/*
* Copyright (c) 2024 Vaughn Nugent
* 
* Library: VNLib
* Package: ObjectCacheServer
* File: PeerEventQueueManager.cs 
*
* PeerEventQueueManager.cs is part of ObjectCacheServer which is 
* part of the larger VNLib collection of libraries and utilities.
*
* ObjectCacheServer is free software: you can redistribute it and/or modify 
* it under the terms of the GNU Affero General Public License as 
* published by the Free Software Foundation, either version 3 of the
* License, or (at your option) any later version.
*
* ObjectCacheServer 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 https://www.gnu.org/licenses/.
*/

using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Threading.Channels;
using System.Collections.Generic;

using VNLib.Plugins;
using VNLib.Utils.Async;
using VNLib.Utils.Logging;
using VNLib.Plugins.Extensions.Loading;
using VNLib.Plugins.Extensions.Loading.Events;


namespace VNLib.Data.Caching.ObjectCache.Server.Cache
{
    internal sealed class PeerEventQueueManager : ICacheEventQueueManager, IIntervalScheduleable
    {
        private readonly int MaxQueueDepth;

        private readonly object SubLock = new();
        private readonly LinkedList<PeerEventListenerQueue> Subscribers = [];

        private readonly object StoreLock = new();
        private readonly Dictionary<string, PeerEventListenerQueue> QueueStore = new(StringComparer.OrdinalIgnoreCase);

        public PeerEventQueueManager(PluginBase plugin, ServerClusterConfig config)
        {
            MaxQueueDepth = config.MaxQueueDepth;

            /*
           * Schedule purge interval to clean up stale queues
           */
            plugin.ScheduleInterval(this, config.EventQueuePurgeInterval);
            
            //Cleanup disposeables on unload
            _ = plugin.RegisterForUnload(() =>
            {
                QueueStore.Clear();
                Subscribers.Clear();
            });
        }

        ///<inheritdoc/>
        public IPeerEventQueue Subscribe(ICachePeer peer)
        {
            PeerEventListenerQueue? nq;

            bool isNew = false;

            //Enter sync lock
            lock (StoreLock)
            {
                //Try to recover the queue for the node
                if (!QueueStore.TryGetValue(peer.NodeId, out nq))
                {
                    //Create new queue since an existing queue was not found
                    nq = new(peer.NodeId, MaxQueueDepth);
                    QueueStore.Add(peer.NodeId, nq);
                    isNew = true;
                }

                //Increment listener count since a new listener has attached
                nq.Listeners++;
            }

            //Publish new peer to subscribers list
            if (isNew)
            {
                lock (SubLock)
                {
                    //Add peer to subscribers list
                    Subscribers.AddLast(nq);
                }
            }

            //Return the node's queue
            return nq;
        }

        ///<inheritdoc/>
        public void Unsubscribe(ICachePeer peer)
        {
            /*
             * The reason I am not purging queues that no longer have listeners
             * now is because it is possible that a listener needed to detach because of 
             * a network issue and will be reconnecting shortly. If the node doesnt 
             * come back before the next purge interval, it's events will be purged.
             * 
             * Point is: there is a reason for the garbage collection style purging
             */

            //Detach a listener for a node
            lock (StoreLock)
            {
                //Get the queue and decrement the listener count
                PeerEventListenerQueue nq = QueueStore[peer.NodeId];
                nq.Listeners--;
            }
        }

        ///<inheritdoc/>
        public void PublishSingle(ChangeEvent change)
        {
            //Wait to enter the sub lock
            lock (SubLock)
            {
                //Loop through ll the fast way
                LinkedListNode<PeerEventListenerQueue>? q = Subscribers.First;

                while (q != null)
                {
                    //Pub single event node
                    q.Value.PublishChange(change);

                    //Get next queue
                    q = q.Next;
                }
            }
        }

        ///<inheritdoc/>
        public void PublishMultiple(Span<ChangeEvent> changes)
        {
            //Wait to enter the sub lock
            lock (SubLock)
            {
                //Loop through ll the fast way
                LinkedListNode<PeerEventListenerQueue>? q = Subscribers.First;

                while (q != null)
                {
                    //Publish multiple
                    q.Value.PublishChanges(changes);

                    //Get next queue
                    q = q.Next;
                }
            }
        }

        ///<inheritdoc/>
        public void PurgeStaleSubscribers()
        {
            //Enter locks
            lock (SubLock)
            {
                lock (StoreLock)
                {
                    //Get all stale queues (queues without listeners)
                    PeerEventListenerQueue[] staleQueues = QueueStore.Values.Where(static nq => nq.Listeners == 0).ToArray();

                    foreach (PeerEventListenerQueue nq in staleQueues)
                    {
                        //Remove from store
                        QueueStore.Remove(nq.NodeId);

                        //remove from subscribers
                        Subscribers.Remove(nq);
                    }
                }
            }
        }

        //Interval to purge stale subscribers
        Task IIntervalScheduleable.OnIntervalAsync(ILogProvider log, CancellationToken cancellationToken)
        {
            log.Debug("Purging stale peer event queues");

            PurgeStaleSubscribers();

            return Task.CompletedTask;
        }


        /*
         * Holds queues for each node and keeps track of the number of listeners
         * attached to the queue
         * 
         * The role of this class is to store change events for a given peer node,
         * and return them when the peer requests them. It also keeps track of the
         * number of active listeners (server connections) to the queue.
         */

        private sealed class PeerEventListenerQueue(string nodeId, int maxDepth) : IPeerEventQueue
        {
            public int Listeners;

            public string NodeId => nodeId;

            /*
             * Create a bounded channel that acts as a lru and evicts 
             * the oldest item when the queue is full
             * 
             * There will also only ever be a single thread writing events 
             * to the queue
             */
            private readonly AsyncQueue<ChangeEvent> Queue = new(new BoundedChannelOptions(maxDepth)
            {
                AllowSynchronousContinuations = true,
                SingleReader = false,
                SingleWriter = true,
                //Drop oldest item in queue if full
                FullMode = BoundedChannelFullMode.DropOldest,
            });

            public void PublishChange(ChangeEvent change) => Queue.TryEnqueue(change);

            public void PublishChanges(Span<ChangeEvent> changes)
            {
                for (int i = 0; i < changes.Length; i++)
                {
                    Queue.TryEnqueue(changes[i]);
                }
            }

            ///<inheritdoc/>
            public ValueTask<ChangeEvent> DequeueAsync(CancellationToken cancellation) => Queue.DequeueAsync(cancellation);

            ///<inheritdoc/>
            public bool TryDequeue(out ChangeEvent change) => Queue.TryDequeue(out change);
        }
    }
}