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Apache Lucene (module: classification)
/*
* 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.classification;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.MultiTerms;
import org.apache.lucene.index.Term;
import org.apache.lucene.index.Terms;
import org.apache.lucene.index.TermsEnum;
import org.apache.lucene.search.BooleanClause;
import org.apache.lucene.search.BooleanQuery;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.Query;
import org.apache.lucene.search.TermQuery;
import org.apache.lucene.search.TopDocs;
import org.apache.lucene.search.similarities.BM25Similarity;
import org.apache.lucene.util.BytesRef;
/**
* A classifier approximating naive bayes classifier by using pure queries on BM25.
*
* @lucene.experimental
*/
public class BM25NBClassifier implements Classifier {
/**
* {@link IndexReader} used to access the {@link Classifier}'s
* index
*/
private final IndexReader indexReader;
/**
* names of the fields to be used as input text
*/
private final String[] textFieldNames;
/**
* name of the field to be used as a class / category output
*/
private final String classFieldName;
/**
* {@link Analyzer} to be used for tokenizing unseen input text
*/
private final Analyzer analyzer;
/**
* {@link IndexSearcher} to run searches on the index for retrieving frequencies
*/
private final IndexSearcher indexSearcher;
/**
* {@link Query} used to eventually filter the document set to be used to classify
*/
private final Query query;
/**
* Creates a new NaiveBayes classifier.
*
* @param indexReader the reader on the index to be used for classification
* @param analyzer an {@link Analyzer} used to analyze unseen text
* @param query a {@link Query} to eventually filter the docs used for training the classifier, or {@code null}
* if all the indexed docs should be used
* @param classFieldName the name of the field used as the output for the classifier NOTE: must not be heavely analyzed
* as the returned class will be a token indexed for this field
* @param textFieldNames the name of the fields used as the inputs for the classifier, NO boosting supported per field
*/
public BM25NBClassifier(IndexReader indexReader, Analyzer analyzer, Query query, String classFieldName, String... textFieldNames) {
this.indexReader = indexReader;
this.indexSearcher = new IndexSearcher(this.indexReader);
this.indexSearcher.setSimilarity(new BM25Similarity());
this.textFieldNames = textFieldNames;
this.classFieldName = classFieldName;
this.analyzer = analyzer;
this.query = query;
}
@Override
public ClassificationResult assignClass(String inputDocument) throws IOException {
return assignClassNormalizedList(inputDocument).get(0);
}
@Override
public List> getClasses(String text) throws IOException {
List> assignedClasses = assignClassNormalizedList(text);
Collections.sort(assignedClasses);
return assignedClasses;
}
@Override
public List> getClasses(String text, int max) throws IOException {
List> assignedClasses = assignClassNormalizedList(text);
Collections.sort(assignedClasses);
return assignedClasses.subList(0, max);
}
/**
* Calculate probabilities for all classes for a given input text
*
* @param inputDocument the input text as a {@code String}
* @return a {@code List} of {@code ClassificationResult}, one for each existing class
* @throws IOException if assigning probabilities fails
*/
private List> assignClassNormalizedList(String inputDocument) throws IOException {
List> assignedClasses = new ArrayList<>();
Terms classes = MultiTerms.getTerms(indexReader, classFieldName);
TermsEnum classesEnum = classes.iterator();
BytesRef next;
String[] tokenizedText = tokenize(inputDocument);
while ((next = classesEnum.next()) != null) {
if (next.length > 0) {
Term term = new Term(this.classFieldName, next);
assignedClasses.add(new ClassificationResult<>(term.bytes(), calculateLogPrior(term) + calculateLogLikelihood(tokenizedText, term)));
}
}
return normClassificationResults(assignedClasses);
}
/**
* Normalize the classification results based on the max score available
*
* @param assignedClasses the list of assigned classes
* @return the normalized results
*/
private ArrayList> normClassificationResults(List> assignedClasses) {
// normalization; the values transforms to a 0-1 range
ArrayList> returnList = new ArrayList<>();
if (!assignedClasses.isEmpty()) {
Collections.sort(assignedClasses);
// this is a negative number closest to 0 = a
double smax = assignedClasses.get(0).getScore();
double sumLog = 0;
// log(sum(exp(x_n-a)))
for (ClassificationResult cr : assignedClasses) {
// getScore-smax <=0 (both negative, smax is the smallest abs()
sumLog += Math.exp(cr.getScore() - smax);
}
// loga=a+log(sum(exp(x_n-a))) = log(sum(exp(x_n)))
double loga = smax;
loga += Math.log(sumLog);
// 1/sum*x = exp(log(x))*1/sum = exp(log(x)-log(sum))
for (ClassificationResult cr : assignedClasses) {
double scoreDiff = cr.getScore() - loga;
returnList.add(new ClassificationResult<>(cr.getAssignedClass(), Math.exp(scoreDiff)));
}
}
return returnList;
}
/**
* tokenize a String on this classifier's text fields and analyzer
*
* @param text the String representing an input text (to be classified)
* @return a String array of the resulting tokens
* @throws IOException if tokenization fails
*/
private String[] tokenize(String text) throws IOException {
Collection result = new LinkedList<>();
for (String textFieldName : textFieldNames) {
try (TokenStream tokenStream = analyzer.tokenStream(textFieldName, text)) {
CharTermAttribute charTermAttribute = tokenStream.addAttribute(CharTermAttribute.class);
tokenStream.reset();
while (tokenStream.incrementToken()) {
result.add(charTermAttribute.toString());
}
tokenStream.end();
}
}
return result.toArray(new String[result.size()]);
}
private double calculateLogLikelihood(String[] tokens, Term term) throws IOException {
double result = 0d;
for (String word : tokens) {
result += Math.log(getTermProbForClass(term, word));
}
return result;
}
private double getTermProbForClass(Term classTerm, String... words) throws IOException {
BooleanQuery.Builder builder = new BooleanQuery.Builder();
builder.add(new BooleanClause(new TermQuery(classTerm), BooleanClause.Occur.MUST));
for (String textFieldName : textFieldNames) {
for (String word : words) {
builder.add(new BooleanClause(new TermQuery(new Term(textFieldName, word)), BooleanClause.Occur.SHOULD));
}
}
if (query != null) {
builder.add(query, BooleanClause.Occur.MUST);
}
TopDocs search = indexSearcher.search(builder.build(), 1);
return search.totalHits.value > 0 ? search.scoreDocs[0].score : 1;
}
private double calculateLogPrior(Term term) throws IOException {
TermQuery termQuery = new TermQuery(term);
BooleanQuery.Builder bq = new BooleanQuery.Builder();
bq.add(termQuery, BooleanClause.Occur.MUST);
if (query != null) {
bq.add(query, BooleanClause.Occur.MUST);
}
TopDocs topDocs = indexSearcher.search(bq.build(), 1);
return topDocs.totalHits.value > 0 ? Math.log(topDocs.scoreDocs[0].score) : 0;
}
}