package org.apache.lucene.search;
/**
* Copyright 2004 The Apache Software Foundation
*
* Licensed 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.index.IndexReader;
import org.apache.lucene.index.Term;
import java.io.IOException;
/** Subclass of FilteredTermEnum for enumerating all terms that are similiar
* to the specified filter term.
*
* <p>Term enumerations are always ordered by Term.compareTo(). Each term in
* the enumeration is greater than all that precede it.
*/
public final class FuzzyTermEnum extends FilteredTermEnum {
/* This should be somewhere around the average long word.
* If it is longer, we waste time and space. If it is shorter, we waste a
* little bit of time growing the array as we encounter longer words.
*/
private static final int TYPICAL_LONGEST_WORD_IN_INDEX = 19;
/* Allows us save time required to create a new array
* everytime similarity is called.
*/
private int[][] d;
private float similarity;
private boolean endEnum = false;
private Term searchTerm = null;
private final String field;
private final String text;
private final String prefix;
private final float minimumSimilarity;
private final float scale_factor;
private final int[] maxDistances = new int[TYPICAL_LONGEST_WORD_IN_INDEX];
/**
* Creates a FuzzyTermEnum with an empty prefix and a minSimilarity of 0.5f.
* <p>
* After calling the constructor the enumeration is already pointing to the first
* valid term if such a term exists.
*
* @param reader
* @param term
* @throws IOException
* @see #FuzzyTermEnum(IndexReader, Term, float, int)
*/
public FuzzyTermEnum(IndexReader reader, Term term) throws IOException {
this(reader, term, FuzzyQuery.defaultMinSimilarity, FuzzyQuery.defaultPrefixLength);
}
/**
* Creates a FuzzyTermEnum with an empty prefix.
* <p>
* After calling the constructor the enumeration is already pointing to the first
* valid term if such a term exists.
*
* @param reader
* @param term
* @param minSimilarity
* @throws IOException
* @see #FuzzyTermEnum(IndexReader, Term, float, int)
*/
public FuzzyTermEnum(IndexReader reader, Term term, float minSimilarity) throws IOException {
this(reader, term, minSimilarity, FuzzyQuery.defaultPrefixLength);
}
/**
* Constructor for enumeration of all terms from specified <code>reader</code> which share a prefix of
* length <code>prefixLength</code> with <code>term</code> and which have a fuzzy similarity >
* <code>minSimilarity</code>.
* <p>
* After calling the constructor the enumeration is already pointing to the first
* valid term if such a term exists.
*
* @param reader Delivers terms.
* @param term Pattern term.
* @param minSimilarity Minimum required similarity for terms from the reader. Default value is 0.5f.
* @param prefixLength Length of required common prefix. Default value is 0.
* @throws IOException
*/
public FuzzyTermEnum(IndexReader reader, Term term, final float minSimilarity, final int prefixLength) throws IOException {
super();
if (minSimilarity >= 1.0f)
throw new IllegalArgumentException("minimumSimilarity cannot be greater than or equal to 1");
else if (minSimilarity < 0.0f)
throw new IllegalArgumentException("minimumSimilarity cannot be less than 0");
if(prefixLength < 0)
throw new IllegalArgumentException("prefixLength cannot be less than 0");
this.minimumSimilarity = minSimilarity;
this.scale_factor = 1.0f / (1.0f - minimumSimilarity);
this.searchTerm = term;
this.field = searchTerm.field();
//The prefix could be longer than the word.
//It's kind of silly though. It means we must match the entire word.
final int fullSearchTermLength = searchTerm.text().length();
final int realPrefixLength = prefixLength > fullSearchTermLength ? fullSearchTermLength : prefixLength;
this.text = searchTerm.text().substring(realPrefixLength);
this.prefix = searchTerm.text().substring(0, realPrefixLength);
initializeMaxDistances();
this.d = initDistanceArray();
setEnum(reader.terms(new Term(searchTerm.field(), prefix)));
}
/**
* The termCompare method in FuzzyTermEnum uses Levenshtein distance to
* calculate the distance between the given term and the comparing term.
*/
protected final boolean termCompare(Term term) {
if (field == term.field() && term.text().startsWith(prefix)) {
final String target = term.text().substring(prefix.length());
this.similarity = similarity(target);
return (similarity > minimumSimilarity);
}
endEnum = true;
return false;
}
public final float difference() {
return (float)((similarity - minimumSimilarity) * scale_factor);
}
public final boolean endEnum() {
return endEnum;
}
/******************************
* Compute Levenshtein distance
******************************/
/**
* Finds and returns the smallest of three integers
*/
private static final int min(int a, int b, int c) {
final int t = (a < b) ? a : b;
return (t < c) ? t : c;
}
private final int[][] initDistanceArray(){
return new int[this.text.length() + 1][TYPICAL_LONGEST_WORD_IN_INDEX];
}
/**
* <p>Similarity returns a number that is 1.0f or less (including negative numbers)
* based on how similar the Term is compared to a target term. It returns
* exactly 0.0f when
* <pre>
* editDistance < maximumEditDistance</pre>
* Otherwise it returns:
* <pre>
* 1 - (editDistance / length)</pre>
* where length is the length of the shortest term (text or target) including a
* prefix that are identical and editDistance is the Levenshtein distance for
* the two words.</p>
*
* <p>Embedded within this algorithm is a fail-fast Levenshtein distance
* algorithm. The fail-fast algorithm differs from the standard Levenshtein
* distance algorithm in that it is aborted if it is discovered that the
* mimimum distance between the words is greater than some threshold.
*
* <p>To calculate the maximum distance threshold we use the following formula:
* <pre>
* (1 - minimumSimilarity) * length</pre>
* where length is the shortest term including any prefix that is not part of the
* similarity comparision. This formula was derived by solving for what maximum value
* of distance returns false for the following statements:
* <pre>
* similarity = 1 - ((float)distance / (float) (prefixLength + Math.min(textlen, targetlen)));
* return (similarity > minimumSimilarity);</pre>
* where distance is the Levenshtein distance for the two words.
* </p>
* <p>Levenshtein distance (also known as edit distance) is a measure of similiarity
* between two strings where the distance is measured as the number of character
* deletions, insertions or substitutions required to transform one string to
* the other string.
* @param target the target word or phrase
* @return the similarity, 0.0 or less indicates that it matches less than the required
* threshold and 1.0 indicates that the text and target are identical
*/
private synchronized final float similarity(final String target) {
final int m = target.length();
final int n = text.length();
if (n == 0) {
//we don't have anything to compare. That means if we just add
//the letters for m we get the new word
return prefix.length() == 0 ? 0.0f : 1.0f - ((float) m / prefix.length());
}
if (m == 0) {
return prefix.length() == 0 ? 0.0f : 1.0f - ((float) n / prefix.length());
}
final int maxDistance = getMaxDistance(m);
if (maxDistance < Math.abs(m-n)) {
//just adding the characters of m to n or vice-versa results in
//too many edits
//for example "pre" length is 3 and "prefixes" length is 8. We can see that
//given this optimal circumstance, the edit distance cannot be less than 5.
//which is 8-3 or more precisesly Math.abs(3-8).
//if our maximum edit distance is 4, then we can discard this word
//without looking at it.
return 0.0f;
}
//let's make sure we have enough room in our array to do the distance calculations.
if (d[0].length <= m) {
growDistanceArray(m);
}
// init matrix d
for (int i = 0; i <= n; i++) d[i][0] = i;
for (int j = 0; j <= m; j++) d[0][j] = j;
// start computing edit distance
for (int i = 1; i <= n; i++) {
int bestPossibleEditDistance = m;
final char s_i = text.charAt(i - 1);
for (int j = 1; j <= m; j++) {
if (s_i != target.charAt(j-1)) {
d[i][j] = min(d[i-1][j], d[i][j-1], d[i-1][j-1])+1;
}
else {
d[i][j] = min(d[i-1][j]+1, d[i][j-1]+1, d[i-1][j-1]);
}
bestPossibleEditDistance = Math.min(bestPossibleEditDistance, d[i][j]);
}
//After calculating row i, the best possible edit distance
//can be found by found by finding the smallest value in a given column.
//If the bestPossibleEditDistance is greater than the max distance, abort.
if (i > maxDistance && bestPossibleEditDistance > maxDistance) { //equal is okay, but not greater
//the closest the target can be to the text is just too far away.
//this target is leaving the party early.
return 0.0f;
}
}
// this will return less than 0.0 when the edit distance is
// greater than the number of characters in the shorter word.
// but this was the formula that was previously used in FuzzyTermEnum,
// so it has not been changed (even though minimumSimilarity must be
// greater than 0.0)
return 1.0f - ((float)d[n][m] / (float) (prefix.length() + Math.min(n, m)));
}
/**
* Grow the second dimension of the array, so that we can calculate the
* Levenshtein difference.
*/
private void growDistanceArray(int m) {
for (int i = 0; i < d.length; i++) {
d[i] = new int[m+1];
}
}
/**
* The max Distance is the maximum Levenshtein distance for the text
* compared to some other value that results in score that is
* better than the minimum similarity.
* @param m the length of the "other value"
* @return the maximum levenshtein distance that we care about
*/
private final int getMaxDistance(int m) {
return (m < maxDistances.length) ? maxDistances[m] : calculateMaxDistance(m);
}
private void initializeMaxDistances() {
for (int i = 0; i < maxDistances.length; i++) {
maxDistances[i] = calculateMaxDistance(i);
}
}
private int calculateMaxDistance(int m) {
return (int) ((1-minimumSimilarity) * (Math.min(text.length(), m) + prefix.length()));
}
public void close() throws IOException {
super.close(); //call super.close() and let the garbage collector do its work.
}
}
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