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.Token;
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.TermAttribute;
/**
* 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 TermAttribute termAtt;
private OffsetAttribute offsetAtt;
/**
* 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;
this.termAtt = addAttribute(TermAttribute.class);
this.offsetAtt = addAttribute(OffsetAttribute.class);
}
/**
* 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 = (char[]) termAtt.termBuffer().clone();
curTermLength = termAtt.termLength();
curGramSize = minGram;
curPos = 0;
tokStart = offsetAtt.startOffset();
}
}
while (curGramSize <= maxGram) {
while (curPos+curGramSize <= curTermLength) { // while there is input
clearAttributes();
termAtt.setTermBuffer(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;
}
}