xorpostlist.h

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/* Copyright (C) 2007,2009,2010,2011,2012,2017 Olly Betts
 * Copyright (C) 2009 Lemur Consulting Ltd
 *
 * 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, see
 * <https://www.gnu.org/licenses/>.
 */
 
#ifndef XAPIAN_INCLUDED_XORPOSTLIST_H
#define XAPIAN_INCLUDED_XORPOSTLIST_H
 
#include "backends/postlist.h"
#include "postlisttree.h"
 
#include <algorithm>
 
class XorPostList : public PostList {
    XorPostList& operator=(const XorPostList&) = delete;
 
    XorPostList(const XorPostList&) = delete;
 
    Xapian::docid did = 0;
 
    size_t n_kids;
 
    PostList** plist = nullptr;
 
    PostListTree *matcher;
 
    void erase_sublist(size_t i) {
        delete plist[i];
        --n_kids;
        for (size_t j = i; j < n_kids; ++j) {
            plist[j] = plist[j + 1];
        }
        matcher->force_recalc();
    }
 
  public:
    template<class RandomItor>
    XorPostList(RandomItor pl_begin, RandomItor pl_end,
                PostListTree* matcher_, Xapian::doccount db_size)
        : n_kids(pl_end - pl_begin), matcher(matcher_)
    {
        plist = new PostList * [n_kids];
        std::copy(pl_begin, pl_end, plist);
 
        // We shortcut an empty shard and avoid creating a postlist tree for it.
        Assert(db_size);
        // We calculate the estimate assuming independence.  The simplest
        // way to calculate this seems to be a series of (n_kids - 1) pairwise
        // calculations, which gives the same answer regardless of the order.
        double scale = 1.0 / db_size;
        double P_est = plist[0]->get_termfreq() * scale;
        for (size_t i = 1; i < n_kids; ++i) {
            double P_i = plist[i]->get_termfreq() * scale;
            P_est += P_i - 2.0 * P_est * P_i;
        }
        termfreq = static_cast<Xapian::doccount>(P_est * db_size + 0.5);
    }
 
    ~XorPostList();
 
    Xapian::docid get_docid() const;
 
    double get_weight(Xapian::termcount doclen,
                      Xapian::termcount unique_terms,
                      Xapian::termcount wdfdocmax) const;
 
    bool at_end() const;
 
    double recalc_maxweight();
 
    PositionList * read_position_list() {
        return NULL;
    }
 
    PostList* next(double w_min);
 
    PostList* skip_to(Xapian::docid, double w_min);
 
    void get_docid_range(Xapian::docid& first, Xapian::docid& last) const;
 
    std::string get_description() const;
 
    Xapian::termcount get_wdf() const;
 
    Xapian::termcount count_matching_subqs() const;
};
 
#endif // XAPIAN_INCLUDED_XORPOSTLIST_H