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Lucene example source code file (SynonymTokenFilter.java)

This example Lucene source code file (SynonymTokenFilter.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Lucene tags/keywords

eos, illegalargumentexception, illegalargumentexception, io, ioexception, ioexception, object, override, override, string, string, synonym_token_type, synonymmap, synonymtokenfilter, tokenfilter

The Lucene SynonymTokenFilter.java source code

package org.apache.lucene.wordnet;

/**
 * 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.
 */

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.PositionIncrementAttribute;
import org.apache.lucene.analysis.tokenattributes.TypeAttribute;
import org.apache.lucene.util.AttributeSource;

/**
 * Injects additional tokens for synonyms of token terms fetched from the
 * underlying child stream; the child stream must deliver lowercase tokens
 * for synonyms to be found.
 *
 */
public class SynonymTokenFilter extends TokenFilter {
    
  /** The Token.type used to indicate a synonym to higher level filters. */
  public static final String SYNONYM_TOKEN_TYPE = "SYNONYM";

  private final SynonymMap synonyms;
  private final int maxSynonyms;
  
  private String[] stack = null;
  private int index = 0;
  private AttributeSource.State current = null;
  private int todo = 0;
  
  private final CharTermAttribute termAtt = addAttribute(CharTermAttribute.class);
  private final TypeAttribute typeAtt = addAttribute(TypeAttribute.class);
  private final PositionIncrementAttribute posIncrAtt = addAttribute(PositionIncrementAttribute.class);
  
  /**
   * Creates an instance for the given underlying stream and synonym table.
   * 
   * @param input
   *            the underlying child token stream
   * @param synonyms
   *            the map used to extract synonyms for terms
   * @param maxSynonyms
   *            the maximum number of synonym tokens to return per underlying
   *            token word (a value of Integer.MAX_VALUE indicates unlimited)
   */
  public SynonymTokenFilter(TokenStream input, SynonymMap synonyms, int maxSynonyms) {
    super(input);
    if (input == null)
      throw new IllegalArgumentException("input must not be null");
    if (synonyms == null)
      throw new IllegalArgumentException("synonyms must not be null");
    if (maxSynonyms < 0) 
      throw new IllegalArgumentException("maxSynonyms must not be negative");
    
    this.synonyms = synonyms;
    this.maxSynonyms = maxSynonyms;
  }
  
  /** Returns the next token in the stream, or null at EOS. */
  @Override
  public final boolean incrementToken() throws IOException {
    while (todo > 0 && index < stack.length) { // pop from stack
      if (createToken(stack[index++], current)) {
        todo--;
        return true;
      }
    }
    
    if (!input.incrementToken()) return false; // EOS; iterator exhausted 
    
    stack = synonyms.getSynonyms(termAtt.toString()); // push onto stack
    if (stack.length > maxSynonyms) randomize(stack);
    index = 0;
    current = captureState();
    todo = maxSynonyms;
    return true;
  }
  
  /**
   * Creates and returns a token for the given synonym of the current input
   * token; Override for custom (stateless or stateful) behavior, if desired.
   * 
   * @param synonym 
   *            a synonym for the current token's term
   * @param current
   *            the current token from the underlying child stream
   * @return a new token, or null to indicate that the given synonym should be
   *         ignored
   */
  protected boolean createToken(String synonym, AttributeSource.State current) {
    restoreState(current);
    termAtt.setEmpty().append(synonym);
    typeAtt.setType(SYNONYM_TOKEN_TYPE);
    posIncrAtt.setPositionIncrement(0);
    return true;
  }
  
  /**
   * Randomize synonyms to later sample a subset. Uses constant random seed
   * for reproducibility. Uses "DRand", a simple, fast, uniform pseudo-random
   * number generator with medium statistical quality (multiplicative
   * congruential method), producing integers in the range [Integer.MIN_VALUE,
   * Integer.MAX_VALUE].
   */
  private static void randomize(Object[] arr) {
    int seed = 1234567; // constant
    int randomState = 4*seed + 1;
//    Random random = new Random(seed); // unnecessary overhead
    int len = arr.length;
    for (int i=0; i < len-1; i++) {
      randomState *= 0x278DDE6D; // z(i+1)=a*z(i) (mod 2**32)
      int r = randomState % (len-i);
      if (r < 0) r = -r; // e.g. -9 % 2 == -1
//      int r = random.nextInt(len-i);
      
      // swap arr[i, i+r]
      Object tmp = arr[i];
      arr[i] = arr[i + r];
      arr[i + r] = tmp;
    }   
  }

  @Override
  public void reset() throws IOException {
    super.reset();
    stack = null;
    index = 0;
    current = null;
    todo = 0;
  }
}

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