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TwoStageTermScoreFunction.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
 *
 *==========================================================================
*/


//
// TwoStageTermScoreFunction.hpp
//
// 16 April 2004 -- tds
//

#ifndef INDRI_TWOSTAGETERMSCOREFUNCTION_HPP
#define INDRI_TWOSTAGETERMSCOREFUNCTION_HPP

class TwoStageTermScoreFunction : public TermScoreFunction {
private:
  double _mu;
  double _lambda;
  double _collectionFrequency;

public:
  TwoStageTermScoreFunction( double mu, double lambda, double collectionFrequency ) :
    _mu(mu),
    _lambda(lambda),
    _collectionFrequency(collectionFrequency) {
  }

  double scoreOccurrence( int occurrences, int contextSize ) {

    //                    [  c(w;d) + \mu * p(w|C)   ]
    //    ( 1 - \lambda ) [ ------------------------ ] + \lambda * p(w|C)
    //                    [       |d| + \mu          ]

    double dirichlet = ((double(occurrences) + _mu*_collectionFrequency) / (double(contextSize) + _mu));
    double p = ( 1-_lambda ) * dirichlet + _lambda * _collectionFrequency;
    return log(p);
  }

  double scoreOccurrence( int occurrences, int contextSize, int documentOccurrences, int documentLength ) {
    double documentFrequency = double(documentOccurrences) / double(documentLength);
    double dirichlet = ((double(occurrences) + _mu*documentFrequency) / (double(contextSize) + _mu));
    double p = ( 1-_lambda ) * dirichlet + _lambda * _collectionFrequency;
    return log(p);
  }
};

#endif // INDRI_TWOSTAGETERMSCOREFUNCTION_HPP

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