/* -*-c++-*- */
/* osgEarth - Geospatial SDK for OpenSceneGraph
 * Copyright 2020 Pelican Mapping
 * http://osgearth.org
 *
 * osgEarth is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser 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 Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>
 */

#ifndef OSGEARTH_REVISIONING_H
#define OSGEARTH_REVISIONING_H 1

#include <osgEarth/Common>
#include <osg/observer_ptr>
#include <vector>
#include <atomic>

namespace osgEarth { namespace Util
{
    /**
     * A tracking revision. See "class Revisioned" below for details.
     */
    struct Revision // header-only; no export
    {
        Revision() : _value(-1) { }
        Revision(int init) : _value(init) { }
        void reset() { _value = -1; }
        operator int() const { return _value; }
        int operator ++() { return ++_value; }
        int operator --() { return --_value; }
    private:
        int _value;
    };


    /**
     * Base class for a revisioned object. A Revisioned object is one that keeps track
     * of its state with a version number. Other objects can then hold a Revision object
     * (see above) and use it to see if they are up to date with this object, thereby
     * enabling passive data model synchronization.
     *
     * A Revisioned object is useful in the case where one or many objects want to 
     * track the state of a single object. If you want the opposite - where one
     * object tracks the state of a multitude (or a hierarchy) of data model objects,
     * use TrackDirty instead.
     */
    class Revisioned /* no export; header-only */
    {
    public:
        /**
         * Marks this object as dirty by increasing the revision number.
         * If the object has parents, it will mark those dirty as well.
         */
        void dirty()
        {
            ++_revision;
        }

        /**
         * Synchronizes the external revision number with this revision, effectively
         * bringing the external object up to date.
         */
        virtual void sync( Revision& externalRevision ) const
        {
            externalRevision = _revision;
        }

        /** 
         * Returns true if the external object is at a different revision that this object.
         */
        bool outOfSyncWith( const Revision& externalRevision) const
        {
            return !inSyncWith( externalRevision );
        }

        /**
         * Returns true if the external object is at the same revision as this object.
         */
        virtual bool inSyncWith( const Revision& externalRevision ) const
        {
            return _alwaysDirty ? false : _revision == externalRevision;
        }

        Revisioned() : _alwaysDirty(false) { }

        /** dtor */
        virtual ~Revisioned() { }

        /** Marks this object as always being dirty (i.e. inSyncWith() will always return false) */
        void setAlwaysDirty( bool value )
        {
            _alwaysDirty = value;
        }

    private:
        Revision _revision;
        bool     _alwaysDirty;
    };


    /**
     * A DirtyNotifier object is an object that can mark itself, and optionally
     * its dependent parents, dirty. This is analagous to OSG's "dirtyBound"
     * concept that can propagate up through a scene graph.
     *
     * The use case is when you want to detect changes somewhere in an object
     * group or hierarchy, and then update something based on the modular
     * changes. This is the opposite of the Revisioned pattern (in which many
     * objects are tracking the state of one.)
     *
     * Note: objects always start out dirty.
     */
    class OSGEARTH_EXPORT DirtyNotifier
    {
    public:
        DirtyNotifier();
        virtual ~DirtyNotifier() { }

        /**
         * Marks the object dirty and notifies parents
         */
        virtual void setDirty();

        /**
         * Marks the object not dirty.
         */
        virtual void resetDirty() { _counter->_count = 0; }

        /**
         * Is this object dirty?
         */
        virtual bool isDirty() const { return _counter->_count > 0; }

        /**
         * Adds a dependent parent that will be dirtied if this object if dirtied.
         */
        virtual void addParent( DirtyNotifier* parent );

        /**
         * Removes a dependent parent previously added by addParent.
         */
        virtual void removeParent( DirtyNotifier* parent );

    private:
        // this pattern is used to we can track parents with an observer pointer.
        struct DirtyCounter : public osg::Referenced
        {
            DirtyCounter(DirtyNotifier* owner) : _owner(owner), _count(1) { }
            DirtyNotifier* _owner;
            int            _count;
        };
        osg::ref_ptr<DirtyCounter>                     _counter;
        std::vector< osg::observer_ptr<DirtyCounter> > _parents;
    };


    /**
     * A simple but thread-safe "dirty" object that support only one client.
     */
    template<typename T>
    struct SimpleMutable
    {
        SimpleMutable() : _dirty(1) { }
        SimpleMutable(const T& value) : _value(value), _dirty(1) { }
        operator const T&() const { return _value; }
        const T* operator ->() const { return &_value; }
        SimpleMutable& operator = (const T& value) { _value = value; _dirty.exchange(1); return *this; }
        bool changed(bool reset=true) const { unsigned r = reset? _dirty.exchange(0) : (unsigned)_dirty; return r==1; }
        void clean() { _dirty.exchange(0); }
    private:
        T _value;
        mutable std::atomic_int _dirty;
    };

} }

#endif // OSGEARTH_REVISIONING_H