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This is the highlighter for apache lucene java
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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.search.highlight;

import java.io.IOException;
import java.util.HashSet;
import java.util.Set;
import java.util.function.Predicate;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.MultiReader;
import org.apache.lucene.index.Term;
import org.apache.lucene.search.BooleanClause;
import org.apache.lucene.search.BoostQuery;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.Query;
import org.apache.lucene.search.QueryVisitor;

/**
 * Utility class used to extract the terms used in a query, plus any weights. This class will not
 * find terms for MultiTermQuery, TermRangeQuery and PrefixQuery classes so the caller must pass a
 * rewritten query (see Query.rewrite) to obtain a list of expanded terms.
 */
public final class QueryTermExtractor {

  /** for term extraction */
  private static final IndexSearcher EMPTY_INDEXSEARCHER;

  static {
    try {
      IndexReader emptyReader = new MultiReader();
      EMPTY_INDEXSEARCHER = new IndexSearcher(emptyReader);
      EMPTY_INDEXSEARCHER.setQueryCache(null);
    } catch (IOException bogus) {
      throw new RuntimeException(bogus);
    }
  }

  /**
   * Extracts all terms texts of a given Query into an array of WeightedTerms
   *
   * @param query Query to extract term texts from
   * @return an array of the terms used in a query, plus their weights.
   */
  public static final WeightedTerm[] getTerms(Query query) {
    return getTerms(query, false);
  }

  /**
   * Extracts all terms texts of a given Query into an array of WeightedTerms
   *
   * @param query Query to extract term texts from
   * @param reader used to compute IDF which can be used to a) score selected fragments better b)
   *     use graded highlights eg changing intensity of font color
   * @param fieldName the field on which Inverse Document Frequency (IDF) calculations are based
   * @return an array of the terms used in a query, plus their weights.
   */
  public static final WeightedTerm[] getIdfWeightedTerms(
      Query query, IndexReader reader, String fieldName) {
    WeightedTerm[] terms = getTerms(query, false, fieldName);
    int totalNumDocs = reader.maxDoc();
    for (int i = 0; i < terms.length; i++) {
      try {
        int docFreq = reader.docFreq(new Term(fieldName, terms[i].term));
        // IDF algorithm taken from ClassicSimilarity class
        float idf = (float) (Math.log(totalNumDocs / (double) (docFreq + 1)) + 1.0);
        terms[i].weight *= idf;
      } catch (
          @SuppressWarnings("unused")
          IOException e) {
        // ignore
      }
    }
    return terms;
  }

  /**
   * Extracts all terms texts of a given Query into an array of WeightedTerms
   *
   * @param query Query to extract term texts from
   * @param prohibited true to extract "prohibited" terms, too
   * @param fieldName The fieldName used to filter query terms
   * @return an array of the terms used in a query, plus their weights.
   */
  public static WeightedTerm[] getTerms(Query query, boolean prohibited, String fieldName) {
    HashSet terms = new HashSet<>();
    Predicate fieldSelector = fieldName == null ? f -> true : fieldName::equals;
    query.visit(new BoostedTermExtractor(1, terms, prohibited, fieldSelector));
    return terms.toArray(new WeightedTerm[0]);
  }

  /**
   * Extracts all terms texts of a given Query into an array of WeightedTerms
   *
   * @param query Query to extract term texts from
   * @param prohibited true to extract "prohibited" terms, too
   * @return an array of the terms used in a query, plus their weights.
   */
  public static final WeightedTerm[] getTerms(Query query, boolean prohibited) {
    return getTerms(query, prohibited, null);
  }

  private static class BoostedTermExtractor extends QueryVisitor {

    final float boost;
    final Set terms;
    final boolean includeProhibited;
    final Predicate fieldSelector;

    private BoostedTermExtractor(
        float boost,
        Set terms,
        boolean includeProhibited,
        Predicate fieldSelector) {
      this.boost = boost;
      this.terms = terms;
      this.includeProhibited = includeProhibited;
      this.fieldSelector = fieldSelector;
    }

    @Override
    public boolean acceptField(String field) {
      return fieldSelector.test(field);
    }

    @Override
    public void consumeTerms(Query query, Term... terms) {
      for (Term term : terms) {
        this.terms.add(new WeightedTerm(boost, term.text()));
      }
    }

    @Override
    public QueryVisitor getSubVisitor(BooleanClause.Occur occur, Query parent) {
      if (parent instanceof BoostQuery) {
        float newboost = boost * ((BoostQuery) parent).getBoost();
        return new BoostedTermExtractor(newboost, terms, includeProhibited, fieldSelector);
      }
      if (occur == BooleanClause.Occur.MUST_NOT && includeProhibited == false) {
        return QueryVisitor.EMPTY_VISITOR;
      }
      return this;
    }
  }
}