xapian-core/expand/tradeweight.cc

   1 /** @file
   2  * @brief Xapian::TradEWeight class - The TradWeight scheme for query expansion.
   3  */
   4 /* Copyright (C) 2013 Aarsh Shah
   5  *
   6  * This program is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU General Public License as
   8  * published by the Free Software Foundation; either version 2 of the
   9  * License, or (at your option) any later version.
  10  *
  11  * This program is distributed in the hope that it will be useful
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License
  17  * along with this program; if not, write to the Free Software
  18  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
  19  */
  20
  21 #include <config.h>
  22
  23 #include "expandweight.h"
  24 #include <cmath>
  25
  26 using namespace std;
  27
  28 namespace Xapian {
  29 namespace Internal {
  30
  31 double
  32 TradEWeight::get_weight() const
  33 {
  34     double reldocs_without_term = get_rsize() - stats.rtermfreq;
  35     double num, denom;
  36     num = (stats.rtermfreq + 0.5) * (get_dbsize() - stats.termfreq - reldocs_without_term + 0.5);
  37
  38     denom = (stats.termfreq - stats.rtermfreq + 0.5) * (reldocs_without_term + 0.5);
  39
  40     double tw = log(num / denom);
  41     return stats.multiplier * tw;
  42 }
  43
  44 }
  45 }