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Lucene example source code file (NGramTokenFilter.java)

This example Lucene source code file (NGramTokenFilter.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

chartermattribute, default_max_ngram_size, default_max_ngram_size, default_min_ngram_size, default_min_ngram_size, illegalargumentexception, io, ioexception, ioexception, ngramtokenfilter, ngramtokenfilter, offsetattribute, override, tokenfilter

The Lucene NGramTokenFilter.java source code

package org.apache.lucene.analysis.ngram;

/**
 * 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.OffsetAttribute;
import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;

/**
 * Tokenizes the input into n-grams of the given size(s).
 */
public final class NGramTokenFilter extends TokenFilter {
  public static final int DEFAULT_MIN_NGRAM_SIZE = 1;
  public static final int DEFAULT_MAX_NGRAM_SIZE = 2;

  private int minGram, maxGram;
  
  private char[] curTermBuffer;
  private int curTermLength;
  private int curGramSize;
  private int curPos;
  private int tokStart;
  
  private final CharTermAttribute termAtt = addAttribute(CharTermAttribute.class);
  private final OffsetAttribute offsetAtt = addAttribute(OffsetAttribute.class);

  /**
   * Creates NGramTokenFilter with given min and max n-grams.
   * @param input {@link TokenStream} holding the input to be tokenized
   * @param minGram the smallest n-gram to generate
   * @param maxGram the largest n-gram to generate
   */
  public NGramTokenFilter(TokenStream input, int minGram, int maxGram) {
    super(input);
    if (minGram < 1) {
      throw new IllegalArgumentException("minGram must be greater than zero");
    }
    if (minGram > maxGram) {
      throw new IllegalArgumentException("minGram must not be greater than maxGram");
    }
    this.minGram = minGram;
    this.maxGram = maxGram;
  }

  /**
   * Creates NGramTokenFilter with default min and max n-grams.
   * @param input {@link TokenStream} holding the input to be tokenized
   */
  public NGramTokenFilter(TokenStream input) {
    this(input, DEFAULT_MIN_NGRAM_SIZE, DEFAULT_MAX_NGRAM_SIZE);
  }

  /** Returns the next token in the stream, or null at EOS. */
  @Override
  public final boolean incrementToken() throws IOException {
    while (true) {
      if (curTermBuffer == null) {
        if (!input.incrementToken()) {
          return false;
        } else {
          curTermBuffer = termAtt.buffer().clone();
          curTermLength = termAtt.length();
          curGramSize = minGram;
          curPos = 0;
          tokStart = offsetAtt.startOffset();
        }
      }
      while (curGramSize <= maxGram) {
        while (curPos+curGramSize <= curTermLength) {     // while there is input
          clearAttributes();
          termAtt.copyBuffer(curTermBuffer, curPos, curGramSize);
          offsetAtt.setOffset(tokStart + curPos, tokStart + curPos + curGramSize);
          curPos++;
          return true;
        }
        curGramSize++;                         // increase n-gram size
        curPos = 0;
      }
      curTermBuffer = null;
    }
  }

  @Override
  public void reset() throws IOException {
    super.reset();
    curTermBuffer = null;
  }
}

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