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 * 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
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 *
 *     http://www.apache.org/licenses/LICENSE-2.0
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package org.apache.lucene.expressions;


import java.io.IOException;
import java.util.List;

import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.index.ReaderUtil;
import org.apache.lucene.search.DoubleValues;
import org.apache.lucene.search.Explanation;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.Rescorer;
import org.apache.lucene.search.Sort;
import org.apache.lucene.search.SortRescorer;

A Rescorer that uses an expression to re-score first pass hits. Functionally this is the same as SortRescorer (if you build the Sort using Expression.getSortField), except for the explain method which gives more detail by showing the value of each variable. 
@lucene.experimental
/**
 * A {@link Rescorer} that uses an expression to re-score
 * first pass hits.  Functionally this is the same as {@link
 * SortRescorer} (if you build the {@link Sort} using {@link
 * Expression#getSortField}), except for the explain method
 * which gives more detail by showing the value of each
 * variable.
 * 
 * @lucene.experimental
 */

class ExpressionRescorer extends SortRescorer {

  private final Expression expression;
  private final Bindings bindings;

  
Uses the provided Expression to assign second pass scores. /** Uses the provided {@link Expression} to assign second
   *  pass scores. */
  public ExpressionRescorer(Expression expression, Bindings bindings) {
    super(new Sort(expression.getSortField(bindings, true)));
    this.expression = expression;
    this.bindings = bindings;
  }

  private static DoubleValues scores(int doc, float score) {
    return new DoubleValues() {
      @Override
      public double doubleValue() throws IOException {
        return score;
      }

      @Override
      public boolean advanceExact(int target) throws IOException {
        assert doc == target;
        return true;
      }
    };
  }

  @Override
  public Explanation explain(IndexSearcher searcher, Explanation firstPassExplanation, int docID) throws IOException {
    Explanation superExpl = super.explain(searcher, firstPassExplanation, docID);

    List<LeafReaderContext> leaves = searcher.getIndexReader().leaves();
    int subReader = ReaderUtil.subIndex(docID, leaves);
    LeafReaderContext readerContext = leaves.get(subReader);
    int docIDInSegment = docID - readerContext.docBase;

    return expression.getDoubleValuesSource(bindings).explain(readerContext, docIDInSegment, superExpl);
  }
}