org.apache.lucene.analysis.ngram.NGramTokenizer Maven / Gradle / Ivy
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.Tokenizer;
import java.io.IOException;
import java.io.Reader;
/**
* Tokenizes the input into n-grams of the given size(s).
*/
public class NGramTokenizer extends Tokenizer {
public static final int DEFAULT_MIN_NGRAM_SIZE = 1;
public static final int DEFAULT_MAX_NGRAM_SIZE = 2;
private int minGram, maxGram;
private int gramSize;
private int pos = 0;
private int inLen;
private String inStr;
private boolean started = false;
/**
* Creates NGramTokenizer with given min and max n-grams.
* @param input Reader holding the input to be tokenized
* @param minGram the smallest n-gram to generate
* @param maxGram the largest n-gram to generate
*/
public NGramTokenizer(Reader 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 NGramTokenizer with default min and max n-grams.
* @param input Reader holding the input to be tokenized
*/
public NGramTokenizer(Reader 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(final Token reusableToken) throws IOException {
assert reusableToken != null;
if (!started) {
started = true;
gramSize = minGram;
char[] chars = new char[1024];
input.read(chars);
inStr = new String(chars).trim(); // remove any trailing empty strings
inLen = inStr.length();
}
if (pos+gramSize > inLen) { // if we hit the end of the string
pos = 0; // reset to beginning of string
gramSize++; // increase n-gram size
if (gramSize > maxGram) // we are done
return null;
if (pos+gramSize > inLen)
return null;
}
int oldPos = pos;
pos++;
return reusableToken.reinit(inStr, oldPos, gramSize, oldPos, oldPos+gramSize);
}
}