lucene/analysis/common/src/java/org/apache/lucene/analysis/en/EnglishAnalyzer.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.Reader;
import java.util.Arrays;
import java.util.List;
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.miscellaneous.SetKeywordMarkerFilter;
import org.apache.lucene.analysis.standard.StandardTokenizer;

/**
 * {@link Analyzer} for English.
 *
 * @since 3.1
 */
public final class EnglishAnalyzer extends StopwordAnalyzerBase {

  /**
   * An unmodifiable set containing some common English words that are not usually useful for
   * searching.
   */
  public static final CharArraySet ENGLISH_STOP_WORDS_SET;

  static {
    final List<String> stopWords =
        Arrays.asList(
            "a", "an", "and", "are", "as", "at", "be", "but", "by", "for", "if", "in", "into", "is",
            "it", "no", "not", "of", "on", "or", "such", "that", "the", "their", "then", "there",
            "these", "they", "this", "to", "was", "will", "with");
    final CharArraySet stopSet = new CharArraySet(stopWords, false);
    ENGLISH_STOP_WORDS_SET = CharArraySet.unmodifiableSet(stopSet);
  }

  private final CharArraySet stemExclusionSet;

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

  /** Builds an analyzer with the default stop words: {@link #getDefaultStopSet}. */
  public EnglishAnalyzer() {
    this(ENGLISH_STOP_WORDS_SET);
  }

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

  /**
   * Builds an analyzer with the given stop words. If a non-empty stem exclusion set is provided
   * this analyzer will add a {@link SetKeywordMarkerFilter} before stemming.
   *
   * @param stopwords a stopword set
   * @param stemExclusionSet a set of terms not to be stemmed
   */
  public EnglishAnalyzer(CharArraySet stopwords, CharArraySet stemExclusionSet) {
    super(stopwords);
    this.stemExclusionSet = CharArraySet.unmodifiableSet(CharArraySet.copy(stemExclusionSet));
  }

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

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