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.TokenFilter;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.tokenattributes.OffsetAttribute;
import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;
import java.io.IOException;
/**
* Tokenizes the given token into n-grams of given size(s).
* <p>
* This {@link TokenFilter} create n-grams from the beginning edge or ending edge of a input token.
* </p>
*/
public final class EdgeNGramTokenFilter extends TokenFilter {
public static final Side DEFAULT_SIDE = Side.FRONT;
public static final int DEFAULT_MAX_GRAM_SIZE = 1;
public static final int DEFAULT_MIN_GRAM_SIZE = 1;
/** Specifies which side of the input the n-gram should be generated from */
public static enum Side {
/** Get the n-gram from the front of the input */
FRONT {
@Override
public String getLabel() { return "front"; }
},
/** Get the n-gram from the end of the input */
BACK {
@Override
public String getLabel() { return "back"; }
};
public abstract String getLabel();
// Get the appropriate Side from a string
public static Side getSide(String sideName) {
if (FRONT.getLabel().equals(sideName)) {
return FRONT;
}
if (BACK.getLabel().equals(sideName)) {
return BACK;
}
return null;
}
}
private final int minGram;
private final int maxGram;
private Side side;
private char[] curTermBuffer;
private int curTermLength;
private int curGramSize;
private int tokStart;
private final CharTermAttribute termAtt = addAttribute(CharTermAttribute.class);
private final OffsetAttribute offsetAtt = addAttribute(OffsetAttribute.class);
/**
* Creates EdgeNGramTokenFilter that can generate n-grams in the sizes of the given range
*
* @param input {@link TokenStream} holding the input to be tokenized
* @param side the {@link Side} from which to chop off an n-gram
* @param minGram the smallest n-gram to generate
* @param maxGram the largest n-gram to generate
*/
public EdgeNGramTokenFilter(TokenStream input, Side side, int minGram, int maxGram) {
super(input);
if (side == null) {
throw new IllegalArgumentException("sideLabel must be either front or back");
}
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.side = side;
}
/**
* Creates EdgeNGramTokenFilter that can generate n-grams in the sizes of the given range
*
* @param input {@link TokenStream} holding the input to be tokenized
* @param sideLabel the name of the {@link Side} from which to chop off an n-gram
* @param minGram the smallest n-gram to generate
* @param maxGram the largest n-gram to generate
*/
public EdgeNGramTokenFilter(TokenStream input, String sideLabel, int minGram, int maxGram) {
this(input, Side.getSide(sideLabel), minGram, maxGram);
}
@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;
tokStart = offsetAtt.startOffset();
}
}
if (curGramSize <= maxGram) {
if (! (curGramSize > curTermLength // if the remaining input is too short, we can't generate any n-grams
|| curGramSize > maxGram)) { // if we have hit the end of our n-gram size range, quit
// grab gramSize chars from front or back
int start = side == Side.FRONT ? 0 : curTermLength - curGramSize;
int end = start + curGramSize;
clearAttributes();
offsetAtt.setOffset(tokStart + start, tokStart + end);
termAtt.copyBuffer(curTermBuffer, start, curGramSize);
curGramSize++;
return true;
}
}
curTermBuffer = null;
}
}
@Override
public void reset() throws IOException {
super.reset();
curTermBuffer = null;
}
}