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/** @file exactphrasepostlist.cc
* @brief Return docs containing terms forming a particular exact phrase.
*/
/* Copyright (C) 2006,2007,2009,2010,2011,2014,2015,2017 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
*/
#include <config.h>
#include "exactphrasepostlist.h"
#include "debuglog.h"
#include "backends/positionlist.h"
#include "omassert.h"
#include <algorithm>
#include <vector>
using namespace std;
ExactPhrasePostList::ExactPhrasePostList(PostList *source_,
const vector<PostList*>::const_iterator &terms_begin,
const vector<PostList*>::const_iterator &terms_end)
: SelectPostList(source_), terms(terms_begin, terms_end)
{
size_t n = terms.size();
Assert(n > 1);
poslists = new PositionList*[n];
try {
order = new unsigned[n];
} catch (...) {
delete [] poslists;
throw;
}
for (size_t i = 0; i < n; ++i) order[i] = unsigned(i);
}
ExactPhrasePostList::~ExactPhrasePostList()
{
delete [] poslists;
delete [] order;
}
void
ExactPhrasePostList::start_position_list(unsigned i)
{
unsigned index = order[i];
poslists[i] = terms[index]->read_position_list();
poslists[i]->index = index;
}
class TermCompare {
vector<PostList *> & terms;
public:
explicit TermCompare(vector<PostList *> & terms_) : terms(terms_) { }
bool operator()(unsigned a, unsigned b) const {
return terms[a]->get_wdf() < terms[b]->get_wdf();
}
};
bool
ExactPhrasePostList::test_doc()
{
LOGCALL(MATCH, bool, "ExactPhrasePostList::test_doc", NO_ARGS);
// We often don't need to read all the position lists, so rather than using
// the shortest position lists first, we approximate by using the terms
// with the lowest wdf first. This will typically give the same or a very
// similar order.
sort(order, order + terms.size(), TermCompare(terms));
// If the first term we check only occurs too close to the start of the
// document, we only need to read one term's positions. E.g. search for
// "ripe mango" when the only occurrence of 'mango' in the current document
// is at position 0.
start_position_list(0);
poslists[0]->skip_to(poslists[0]->index);
if (poslists[0]->at_end()) RETURN(false);
// If we get here, we'll need to read the positionlists for at least two
// terms, so check the true positionlist length for the two terms with the
// lowest wdf and if necessary swap them so the true shorter one is first.
start_position_list(1);
if (poslists[0]->get_size() > poslists[1]->get_size()) {
poslists[1]->skip_to(poslists[1]->index);
if (poslists[1]->at_end()) RETURN(false);
swap(poslists[0], poslists[1]);
}
unsigned read_hwm = 1;
Xapian::termpos idx0 = poslists[0]->index;
do {
Xapian::termpos base = poslists[0]->get_position() - idx0;
unsigned i = 1;
while (true) {
if (i > read_hwm) {
read_hwm = i;
start_position_list(i);
// FIXME: consider comparing with poslist[0] and swapping
// if less common. Should we allow for the number of positions
// we've read from poslist[0] already?
}
Xapian::termpos idx = poslists[i]->index;
Xapian::termpos required = base + idx;
poslists[i]->skip_to(required);
if (poslists[i]->at_end()) RETURN(false);
Xapian::termpos got = poslists[i]->get_position();
if (got == required) {
if (++i == terms.size()) RETURN(true);
continue;
}
poslists[0]->skip_to(got - idx + idx0);
break;
}
} while (!poslists[0]->at_end());
RETURN(false);
}
Xapian::termcount
ExactPhrasePostList::get_wdf() const
{
// Calculate an estimate for the wdf of an exact phrase postlist.
//
// We use the minimum wdf of a sub-postlist as our estimate. See the
// comment in NearPostList::get_wdf() for justification of this estimate.
vector<PostList *>::const_iterator i = terms.begin();
Xapian::termcount wdf = (*i)->get_wdf();
while (++i != terms.end()) {
wdf = min(wdf, (*i)->get_wdf());
}
return wdf;
}
Xapian::doccount
ExactPhrasePostList::get_termfreq_est() const
{
// It's hard to estimate how many times the exact phrase will occur as
// it depends a lot on the phrase, but usually the exact phrase will
// occur significantly less often than the individual terms.
//
// We divide by 4 here rather than by 2 as we do for NearPostList and
// PhrasePostList, as a very rough heuristic to represent the fact that the
// words must occur exactly in order, and phrases are therefore rarer than
// near matches and (non-exact) phrase matches.
return source->get_termfreq_est() / 4;
}
TermFreqs
ExactPhrasePostList::get_termfreq_est_using_stats(
const Xapian::Weight::Internal & stats) const
{
LOGCALL(MATCH, TermFreqs, "ExactPhrasePostList::get_termfreq_est_using_stats", stats);
// No idea how to estimate this - do the same as get_termfreq_est() for
// now.
TermFreqs result(source->get_termfreq_est_using_stats(stats));
result.termfreq /= 4;
result.reltermfreq /= 4;
RETURN(result);
}
string
ExactPhrasePostList::get_description() const
{
return "(ExactPhrase " + source->get_description() + ")";
}
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