xapian-core/expand/bo1eweight.cc

   1 /** @file
   2  * @brief Xapian::Bo1EWeight class - The Bo1 scheme of the DFR framework
   3  *                                   for query expansion.
   4  */
   5 /* Copyright (C) 2013 Aarsh Shah
   6  *
   7  * This program is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU General Public License as
   9  * published by the Free Software Foundation; either version 2 of the
  10  * License, or (at your option) any later version.
  11  *
  12  * This program is distributed in the hope that it will be useful
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  15  * GNU General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU General Public License
  18  * along with this program; if not, write to the Free Software
  19  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
  20  */
  21
  22 #include <config.h>
  23
  24 #include "expandweight.h"
  25 #include "common/log2.h"
  26
  27 using namespace std;
  28
  29 namespace Xapian {
  30 namespace Internal {
  31
  32 double
  33 Bo1EWeight::get_weight() const
  34 {
  35     double F = get_collection_freq();
  36
  37     double N = get_dbsize();
  38
  39     double mean = F / N;
  40     double wt = stats.rcollection_freq * log2((1.0 + mean) / mean) + log2(1.0 + mean);
  41
  42     return wt;
  43 }
  44
  45 }
  46 }