ngramtokenfilter.java

lucene2.2.0版本

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 org.apache.lucene.analysis.Token;
import org.apache.lucene.analysis.TokenFilter;
import org.apache.lucene.analysis.TokenStream;

import java.io.IOException;
import java.util.LinkedList;

/**
 * Tokenizes the input into n-grams of the given size(s).
 * @author Otis Gospodnetic
 */
public 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 LinkedList ngrams;

  /**
   * Creates NGramTokenFilter with given min and max n-grams.
   * @param input 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;
    this.ngrams = new LinkedList();
  }

  /**
   * Creates NGramTokenFilter with default min and max n-grams.
   * @param input 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. */
  public final Token next() throws IOException {
    if (ngrams.size() > 0) {
      return (Token) ngrams.removeFirst();
    }

    Token token = input.next();
    if (token == null) {
      return null;
    }

    ngram(token);
    if (ngrams.size() > 0)
      return (Token) ngrams.removeFirst();
    else
      return null;
  }

  private void ngram(Token token) { 
    String inStr = token.termText();
    int inLen = inStr.length();
    int gramSize = minGram;
    while (gramSize <= maxGram) {
      int pos = 0;                        // reset to beginning of string
      while (pos+gramSize <= inLen) {     // while there is input
        String gram = inStr.substring(pos, pos+gramSize);
        Token tok = new Token(gram, pos, pos+gramSize);
//        tok.setPositionIncrement(pos);
        ngrams.add(tok);
        pos++;
      }
      gramSize++;                         // increase n-gram size
    }
  }
}