils.requireResourceNonNull(
                        SnowballFilter.class.getResourceAsStream(DEFAULT_STOPWORD_FILE),
                        DEFAULT_STOPWORD_FILE),
                    StandardCharsets.UTF_8));
      } catch (IOException ex) {
        // default set should always be present as it is part of the
        // distribution (JAR)
        throw new RuntimeException("Unable to load default stopword set", ex);
      }
    }
  }

  private final CharArraySet stemExclusionSet;

  /**
   * Returns an unmodifiable instance of the default stop-words set.
   *
   * @return an unmodifiable instance of the default stop-words set.
   */
  public static CharArraySet getDefaultStopSet() {
    return DefaultSetHolder.DEFAULT_STOP_SET;
  }

  public RussianAnalyzer() {
    this(DefaultSetHolder.DEFAULT_STOP_SET);
  }

  /**
   * Builds an analyzer with the given stop words
   *
   * @param stopwords a stopword set
   */
  public RussianAnalyzer(CharArraySet stopwords) {
    this(stopwords, CharArraySet.EMPTY_SET);
  }

  /**
   * Builds an analyzer with the given stop words
   *
   * @param stopwords a stopword set
   * @param stemExclusionSet a set of words not to be stemmed
   */
  public RussianAnalyzer(CharArraySet stopwords, CharArraySet stemExclusionSet) {
    super(stopwords);
    this.stemExclusionSet = CharArraySet.unmodifiableSet(CharArraySet.copy(stemExclusionSet));
  }

  /**
   * Creates {@link org.apache.lucene.analysis.Analyzer.TokenStreamComponents} used to tokenize all
   * the text in the provided {@link Reader}.
   *
   * @return {@link org.apache.lucene.analysis.Analyzer.TokenStreamComponents} built from a {@link
   *     StandardTokenizer} filtered with {@link LowerCaseFilter}, {@link StopFilter}, {@link
   *     SetKeywordMarkerFilter} if a stem exclusion set is provided, and {@link SnowballFilter}
   */
  @Override
  protected TokenStreamComponents createComponents(String fieldName) {
    final Tokenizer source = new StandardTokenizer();
    TokenStream result = new LowerCaseFilter(source);
    result = new StopFilter(result, stopwords);
    if (!stemExclusionSet.isEmpty()) result = new SetKeywordMarkerFilter(result, stemExclusionSet);
    result = new SnowballFilter(result, new org.tartarus.snowball.ext.RussianStemmer());
    return new TokenStreamComponents(source, result);
  }

  @Override
  protected TokenStream normalize(String fieldName, TokenStream in) {
    return new LowerCaseFilter(in);
  }
}