/* Copyright (C) 2015
 * swift project Community / Contributors
 *
 * This file is part of swift project. It is subject to the license terms in the LICENSE file found in the top-level
 * directory of this distribution and at http://www.swift-project.org/license.html. No part of swift project,
 * including this file, may be copied, modified, propagated, or distributed except according to the terms
 * contained in the LICENSE file.
 */

//! \file

#ifndef BLACKMISC_DATACACHE_H
#define BLACKMISC_DATACACHE_H

#include "blackmisc/blackmiscexport.h"
#include "blackmisc/valuecache.h"
#include "blackmisc/worker.h"
#include <QUuid>
#include <QFileSystemWatcher>
#include <QLockFile>
#include <future>

namespace BlackMisc
{
    namespace Private
    {
        /*!
         * Decorator for CValuePage which allows incoming remote changes to be queued to allow for more
         * flexibility to control how the queue is popped compared to the usual Qt::QueuedConnection.
         */
        class BLACKMISC_EXPORT CDataPageQueue : public QObject
        {
            Q_OBJECT

        public:
            //! Constructor.
            CDataPageQueue(CValuePage *parent) : QObject(parent), m_page(parent) {}

            //! Add to the queue to synchronize with a change caused by another page.
            //! \threadsafe
            void queueValuesFromCache(const BlackMisc::CValueCachePacket &values, QObject *changedBy);

            //! Synchronize with changes queued by queueValuesFromCache.
            void setQueuedValuesFromCache();

            //! Synchronize with one specific change in the queue, leave the rest for later.
            void setQueuedValueFromCache(const QString &key);

        private:
            CValuePage *m_page = nullptr;
            QList<std::pair<CValueCachePacket, QObject*>> m_queue;
            QMutex m_mutex;
        };
    }

    class CDataCache;

    /*!
     * Encapsulates metastate about how the version of the cache in memory compares to the one on disk.
     * \threadsafe
     */
    class BLACKMISC_EXPORT CDataCacheRevision
    {
    public:
        //! Construct the single instance of the revision metastate.
        CDataCacheRevision(const QString &basename) : m_basename(basename) {}

        //! Non-copyable.
        //! @{
        CDataCacheRevision(const CDataCacheRevision &) = delete;
        CDataCacheRevision &operator =(const CDataCacheRevision &) = delete;
        //! @}

        //! RAII class to keep the revision file locked during update.
        class LockGuard;

        //! Get the state of the disk cache, and prepare to update any values which are out of date.
        //! Return value can be converted to bool, false means update is not started (error, or already up-to-date).
        //! \param timestamps Current in-memory timestamps, to be compared with the on-disk ones.
        //! \param updateUuid Whether to prepare for an actual update, or just interrograte whether one is needed.
        //! \param pinsOnly Only load pinned values.
        LockGuard beginUpdate(const QMap<QString, qint64> &timestamps, bool updateUuid = true, bool pinsOnly = false);

        //! During update, writes a new revision file with new timestamps.
        void writeNewRevision(const QMap<QString, qint64> &timestamps);

        //! Release the revision file lock and mark everything up-to-date (called by LockGuard destructor).
        void finishUpdate();

        //! True if beginUpdate found some values with timestamps newer than in memory.
        bool isPendingRead() const;

        //! Call before beginUpdate if there is a write pending, so update will start even if there is nothing to read.
        void notifyPendingWrite();

        //! During update, returns keys which have on-disk timestamps newer than in-memory. Guaranteed not empty.
        QSet<QString> keysWithNewerTimestamps() const;

        //! During update, returns true if the on-disk timestamp of this key is newer than in-memory.
        bool isNewerValueAvailable(const QString &key, qint64 timestamp);

        //! Return a future which will be made ready when the value is loaded. Future is invalid if value is not loading.
        std::future<void> promiseLoadedValue(const QString &key, qint64 currentTimestamp);

        //! Returns (by move) the container of promises to load values.
        std::vector<std::promise<void>> loadedValuePromises();

        //! Set TTL value that will be written to the revision file.
        void setTimeToLive(const QString &key, int ttl);

        //! Causes the new timestamp to be written to the revision file.
        void overrideTimestamp(const QString &key, qint64 timestamp);

        //! Set the flag which will cause the value to be pre-loaded.
        void pinValue(const QString &key);

    private:
        mutable QMutex m_mutex { QMutex::Recursive };
        bool m_updateInProgress = false;
        bool m_pendingRead = false;
        bool m_pendingWrite = false;
        QString m_basename;
        QLockFile m_lockFile { m_basename + "/.lock" };
        QUuid m_uuid;
        QMap<QString, qint64> m_timestamps;
        QMap<QString, qint64> m_timesToLive;
        QSet<QString> m_pinnedValues;
        std::vector<std::promise<void>> m_promises;

        static QJsonObject toJson(const QMap<QString, qint64> &timestamps);
        static QMap<QString, qint64> fromJson(const QJsonObject &timestamps);
        static QJsonArray toJson(const QSet<QString> &pins);
        static QSet<QString> fromJson(const QJsonArray &pins);
    };

    /*!
     * Worker which performs (de)serialization on behalf of CDataCache, in a separate thread
     * so that the main thread is not blocked by (de)serialization of large objects.
     */
    class BLACKMISC_EXPORT CDataCacheSerializer : public BlackMisc::CContinuousWorker
    {
        Q_OBJECT

    public:
        //! Constructor.
        CDataCacheSerializer(CDataCache *owner, const QString &revisionFileName);

        //! Save values to persistent store. Called whenever a value is changed locally.
        void saveToStore(const BlackMisc::CVariantMap &values, const BlackMisc::CValueCachePacket &baseline);

        //! Load values from persistent store. Called once per second.
        //! Also called by saveToStore, to ensure that remote changes to unrelated values are not lost.
        //! \param baseline A snapshot of the currently loaded values, taken when the load is queued.
        //! \param defer Whether to defer applying the changes. Used when called by saveToStore.
        //! \param pinsOnly Only load pinned values.
        //! \return Usually ignored, but can be held in order to retain the revision file lock.
        CDataCacheRevision::LockGuard loadFromStore(const BlackMisc::CValueCachePacket &baseline, bool defer = false, bool pinsOnly = false);

    signals:
        //! Signal back to the cache when values have been loaded.
        void valuesLoadedFromStore(const BlackMisc::CValueCachePacket &values, const BlackMisc::CIdentifier &originator);

    private:
        const QString &persistentStore() const;
        void applyDeferredChanges();
        void deliverPromises(std::vector<std::promise<void>>);

        CDataCache *const m_cache = nullptr;
        QUuid m_revision;
        const QString m_revisionFileName;
        BlackMisc::CValueCachePacket m_deferredChanges;
    };

    /*!
     * Singleton derived class of CValueCache, for core dynamic data.
     *
     * File-based distribution between processes is built-in to the class.
     */
    class BLACKMISC_EXPORT CDataCache : public BlackMisc::CValueCache
    {
        Q_OBJECT

    public:
        //! Return the singleton instance.
        static CDataCache *instance();

        //! The directory where core data are stored.
        static const QString &persistentStore();

        //! Return the filename where the value with the given key may be stored.
        static QString filenameForKey(const QString &key);

        //! Return all files where data may be stored.
        QStringList enumerateStore() const;

        //! Method used for implementing CData::synchronize.
        bool synchronize(const QString &key);

        //! Method used for implementing TTL.
        void setTimeToLive(const QString &key, int ttl);

        //! Method used for implementing timestamp renewal.
        void renewTimestamp(const QString &key, qint64 timestamp);

        //! Method used for implementing pinning values.
        void pinValue(const QString &key);

    private:
        CDataCache();

        void saveToStoreAsync(const BlackMisc::CValueCachePacket &values);
        void loadFromStoreAsync();

        virtual void connectPage(Private::CValuePage *page) override;

        QFileSystemWatcher m_watcher;
        const QString m_revisionFileName { persistentStore() + "/.rev" };

        CDataCacheSerializer m_serializer { this, m_revisionFileName };
        CDataCacheRevision m_revision { persistentStore() + "/" };
        friend class CDataCacheSerializer; // to access m_revision and protected members of CValueCache
    };

    /*!
     * Class template for accessing a specific value in the CDataCache.
     * \tparam Trait A subclass of BlackMisc::CDataTrait that identifies the value's key and other metadata.
     */
    template <typename Trait>
    class CData : public BlackMisc::CCached<typename Trait::type>
    {
    public:
        //! \copydoc BlackMisc::CCached::NotifySlot
        template <typename T>
        using NotifySlot = typename BlackMisc::CCached<typename Trait::type>::template NotifySlot<T>;

        //! Constructor.
        //! \param owner Will be the parent of the internal QObject used to access the value.
        //! \param slot Slot to call when the value is modified by another object.
        //!             Must be a void, non-const member function of the owner.
        template <typename T>
        CData(T *owner, NotifySlot<T> slot = nullptr) :
            CData::CCached(CDataCache::instance(), Trait::key(), Trait::isValid, Trait::defaultValue(), owner, slot)
        {
            if (Trait::timeToLive() >= 0) { CDataCache::instance()->setTimeToLive(Trait::key(), Trait::timeToLive()); }
            if (Trait::isPinned()) { CDataCache::instance()->pinValue(Trait::key()); }
        }

        //! Reset the data to its default value.
        void setDefault() { this->set(Trait::defaultValue()); }

        //! Return the file that is used for persistence for this value.
        QString getFilename() const { return CDataCache::filenameForKey(this->getKey()); }

        //! True if the current timestamp is older than the TTL (time to live).
        bool isStale() const { return Trait::timeToLive() >= 0 && this->getTimestamp() + Trait::timeToLive() > QDateTime::currentMSecsSinceEpoch(); }

        //! Don't change the value, but write a new timestamp, to extend the life of the value.
        void renewTimestamp(qint64 timestamp) { return CDataCache::instance()->renewTimestamp(this->getKey(), timestamp); }

        //! If the value is currently being loaded, wait for it to finish loading, and call the notification slot, if any.
        void synchronize()
        {
            auto *queue = this->m_page.template findChild<Private::CDataPageQueue *>();
            Q_ASSERT(queue);
            CDataCache::instance()->synchronize(this->getKey());
            queue->setQueuedValueFromCache(this->getKey());
        }

        //! Data cache doesn't support setAndSave (because set() already causes save anyway).
        CStatusMessage setAndSave(const typename Trait::type &value, qint64 timestamp = 0) = delete;

        //! Data cache doesn't support save (because currently set value is saved already).
        CStatusMessage save() = delete;
    };

    /*!
     * Base class for traits to be used as template argument to BlackMisc::CData.
     */
    template <typename T>
    struct CDataTrait
    {
        //! Data type of the value.
        using type = T;

        //! Key string of the value. Reimplemented in derived class.
        static const char *key() { qFatal("Not implemented"); return ""; }

        //! Validator function. Return true if the argument is valid, false otherwise. Default
        //! implementation just returns true. Reimplemented in derived class to support validation of the value.
        static bool isValid(const T &) { return true; }

        //! Return the value to use in case the supplied value does not satisfy the validator.
        //! Default implementation returns a default-constructed value.
        static const T &defaultValue() { static const T def {}; return def; }

        //! Number of milliseconds after which cached value becomes stale.
        //! Default is -1 which means value never becomes stale.
        static int timeToLive() { return -1; }

        //! If true, then value will be synchronously loaded when CDataCache is constructed.
        //! Good for small, important values; bad for large ones.
        static bool isPinned() { return false; }

        //! Deleted default constructor.
        CDataTrait() = delete;

        //! Deleted copy constructor.
        CDataTrait(const CDataTrait &) = delete;

        //! Deleted copy assignment operator.
        CDataTrait &operator =(const CDataTrait &) = delete;
    };
}

#endif