/** @file
 * @brief Merged postlist: items from one list, weights from both
 *
 * AND MAYBE of two posting lists
 * A AND MAYBE B is logically just A, but we keep B around for weight purposes
 */
/* Copyright 1999,2000,2001 BrightStation PLC
 * Copyright 2002 Ananova Ltd
 * Copyright 2003,2004,2009,2011,2017 Olly Betts
 * Copyright 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, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301
 * USA
 */


#ifndef OM_HGUARD_ANDMAYBEPOSTLIST_H
#define OM_HGUARD_ANDMAYBEPOSTLIST_H

#include "branchpostlist.h"

/** A postlist with weights modified by another postlist.
 *
 *  This postlist returns a posting if and only if it is in the left
 *  sub-postlist.
 *
 *  If the posting does not occur in the right postlist, the weight for the
 *  posting is simply that in the left postlist.  If the posting occurs in
 *  both postlists, the weight for the posting is the sum of the weights in
 *  the sub-postlists.
 *
 *  This type of postlist is useful for specifying a set of terms which
 *  must appear in the query result: these terms can be specified as the
 *  left hand argument, with the rest of the query being on the right hand
 *  side, and having the effect of modifying the weights.
 *
 *  The postlist is also used as a "decay product" of other postlist types
 *  during the match process: when a postlist can no longer cause a
 *  document to enter the mset on its own, but can influence relative
 *  rankings, it may be combined using one of these.
 */
class AndMaybePostList : public BranchPostList {
    private:
	Xapian::doccount dbsize; // only need in case we decay to an AndPostList
	Xapian::docid lhead, rhead;
	double lmax, rmax;

	PostList * process_next_or_skip_to(double w_min, PostList *ret);
    public:
	Xapian::doccount get_termfreq_max() const;
	Xapian::doccount get_termfreq_min() const;
	Xapian::doccount get_termfreq_est() const;

	TermFreqs get_termfreq_est_using_stats(
	    const Xapian::Weight::Internal & stats) const;

	Xapian::docid get_docid() const;
	double get_weight() const;
	double get_maxweight() const;

	double recalc_maxweight();

	PostList *next(double w_min);
	PostList *skip_to(Xapian::docid did, double w_min);
	bool at_end() const;

	std::string get_description() const;

	/** Return the document length of the document the current term
	 *  comes from.
	 */
	virtual Xapian::termcount get_doclength() const;

	virtual Xapian::termcount get_unique_terms() const;

	AndMaybePostList(PostList *left_,
			 PostList *right_,
			 MultiMatch *matcher_,
			 Xapian::doccount dbsize_)
		: BranchPostList(left_, right_, matcher_),
		  dbsize(dbsize_), lhead(0), rhead(0)
	{
	    // lmax and rmax will get initialised by a recalc_maxweight
	}

	/// Constructor for use by decomposing OrPostList
	AndMaybePostList(PostList *left_,
			 PostList *right_,
			 MultiMatch *matcher_,
			 Xapian::doccount dbsize_,
			 Xapian::docid lhead_,
			 Xapian::docid rhead_)
		: BranchPostList(left_, right_, matcher_),
		  dbsize(dbsize_), lhead(lhead_), rhead(rhead_)
	{
	    // Initialise the maxweights from the kids so we can avoid forcing
	    // a full maxweight recalc
	    lmax = l->get_maxweight();
	    rmax = r->get_maxweight();
	}

	/** Synchronise the RHS to the LHS after construction.
	 *  Used after constructing from a decomposing OrPostList
	 */
	PostList * sync_rhs(double w_min);

	/** get_wdf() for ANDMAYBE postlists returns the sum of the wdfs of the
	 *  sub postlists which are at the current document - this is desirable
	 *  when the ANDMAYBE is part of a synonym.
	 */
	Xapian::termcount get_wdf() const;

	Xapian::termcount count_matching_subqs() const;

	void gather_position_lists(OrPositionList* orposlist);
};

#endif /* OM_HGUARD_ANDMAYBEPOSTLIST_H */
