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Apache Lucene (module: classification)
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/*
* 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.WildcardQuery;
import org.apache.lucene.util.BytesRef;
/**
* A simplistic Lucene based NaiveBayes classifier, see
* http://en.wikipedia.org/wiki/Naive_Bayes_classifier
*
* @lucene.experimental
*/
public class SimpleNaiveBayesClassifier implements Classifier {
/**
* {@link org.apache.lucene.index.IndexReader} used to access the {@link
* org.apache.lucene.classification.Classifier}'s index
*/
protected final IndexReader indexReader;
/** names of the fields to be used as input text */
protected final String[] textFieldNames;
/** name of the field to be used as a class / category output */
protected final String classFieldName;
/** {@link org.apache.lucene.analysis.Analyzer} to be used for tokenizing unseen input text */
protected final Analyzer analyzer;
/**
* {@link org.apache.lucene.search.IndexSearcher} to run searches on the index for retrieving
* frequencies
*/
protected final IndexSearcher indexSearcher;
/**
* {@link org.apache.lucene.search.Query} used to eventually filter the document set to be used to
* classify
*/
protected 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 havely 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 SimpleNaiveBayesClassifier(
IndexReader indexReader,
Analyzer analyzer,
Query query,
String classFieldName,
String... textFieldNames) {
this.indexReader = indexReader;
this.indexSearcher = new IndexSearcher(this.indexReader);
this.textFieldNames = textFieldNames;
this.classFieldName = classFieldName;
this.analyzer = analyzer;
this.query = query;
}
@Override
public ClassificationResult assignClass(String inputDocument) throws IOException {
List> assignedClasses = assignClassNormalizedList(inputDocument);
ClassificationResult assignedClass = null;
double maxscore = -Double.MAX_VALUE;
for (ClassificationResult c : assignedClasses) {
if (c.score() > maxscore) {
assignedClass = c;
maxscore = c.score();
}
}
return assignedClass;
}
@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
*/
protected List> assignClassNormalizedList(String inputDocument)
throws IOException {
List> assignedClasses = new ArrayList<>();
Terms classes = MultiTerms.getTerms(indexReader, classFieldName);
if (classes != null) {
TermsEnum classesEnum = classes.iterator();
BytesRef next;
String[] tokenizedText = tokenize(inputDocument);
int docsWithClassSize = countDocsWithClass();
while ((next = classesEnum.next()) != null) {
if (next.length > 0) {
Term term = new Term(this.classFieldName, next);
double clVal =
calculateLogPrior(term, docsWithClassSize)
+ calculateLogLikelihood(tokenizedText, term, docsWithClassSize);
assignedClasses.add(new ClassificationResult<>(term.bytes(), clVal));
}
}
}
// normalization; the values transforms to a 0-1 range
return normClassificationResults(assignedClasses);
}
/**
* count the number of documents in the index having at least a value for the 'class' field
*
* @return the no. of documents having a value for the 'class' field
* @throws IOException if accessing to term vectors or search fails
*/
protected int countDocsWithClass() throws IOException {
Terms terms = MultiTerms.getTerms(this.indexReader, this.classFieldName);
int docCount;
if (terms == null || terms.getDocCount() == -1) { // in case codec doesn't support getDocCount
BooleanQuery.Builder q = new BooleanQuery.Builder();
q.add(
new BooleanClause(
new WildcardQuery(
new Term(classFieldName, String.valueOf(WildcardQuery.WILDCARD_STRING))),
BooleanClause.Occur.MUST));
if (query != null) {
q.add(query, BooleanClause.Occur.MUST);
}
docCount = indexSearcher.count(q.build());
} else {
docCount = terms.getDocCount();
}
return docCount;
}
/**
* 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
*/
protected 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[0]);
}
private double calculateLogLikelihood(String[] tokenizedText, Term term, int docsWithClass)
throws IOException {
// for each word
double result = 0d;
for (String word : tokenizedText) {
// search with text:word AND class:c
int hits = getWordFreqForClass(word, term);
// num : count the no of times the word appears in documents of class c (+1)
double num = hits + 1; // +1 is added because of add 1 smoothing
// den : for the whole dictionary, count the no of times a word appears in documents of class
// c (+|V|)
double den = getTextTermFreqForClass(term) + docsWithClass;
// P(w|c) = num/den
double wordProbability = num / den;
result += Math.log(wordProbability);
}
// log(P(d|c)) = log(P(w1|c))+...+log(P(wn|c))
return result;
}
/**
* Returns the average number of unique terms times the number of docs belonging to the input
* class
*
* @param term the term representing the class
* @return the average number of unique terms
* @throws IOException if a low level I/O problem happens
*/
private double getTextTermFreqForClass(Term term) throws IOException {
double avgNumberOfUniqueTerms = 0;
for (String textFieldName : textFieldNames) {
Terms terms = MultiTerms.getTerms(indexReader, textFieldName);
long numPostings = terms.getSumDocFreq(); // number of term/doc pairs
avgNumberOfUniqueTerms +=
numPostings / (double) terms.getDocCount(); // avg # of unique terms per doc
}
int docsWithC = indexReader.docFreq(term);
return avgNumberOfUniqueTerms
* docsWithC; // avg # of unique terms in text fields per doc * # docs with c
}
/**
* Returns the number of documents of the input class ( from the whole index or from a subset)
* that contains the word ( in a specific field or in all the fields if no one selected)
*
* @param word the token produced by the analyzer
* @param term the term representing the class
* @return the number of documents of the input class
* @throws IOException if a low level I/O problem happens
*/
private int getWordFreqForClass(String word, Term term) throws IOException {
BooleanQuery.Builder booleanQuery = new BooleanQuery.Builder();
BooleanQuery.Builder subQuery = new BooleanQuery.Builder();
for (String textFieldName : textFieldNames) {
subQuery.add(
new BooleanClause(
new TermQuery(new Term(textFieldName, word)), BooleanClause.Occur.SHOULD));
}
booleanQuery.add(new BooleanClause(subQuery.build(), BooleanClause.Occur.MUST));
booleanQuery.add(new BooleanClause(new TermQuery(term), BooleanClause.Occur.MUST));
if (query != null) {
booleanQuery.add(query, BooleanClause.Occur.MUST);
}
return indexSearcher.count(booleanQuery.build());
}
private double calculateLogPrior(Term term, int docsWithClassSize) throws IOException {
return Math.log(docCount(term)) - Math.log(docsWithClassSize);
}
private int docCount(Term term) throws IOException {
return indexReader.docFreq(term);
}
/**
* Normalize the classification results based on the max score available
*
* @param assignedClasses the list of assigned classes
* @return the normalized results
*/
protected 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).score();
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.score() - 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.score() - loga;
returnList.add(new ClassificationResult<>(cr.assignedClass(), Math.exp(scoreDiff)));
}
}
return returnList;
}
}