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StructQryDocRep.hpp

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/*==========================================================================
 * Copyright (c) 2002 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
 *
 *==========================================================================
*/

#ifndef _STRUCTQRYDOCREP_HPP
#define _STRUCTQRYDOCREP_HPP
#include "DocumentRep.hpp"
#include <cmath>

class StructQryDocRep : public DocumentRep {
public:
  StructQryDocRep(DOCID_T docID, double *idfValue, int docLength, int docCount,
                  double docLengthAverage, double db) : 
    DocumentRep(docID), did(docID), idf(idfValue), end(docLength),
    docEnd(docLength), size(docLength), start(0),
    dla(docLengthAverage), defaultBelief(db) {
    oneMinusDB = 1 - defaultBelief;
    denom = log(docCount + 1.0);
    numer = docCount + 0.5;
  }
  virtual ~StructQryDocRep() {}
  virtual double termWeight(TERMID_T termID, const DocInfo *info) const { return 0;}
  virtual double termWeight(TERMID_T termID, double dtf, int df) const{
    if (idf)
      return beliefScore(dtf, idf[termID]);
    else
      return beliefScore(dtf, computeIdfScore(df));
  }
  virtual double scoreConstant() const { return 0;}

  void startPassageIteration(int sz) const {
    size = sz;
    increment = size/2;
    start = 0;
    end = size < docEnd ? size : docEnd;
  }
  bool hasMorePassage() const {
    // still some terms in the list.
    return(start < docEnd);
  }

  void nextPassage() const{
    if(start + increment < docEnd)
      start += increment;
    else
      start = docEnd;
    end = (start + size) < docEnd ? (start + size) : docEnd;
  }

  double computeIdfScore(double df) const {
    return log(numer/df)/denom;
  }

  double beliefScore(double df, double idf) const {
    return (defaultBelief + oneMinusDB
            * (df / (df + 0.5 + 1.5* ((end - start)/dla))) * idf);
  }

  DOCID_T did;
  mutable int start; 
  mutable int end;

private:
  double *idf;
  mutable int size; 
  mutable int increment; 
  int docEnd;  
  double dla;
  double numer, denom;
  double defaultBelief, oneMinusDB;
};
#endif

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