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/** @file chert_termlisttable.cc
* @brief Subclass of ChertTable which holds termlists.
*/
/* Copyright (C) 2007,2008 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 "chert_termlisttable.h"
#include <xapian/document.h>
#include <xapian/error.h>
#include <xapian/termiterator.h>
#include "debuglog.h"
#include "omassert.h"
#include "pack.h"
#include "stringutils.h"
#include <string>
using namespace std;
void
ChertTermListTable::set_termlist(Xapian::docid did,
const Xapian::Document & doc,
chert_doclen_t doclen)
{
LOGCALL_VOID(DB, "ChertTermListTable::set_termlist", did | doc | doclen);
string tag;
pack_uint(tag, doclen);
Xapian::doccount termlist_size = doc.termlist_count();
if (termlist_size == 0) {
// doclen is sum(wdf) so should be zero if there are no terms.
Assert(doclen == 0);
Assert(doc.termlist_begin() == doc.termlist_end());
add(make_key(did), string());
return;
}
Xapian::TermIterator t = doc.termlist_begin();
if (t != doc.termlist_end()) {
pack_uint(tag, termlist_size);
string prev_term = *t;
tag += prev_term.size();
tag += prev_term;
pack_uint(tag, t.get_wdf());
--termlist_size;
while (++t != doc.termlist_end()) {
const string & term = *t;
// If there's a shared prefix with the previous term, we don't
// store it explicitly, but just store the length of the shared
// prefix. In general, this is a big win.
size_t reuse = common_prefix_length(prev_term, term);
// reuse must be <= prev_term.size(), and we know that value while
// decoding. So if the wdf is small enough that we can multiply it
// by (prev_term.size() + 1), add reuse and fit the result in a
// byte, then we can pack reuse and the wdf into a single byte and
// save ourselves a byte. We actually need to add one to the wdf
// before multiplying so that a wdf of 0 can be detected by the
// decoder.
size_t packed = 0;
Xapian::termcount wdf = t.get_wdf();
// If wdf >= 128, then we aren't going to be able to pack it in so
// don't even try to avoid the calculation overflowing and making
// us think we can.
if (wdf < 127)
packed = (wdf + 1) * (prev_term.size() + 1) + reuse;
if (packed && packed < 256) {
// We can pack the wdf into the same byte.
tag += char(packed);
tag += char(term.size() - reuse);
tag.append(term.data() + reuse, term.size() - reuse);
} else {
tag += char(reuse);
tag += char(term.size() - reuse);
tag.append(term.data() + reuse, term.size() - reuse);
// FIXME: pack wdf after reuse next time we rejig the format
// incompatibly.
pack_uint(tag, wdf);
}
prev_term = *t;
--termlist_size;
}
}
Assert(termlist_size == 0);
add(make_key(did), tag);
}
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