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Apache Lucene (module: suggest)
/*
* 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.
*/
package org.apache.lucene.search.spell;
/**
* N-Gram version of edit distance based on paper by Grzegorz Kondrak,
* "N-gram similarity and distance". Proceedings of the Twelfth International
* Conference on String Processing and Information Retrieval (SPIRE 2005), pp. 115-126,
* Buenos Aires, Argentina, November 2005.
* http://www.cs.ualberta.ca/~kondrak/papers/spire05.pdf
*
* This implementation uses the position-based optimization to compute partial
* matches of n-gram sub-strings and adds a null-character prefix of size n-1
* so that the first character is contained in the same number of n-grams as
* a middle character. Null-character prefix matches are discounted so that
* strings with no matching characters will return a distance of 0.
*
*/
public class NGramDistance implements StringDistance {
private int n;
/**
* Creates an N-Gram distance measure using n-grams of the specified size.
* @param size The size of the n-gram to be used to compute the string distance.
*/
public NGramDistance(int size) {
this.n = size;
}
/**
* Creates an N-Gram distance measure using n-grams of size 2.
*/
public NGramDistance() {
this(2);
}
@Override
public float getDistance(String source, String target) {
final int sl = source.length();
final int tl = target.length();
if (sl == 0 || tl == 0) {
if (sl == tl) {
return 1;
}
else {
return 0;
}
}
int cost = 0;
if (sl < n || tl < n) {
for (int i=0,ni=Math.min(sl,tl);i