expand/expandweight.cc

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/* Copyright (C) 2007,2008 Olly Betts
 * Copyright (C) 2011 Action Without Borders
 *
 * 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
 */

#include <config.h>

#include "expandweight.h"

#include "debuglog.h"
#include "omassert.h"
#include "termlist.h"

#include <cmath>

using namespace std;

namespace Xapian {
namespace Internal {

Xapian::weight
ExpandWeight::get_weight(TermList * merger, const string & term) const
{
    LOGCALL(MATCH, Xapian::weight, "ExpandWeight::get_weight", merger | term);

    // Accumulate the stats for this term across all relevant documents.
    ExpandStats stats(avlen, expand_k);
    merger->accumulate_stats(stats);

    double termfreq = stats.termfreq;
    double rtermfreq = stats.rtermfreq;

    LOGVALUE(EXPAND, rsize);
    LOGVALUE(EXPAND, rtermfreq);

    LOGVALUE(EXPAND, dbsize);
    LOGVALUE(EXPAND, stats.dbsize);
    if (stats.dbsize == dbsize) {
        // Either we're expanding from just one database, or we got stats from
        // all the sub-databases (because at least one relevant document from
        // each sub-database contained this term), so termfreq should already
        // be exact.
        AssertEqParanoid(termfreq, db.get_termfreq(term));
    } else {
        AssertRel(stats.dbsize,<,dbsize);
        // We're expanding from more than one database and the stats we've got
        // only cover some of the sub-databases, so termfreq only includes
        // those sub-databases.
        if (use_exact_termfreq) {
            LOGLINE(EXPAND, "Had to request exact termfreq");
            termfreq = db.get_termfreq(term);
        } else {
            // Approximate the termfreq by scaling it up from the databases we
            // do have information from.
            termfreq *= double(dbsize) / double(stats.dbsize);
            LOGLINE(EXPAND, "termfreq is approx " << stats.termfreq << " * " <<
                            dbsize << " / " << stats.dbsize << " = " <<
                            termfreq);
            LOGVALUE(EXPAND, db.get_termfreq(term));
            if (termfreq < rtermfreq) {
                // termfreq must be at least rtermfreq, since there are at
                // least rtermfreq documents indexed by this term.
                LOGLINE(EXPAND, "termfreq must be at least rtermfreq");
                termfreq = rtermfreq;
            } else {
                // termfreq can't be more than (dbsize - rsize + rtermfreq)
                // since the number of relevant documents not indexed by this
                // term can't be more than the number of documents not indexed
                // by this term, so:
                //
                //     rsize - rtermfreq <= dbsize - termfreq
                // <=> termfreq <= dbsize - (rsize - rtermfreq)
                double termfreq_upper_bound = dbsize - (rsize - rtermfreq);
                if (termfreq > termfreq_upper_bound) {
                    LOGLINE(EXPAND, "termfreq can't be more than "
                                    "dbsize - (rsize + rtermfreq)");
                    termfreq = termfreq_upper_bound;
                }
            }
        }
    }
    LOGVALUE(EXPAND, termfreq);

    double reldocs_without_term = rsize - rtermfreq;
    double num, denom;
    num = (rtermfreq + 0.5) * (dbsize - termfreq - reldocs_without_term + 0.5);
    AssertRel(num,>,0);
    denom = (termfreq - rtermfreq + 0.5) * (reldocs_without_term + 0.5);
    AssertRel(denom,>,0);

    Xapian::weight tw = log(num / denom);
    LOGVALUE(EXPAND, tw);
    LOGVALUE(EXPAND, stats.multiplier);
    RETURN(stats.multiplier * tw);
}

}
}

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