common/expandweight.h

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/* Copyright (C) 2007,2008,2009 Olly Betts
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 */

#ifndef XAPIAN_INCLUDED_EXPANDWEIGHT_H
#define XAPIAN_INCLUDED_EXPANDWEIGHT_H

#include <xapian/database.h>

#include "termlist.h"

#include <string>
#include <vector>

namespace Xapian {
namespace Internal {

class ExpandStats {
    // Which databases in a multidb are included in termfreq.
    std::vector<bool> dbs_seen;

    // Average document length in the whole database.
    Xapian::doclength avlen;

    double expand_k;

  public:
    Xapian::doccount dbsize;

    Xapian::doccount termfreq;

    Xapian::weight multiplier;

    Xapian::doccount rtermfreq;

    size_t db_index;

    ExpandStats(Xapian::doclength avlen_, double expand_k_)
        : avlen(avlen_), expand_k(expand_k_),
          dbsize(0), termfreq(0), multiplier(0), rtermfreq(0), db_index(0) {
    }

    void accumulate(Xapian::termcount wdf, Xapian::termcount doclen,
                    Xapian::doccount subtf, Xapian::doccount subdbsize) {
        // Boolean terms may have wdf == 0, but treat that as 1 so such terms
        // get a non-zero weight.
        if (wdf == 0) wdf = 1;

        multiplier += (expand_k + 1) * wdf / (expand_k * doclen / avlen + wdf);
        ++rtermfreq;

        // If we've not seen this sub-database before, then update dbsize and
        // termfreq and note that we have seen it.
        if (db_index >= dbs_seen.size() || !dbs_seen[db_index]) {
            if (db_index >= dbs_seen.size()) dbs_seen.resize(db_index + 1);
            dbs_seen[db_index] = true;
            dbsize += subdbsize;
            termfreq += subtf;
        }
    }
};

class ExpandWeight {
    const Xapian::Database db;

    Xapian::doccount dbsize;

    Xapian::doclength avlen;

    Xapian::doccount rsize;

    bool use_exact_termfreq;

    double expand_k;

public:
    ExpandWeight(const Xapian::Database &db_,
                 Xapian::doccount rsize_,
                 bool use_exact_termfreq_,
                 double expand_k_)
        : db(db_), dbsize(db.get_doccount()), avlen(db.get_avlength()),
          rsize(rsize_), use_exact_termfreq(use_exact_termfreq_),
          expand_k(expand_k_) { } 

    Xapian::weight get_weight(TermList * merger,
                              const std::string & term) const;
};

}
}

#endif // XAPIAN_INCLUDED_EXPANDWEIGHT_H

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