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package org.apache.lucene.search.spell;
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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership.
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* The ASF licenses this file to You under the Apache License, Version 2.0
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* (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at
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* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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* N-Gram version of edit distance based on paper by Grzegorz Kondrak,
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* "N-gram similarity and distance". Proceedings of the Twelfth International
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* Conference on String Processing and Information Retrieval (SPIRE 2005), pp. 115-126,
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* Buenos Aires, Argentina, November 2005.
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* http://www.cs.ualberta.ca/~kondrak/papers/spire05.pdf
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* This implementation uses the position-based optimization to compute partial
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* matches of n-gram sub-strings and adds a null-character prefix of size n-1
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* so that the first character is contained in the same number of n-grams as
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* a middle character. Null-character prefix matches are discounted so that
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* strings with no matching characters will return a distance of 0.
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public class NGramDistance implements StringDistance {
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* Creates an N-Gram distance measure using n-grams of the specified size.
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* @param size The size of the n-gram to be used to compute the string distance.
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public NGramDistance(int size) {
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* Creates an N-Gram distance measure using n-grams of size 2.
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public NGramDistance() {
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public float getDistance(String source, String target) {
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final int sl = source.length();
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final int tl = target.length();
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if (sl == 0 || tl == 0) {
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if (sl < n || tl < n) {
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for (int i=0,ni=Math.min(sl,tl);i<ni;i++) {
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if (source.charAt(i) == target.charAt(i)) {
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return (float) cost/Math.max(sl, tl);
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char[] sa = new char[sl+n-1];
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float p[]; //'previous' cost array, horizontally
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float d[]; // cost array, horizontally
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float _d[]; //placeholder to assist in swapping p and d
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//construct sa with prefix
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for (int i=0;i<sa.length;i++) {
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sa[i] = source.charAt(i-n+1);
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// indexes into strings s and t
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int i; // iterates through source
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int j; // iterates through target
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char[] t_j = new char[n]; // jth n-gram of t
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for (i = 0; i<=sl; i++) {
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for (j = 1; j<=tl; j++) {
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//construct t_j n-gram
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for (int ti=0;ti<n-j;ti++) {
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t_j[ti]=0; //add prefix
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for (int ti=n-j;ti<n;ti++) {
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t_j[ti]=target.charAt(ti-(n-j));
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t_j = target.substring(j-n, j).toCharArray();
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for (i=1; i<=sl; i++) {
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for (int ni=0;ni<n;ni++) {
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if (sa[i-1+ni] != t_j[ni]) {
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else if (sa[i-1+ni] == 0) { //discount matches on prefix
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float ec = (float) cost/tn;
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// minimum of cell to the left+1, to the top+1, diagonally left and up +cost
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d[i] = Math.min(Math.min(d[i-1]+1, p[i]+1), p[i-1]+ec);
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// copy current distance counts to 'previous row' distance counts
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// our last action in the above loop was to switch d and p, so p now
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// actually has the most recent cost counts
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return 1.0f - (p[sl] / Math.max(tl, sl));