// snapshot.h


/**
 *    Copyright (C) 2018-present MongoDB, Inc.
 *
 *    This program is free software: you can redistribute it and/or modify
 *    it under the terms of the Server Side Public License, version 1,
 *    as published by MongoDB, Inc.
 *
 *    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
 *    Server Side Public License for more details.
 *
 *    You should have received a copy of the Server Side Public License
 *    along with this program. If not, see
 *    <http://www.mongodb.com/licensing/server-side-public-license>.
 *
 *    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 Server Side 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/util/assert_util.h"

namespace mongo {

class SnapshotId {
    static const uint64_t kNullId = 0;

public:
    SnapshotId() : _id(kNullId) {}

    // 0 is NULL
    explicit SnapshotId(uint64_t id) : _id(id) {
        invariant(id != kNullId);
    }

    bool isNull() const {
        return _id == kNullId;
    }

    bool operator==(const SnapshotId& other) const {
        return _id == other._id;
    }

    bool operator!=(const SnapshotId& other) const {
        return _id != other._id;
    }

    std::string toString() const {
        return std::to_string(_id);
    }

private:
    uint64_t _id;
};

template <typename T>
class Snapshotted {
public:
    Snapshotted() : _id(), _value() {}

    Snapshotted(SnapshotId id, const T& value) : _id(id), _value(value) {}

    void reset() {
        *this = Snapshotted();
    }

    void setValue(const T& t) {
        _value = t;
    }

    SnapshotId snapshotId() const {
        return _id;
    }
    const T& value() const {
        return _value;
    }
    T& value() {
        return _value;
    }

private:
    SnapshotId _id;
    T _value;
};
}