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JelinekMercerTermScoreFunction.hpp

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/*==========================================================================
 * Copyright (c) 2004 University of Massachusetts.  All Rights Reserved.
 *
 * Use of the Lemur Toolkit for Language Modeling and Information Retrieval
 * is subject to the terms of the software license set forth in the LICENSE
 * file included with this software, and also available at
 * http://www.lemurproject.org/license.html
 *
 *==========================================================================
*/


//
// JelinekMercerTermScoreFunction
//
// 26 January 2004 - tds
//

#ifndef INDRI_JELINEKMERCERTERMSCOREFUNCTION_HPP
#define INDRI_JELINEKMERCERTERMSCOREFUNCTION_HPP

#include <math.h>

class JelinekMercerTermScoreFunction : public TermScoreFunction {
private:
  double _lambda;
  double _backgroundLambda;
  double _collectionFrequency;
  double _collectionComponent;
  double _oneLevelCollectionComponent;
  double _contextLambda;
  double _collectionLambda;
  double _documentLambda;
  double _foregroundLambda;

public:
  JelinekMercerTermScoreFunction( double collectionFrequency, double collectionLambda, double documentLambda = 0.0 ) {
    _contextLambda = (1 - collectionLambda - documentLambda);
    _collectionFrequency = collectionFrequency;
    _collectionLambda = collectionLambda;
    _documentLambda = documentLambda;
    _foregroundLambda = _collectionLambda + _documentLambda;

    assert( _documentLambda >= 0.0 && _documentLambda <= 1.0 );
    assert( _collectionLambda >= 0.0 && _collectionLambda <= 1.0 );
    assert( _contextLambda >= 0.0 && _contextLambda <= 1.0 );
    
    _collectionComponent = _collectionLambda * _collectionFrequency;
  }

  double scoreOccurrence( int occurrences, int contextSize ) {
    //
    //             [                      occurrences                                             ]
    // score = log [ foregroundLambda * ---------------  + collectionLambda * collectionFrequency ]
    //             [                      contextSize                                             ]
    //

    double contextFrequency = contextSize ? double(occurrences) / double(contextSize) : 0.0;
    return log( _foregroundLambda * contextFrequency + _collectionComponent );
  }

  double scoreOccurrence( int occurrences, int contextSize, int documentOccurrences, int documentLength ) {
    double contextFrequency = contextSize ? double(occurrences) / double(contextSize) : 0.0;
    double documentFrequency = documentLength ? double(documentOccurrences) / double(documentLength) : 0.0;
    return log( _contextLambda * contextFrequency + _documentLambda * documentFrequency + _collectionComponent );
  }
};

#endif // INDRI_JELINEKMERCERTERMSCOREFUNCTION_HPP

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