/**
 *    Copyright (C) 2013 10gen Inc.
 *
 *    This program is free software: you can redistribute it and/or  modify
 *    it under the terms of the GNU Affero General Public License, version 3,
 *    as published by the Free Software Foundation.
 *
 *    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 Affero General Public License for more details.
 *
 *    You should have received a copy of the GNU Affero General Public License
 *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 *    As a special exception, the copyright holders give permission to link the
 *    code of portions of this program with the OpenSSL library under certain
 *    conditions as described in each individual source file and distribute
 *    linked combinations including the program with the OpenSSL library. You
 *    must comply with the GNU Affero General Public License in all respects for
 *    all of the code used other than as permitted herein. If you modify file(s)
 *    with this exception, you may extend this exception to your version of the
 *    file(s), but you are not obligated to do so. If you do not wish to do so,
 *    delete this exception statement from your version. If you delete this
 *    exception statement from all source files in the program, then also delete
 *    it in the license file.
 */

#pragma once

#include "mongo/db/diskloc.h"
#include "mongo/db/exec/collection_scan.h"
#include "mongo/db/exec/plan_stage.h"
#include "mongo/db/matcher/expression.h"
#include "mongo/util/timer.h"

namespace mongo {

    class NamespaceDetails;

    /**
     * OplogStart walks a collection backwards to find the first object in the collection that
     * matches the query.  It's used by replication to efficiently find where the oplog should be
     * replayed from.
     *
     * The oplog is always a capped collection.  In capped collections, documents are oriented on
     * disk according to insertion order.  The oplog inserts documents with increasing timestamps.
     * Queries on the oplog look for entries that are after a certain time.  Therefore if we
     * navigate backwards, the last document we encounter that satisfies our query (over the
     * timestamp) is the first document we must scan from to answer the query.
     *
     * Why isn't this a normal reverse table scan, you may ask?  We could be correct if we used a
     * normal reverse collection scan.  However, that's not fast enough.  Since we know all
     * documents are oriented on disk in insertion order, we know all documents in one extent were
     * inserted before documents in a subsequent extent.  As such we can skip through entire extents
     * looking only at the first document.
     *
     * Why is this a stage?  Because we want to yield, and we want to be notified of DiskLoc
     * invalidations.  :(
     */
    class OplogStart : public PlanStage {
    public:
        // Does not take ownership.
        OplogStart(const string& ns, MatchExpression* filter, WorkingSet* ws);
        virtual ~OplogStart();

        virtual StageState work(WorkingSetID* out);
        virtual bool isEOF();

        virtual void invalidate(const DiskLoc& dl, InvalidationType type);
        virtual void prepareToYield();
        virtual void recoverFromYield();

        // PS. don't call this.
        virtual PlanStageStats* getStats() { return NULL; }

        // For testing only.
        void setBackwardsScanTime(int newTime) { _backwardsScanTime = newTime; }
        bool isExtentHopping() { return _extentHopping; }
        bool isBackwardsScanning() { return _backwardsScanning; }
    private:
        // Copied verbatim.
        static DiskLoc prevExtentFirstLoc(NamespaceDetails* nsd, const DiskLoc& rec);

        StageState workBackwardsScan(WorkingSetID* out);

        void switchToExtentHopping();

        StageState workExtentHopping(WorkingSetID* out);

        // If we're backwards scanning we just punt to a collscan.
        scoped_ptr<CollectionScan> _cs;

        // What's our current DiskLoc?  Set by both collscan and extent hopping.
        // Only written by collscan, read and written by extent hopping.
        DiskLoc _curloc;

        // Have we done our heavy init yet?
        bool _needInit;

        // Our first state: going backwards via a collscan.
        bool _backwardsScanning;

        // Our second state: hopping backwards extent by extent.
        bool _extentHopping;

        // Our final state: done.
        bool _done;

        NamespaceDetails* _nsd;

        // We only go backwards via a collscan for a few seconds.
        Timer _timer;

        // WorkingSet is not owned by us.
        WorkingSet* _workingSet;

        string _ns;
        
        MatchExpression* _filter;

        static int _backwardsScanTime;
    };

}  // namespace mongo
