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package org.apache.lucene.search;
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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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import java.io.IOException;
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import org.apache.lucene.index.IndexReader;
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import org.apache.lucene.index.Term;
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/** Subclass of FilteredTermEnum for enumerating all terms that are similar
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* to the specified filter term.
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* <p>Term enumerations are always ordered by Term.compareTo(). Each term in
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* the enumeration is greater than all that precede it.
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public final class FuzzyTermEnum extends FilteredTermEnum {
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/* Allows us save time required to create a new array
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* every time similarity is called.
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private float similarity;
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private boolean endEnum = false;
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private Term searchTerm = null;
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private final String field;
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private final char[] text;
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private final String prefix;
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private final float minimumSimilarity;
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private final float scale_factor;
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* Creates a FuzzyTermEnum with an empty prefix and a minSimilarity of 0.5f.
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* After calling the constructor the enumeration is already pointing to the first
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* valid term if such a term exists.
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* @see #FuzzyTermEnum(IndexReader, Term, float, int)
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public FuzzyTermEnum(IndexReader reader, Term term) throws IOException {
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this(reader, term, FuzzyQuery.defaultMinSimilarity, FuzzyQuery.defaultPrefixLength);
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* Creates a FuzzyTermEnum with an empty prefix.
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* After calling the constructor the enumeration is already pointing to the first
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* valid term if such a term exists.
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* @param minSimilarity
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* @see #FuzzyTermEnum(IndexReader, Term, float, int)
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public FuzzyTermEnum(IndexReader reader, Term term, float minSimilarity) throws IOException {
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this(reader, term, minSimilarity, FuzzyQuery.defaultPrefixLength);
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* Constructor for enumeration of all terms from specified <code>reader</code> which share a prefix of
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* length <code>prefixLength</code> with <code>term</code> and which have a fuzzy similarity >
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* <code>minSimilarity</code>.
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* After calling the constructor the enumeration is already pointing to the first
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* valid term if such a term exists.
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* @param reader Delivers terms.
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* @param term Pattern term.
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* @param minSimilarity Minimum required similarity for terms from the reader. Default value is 0.5f.
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* @param prefixLength Length of required common prefix. Default value is 0.
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public FuzzyTermEnum(IndexReader reader, Term term, final float minSimilarity, final int prefixLength) throws IOException {
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if (minSimilarity >= 1.0f)
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throw new IllegalArgumentException("minimumSimilarity cannot be greater than or equal to 1");
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else if (minSimilarity < 0.0f)
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throw new IllegalArgumentException("minimumSimilarity cannot be less than 0");
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throw new IllegalArgumentException("prefixLength cannot be less than 0");
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this.minimumSimilarity = minSimilarity;
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this.scale_factor = 1.0f / (1.0f - minimumSimilarity);
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this.searchTerm = term;
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this.field = searchTerm.field();
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//The prefix could be longer than the word.
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//It's kind of silly though. It means we must match the entire word.
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final int fullSearchTermLength = searchTerm.text().length();
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final int realPrefixLength = prefixLength > fullSearchTermLength ? fullSearchTermLength : prefixLength;
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this.text = searchTerm.text().substring(realPrefixLength).toCharArray();
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this.prefix = searchTerm.text().substring(0, realPrefixLength);
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this.p = new int[this.text.length+1];
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this.d = new int[this.text.length+1];
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setEnum(reader.terms(new Term(searchTerm.field(), prefix)));
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* The termCompare method in FuzzyTermEnum uses Levenshtein distance to
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* calculate the distance between the given term and the comparing term.
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protected final boolean termCompare(Term term) {
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if (field == term.field() && term.text().startsWith(prefix)) {
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final String target = term.text().substring(prefix.length());
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this.similarity = similarity(target);
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return (similarity > minimumSimilarity);
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public final float difference() {
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return (similarity - minimumSimilarity) * scale_factor;
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public final boolean endEnum() {
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/******************************
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* Compute Levenshtein distance
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******************************/
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* <p>Similarity returns a number that is 1.0f or less (including negative numbers)
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* based on how similar the Term is compared to a target term. It returns
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* editDistance > maximumEditDistance</pre>
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* Otherwise it returns:
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* 1 - (editDistance / length)</pre>
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* where length is the length of the shortest term (text or target) including a
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* prefix that are identical and editDistance is the Levenshtein distance for
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* <p>Embedded within this algorithm is a fail-fast Levenshtein distance
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* algorithm. The fail-fast algorithm differs from the standard Levenshtein
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* distance algorithm in that it is aborted if it is discovered that the
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* minimum distance between the words is greater than some threshold.
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* <p>To calculate the maximum distance threshold we use the following formula:
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* (1 - minimumSimilarity) * length</pre>
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* where length is the shortest term including any prefix that is not part of the
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* similarity comparison. This formula was derived by solving for what maximum value
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* of distance returns false for the following statements:
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* similarity = 1 - ((float)distance / (float) (prefixLength + Math.min(textlen, targetlen)));
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* return (similarity > minimumSimilarity);</pre>
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* where distance is the Levenshtein distance for the two words.
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* <p>Levenshtein distance (also known as edit distance) is a measure of similarity
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* between two strings where the distance is measured as the number of character
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* deletions, insertions or substitutions required to transform one string to
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* @param target the target word or phrase
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* @return the similarity, 0.0 or less indicates that it matches less than the required
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* threshold and 1.0 indicates that the text and target are identical
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private float similarity(final String target) {
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final int m = target.length();
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final int n = text.length;
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//we don't have anything to compare. That means if we just add
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//the letters for m we get the new word
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return prefix.length() == 0 ? 0.0f : 1.0f - ((float) m / prefix.length());
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return prefix.length() == 0 ? 0.0f : 1.0f - ((float) n / prefix.length());
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final int maxDistance = calculateMaxDistance(m);
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if (maxDistance < Math.abs(m-n)) {
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//just adding the characters of m to n or vice-versa results in
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//for example "pre" length is 3 and "prefixes" length is 8. We can see that
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//given this optimal circumstance, the edit distance cannot be less than 5.
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//which is 8-3 or more precisely Math.abs(3-8).
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//if our maximum edit distance is 4, then we can discard this word
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//without looking at it.
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for (int i = 0; i<=n; ++i) {
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// start computing edit distance
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for (int j = 1; j<=m; ++j) { // iterates through target
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int bestPossibleEditDistance = m;
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final char t_j = target.charAt(j-1); // jth character of t
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for (int i=1; i<=n; ++i) { // iterates through text
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// minimum of cell to the left+1, to the top+1, diagonally left and up +(0|1)
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if (t_j != text[i-1]) {
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d[i] = Math.min(Math.min(d[i-1], p[i]), p[i-1]) + 1;
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d[i] = Math.min(Math.min(d[i-1]+1, p[i]+1), p[i-1]);
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bestPossibleEditDistance = Math.min(bestPossibleEditDistance, d[i]);
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//After calculating row i, the best possible edit distance
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//can be found by found by finding the smallest value in a given column.
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//If the bestPossibleEditDistance is greater than the max distance, abort.
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if (j > maxDistance && bestPossibleEditDistance > maxDistance) { //equal is okay, but not greater
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//the closest the target can be to the text is just too far away.
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//this target is leaving the party early.
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// copy current distance counts to 'previous row' distance counts: swap p and d
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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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// this will return less than 0.0 when the edit distance is
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// greater than the number of characters in the shorter word.
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// but this was the formula that was previously used in FuzzyTermEnum,
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// so it has not been changed (even though minimumSimilarity must be
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return 1.0f - ((float)p[n] / (float) (prefix.length() + Math.min(n, m)));
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* The max Distance is the maximum Levenshtein distance for the text
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* compared to some other value that results in score that is
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* better than the minimum similarity.
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* @param m the length of the "other value"
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* @return the maximum levenshtein distance that we care about
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private int calculateMaxDistance(int m) {
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return (int) ((1-minimumSimilarity) * (Math.min(text.length, m) + prefix.length()));
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public void close() throws IOException {
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super.close(); //call super.close() and let the garbage collector do its work.