using System;
using System.Buffers;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using JetBrains.Annotations;
using Microsoft.CodeAnalysis;
using Microsoft.Extensions.ObjectPool;
using Robust.Server.GameObjects;
using Robust.Server.Player;
using Robust.Shared.Enums;
using Robust.Shared.GameObjects;
using Robust.Shared.GameStates;
using Robust.Shared.IoC;
using Robust.Shared.Log;
using Robust.Shared.Map;
using Robust.Shared.Network;
using Robust.Shared.Network.Messages;
using Robust.Shared.Timing;
using Robust.Shared.Utility;

namespace Robust.Server.GameStates
{
    /// <inheritdoc cref="IServerGameStateManager"/>
    [UsedImplicitly]
    public sealed class ServerGameStateManager : IServerGameStateManager, IPostInjectInit
    {
        // Mapping of net UID of clients -> last known acked state.
        private readonly Dictionary<long, GameTick> _ackedStates = new();
        private GameTick _lastOldestAck = GameTick.Zero;

        private PVSSystem _pvs = default!;

        [Dependency] private readonly IServerEntityManager _entityManager = default!;
        [Dependency] private readonly IGameTiming _gameTiming = default!;
        [Dependency] private readonly IServerNetManager _networkManager = default!;
        [Dependency] private readonly IPlayerManager _playerManager = default!;
        [Dependency] private readonly INetworkedMapManager _mapManager = default!;
        [Dependency] private readonly IEntitySystemManager _systemManager = default!;
        [Dependency] private readonly IServerEntityNetworkManager _entityNetworkManager = default!;

        private ISawmill _logger = default!;

        public ushort TransformNetId { get; set; }

        public void PostInject()
        {
            _logger = Logger.GetSawmill("PVS");
        }

        /// <inheritdoc />
        public void Initialize()
        {
            _networkManager.RegisterNetMessage<MsgState>();
            _networkManager.RegisterNetMessage<MsgStateAck>(HandleStateAck);

            _networkManager.Connected += HandleClientConnected;
            _networkManager.Disconnect += HandleClientDisconnect;

            _pvs = EntitySystem.Get<PVSSystem>();
        }

        private void HandleClientConnected(object? sender, NetChannelArgs e)
        {
            if (!_ackedStates.ContainsKey(e.Channel.ConnectionId))
                _ackedStates.Add(e.Channel.ConnectionId, GameTick.Zero);
            else
                _ackedStates[e.Channel.ConnectionId] = GameTick.Zero;
        }

        private void HandleClientDisconnect(object? sender, NetChannelArgs e)
        {
            _ackedStates.Remove(e.Channel.ConnectionId);
        }

        private void HandleStateAck(MsgStateAck msg)
        {
            Ack(msg.MsgChannel.ConnectionId, msg.Sequence);
        }

        private void Ack(long uniqueIdentifier, GameTick stateAcked)
        {
            DebugTools.Assert(_networkManager.IsServer);

            if (_ackedStates.TryGetValue(uniqueIdentifier, out var lastAck))
            {
                if (stateAcked > lastAck) // most of the time this is true
                {
                    _ackedStates[uniqueIdentifier] = stateAcked;
                }
                else if (stateAcked == GameTick.Zero) // client signaled they need a full state
                {
                    //Performance/Abuse: Should this be rate limited?
                    _ackedStates[uniqueIdentifier] = GameTick.Zero;
                }

                //else stateAcked was out of order or client is being silly, just ignore
            }
            else
                DebugTools.Assert("How did the client send us an ack without being connected?");
        }

        /// <inheritdoc />
        public void SendGameStateUpdate()
        {
            DebugTools.Assert(_networkManager.IsServer);

            if (!_networkManager.IsConnected)
            {
                // Prevent deletions piling up if we have no clients.
                _entityManager.CullDeletionHistory(GameTick.MaxValue);
                _pvs.CullDeletionHistory(GameTick.MaxValue);
                _mapManager.CullDeletionHistory(GameTick.MaxValue);
                _pvs.Cleanup(_playerManager.ServerSessions);
                return;
            }

            var inputSystem = _systemManager.GetEntitySystem<InputSystem>();

            var oldestAckValue = GameTick.MaxValue.Value;

            var mainThread = Thread.CurrentThread;
            var parentDeps = IoCManager.Instance!;

            _pvs.ProcessCollections();

            // people not in the game don't get states
            var players = _playerManager.ServerSessions.Where(o => o.Status == SessionStatus.InGame).ToArray();

            //todo paul oh my god make this less shit
            EntityQuery<MetaDataComponent> metadataQuery = default!;
            EntityQuery<TransformComponent> transformQuery = default!;
            HashSet<int>[] playerChunks = default!;
            EntityUid[][] viewerEntities = default!;
            (Dictionary<EntityUid, MetaDataComponent> metadata, RobustTree<EntityUid> tree)?[] chunkCache = default!;

            if (_pvs.CullingEnabled)
            {
                List<(uint, IChunkIndexLocation)> chunks;
                (chunks, playerChunks, viewerEntities) = _pvs.GetChunks(players);
                const int ChunkBatchSize = 2;
                var chunksCount = chunks.Count;
                var chunkBatches = (int) MathF.Ceiling((float) chunksCount / ChunkBatchSize);
                chunkCache =
                    new (Dictionary<EntityUid, MetaDataComponent> metadata, RobustTree<EntityUid> tree)?[chunksCount];

                // Update the reused trees sequentially to avoid having to lock the dictionary per chunk.
                var reuse = ArrayPool<bool>.Shared.Rent(chunksCount);

                transformQuery = _entityManager.GetEntityQuery<TransformComponent>();
                metadataQuery = _entityManager.GetEntityQuery<MetaDataComponent>();
                Parallel.For(0, chunkBatches, i =>
                {
                    var start = i * ChunkBatchSize;
                    var end = Math.Min(start + ChunkBatchSize, chunksCount);

                    for (var j = start; j < end; ++j)
                    {
                        var (visMask, chunkIndexLocation) = chunks[j];
                        reuse[j] = _pvs.TryCalculateChunk(chunkIndexLocation, visMask, transformQuery, metadataQuery, out var chunk);
                        chunkCache[j] = chunk;
                    }
                });

                _pvs.RegisterNewPreviousChunkTrees(chunks, chunkCache, reuse);
                ArrayPool<bool>.Shared.Return(reuse);
            }

            const int BatchSize = 2;
            var batches = (int) MathF.Ceiling((float) players.Length / BatchSize);

            Parallel.For(0, batches, i =>
            {
                var start = i * BatchSize;
                var end = Math.Min(start + BatchSize, players.Length);

                for (var j = start; j < end; ++j)
                {
                    try
                    {
                        SendStateUpdate(j);
                    }
                    catch (Exception e) // Catch EVERY exception
                    {
                        _logger.Log(LogLevel.Error, e, "Caught exception while generating mail.");
                    }
                }
            });

            void SendStateUpdate(int sessionIndex)
            {
                var session = players[sessionIndex];

                // KILL IT WITH FIRE
                if(mainThread != Thread.CurrentThread)
                    IoCManager.InitThread(new DependencyCollection(parentDeps), true);

                var channel = session.ConnectedClient;

                if (!_ackedStates.TryGetValue(channel.ConnectionId, out var lastAck))
                {
                    DebugTools.Assert("Why does this channel not have an entry?");
                }

                var (entStates, deletions) = _pvs.CullingEnabled
                    ? _pvs.CalculateEntityStates(session, lastAck, _gameTiming.CurTick, chunkCache,
                        playerChunks[sessionIndex], metadataQuery, transformQuery, viewerEntities[sessionIndex])
                    : _pvs.GetAllEntityStates(session, lastAck, _gameTiming.CurTick);
                var playerStates = _playerManager.GetPlayerStates(lastAck);
                var mapData = _mapManager.GetStateData(lastAck);

                // lastAck varies with each client based on lag and such, we can't just make 1 global state and send it to everyone
                var lastInputCommand = inputSystem.GetLastInputCommand(session);
                var lastSystemMessage = _entityNetworkManager.GetLastMessageSequence(session);
                var state = new GameState(lastAck, _gameTiming.CurTick, Math.Max(lastInputCommand, lastSystemMessage), entStates, playerStates, deletions, mapData);

                InterlockedHelper.Min(ref oldestAckValue, lastAck.Value);

                // actually send the state
                var stateUpdateMessage = _networkManager.CreateNetMessage<MsgState>();
                stateUpdateMessage.State = state;

                // If the state is too big we let Lidgren send it reliably.
                // This is to avoid a situation where a state is so large that it consistently gets dropped
                // (or, well, part of it).
                // When we send them reliably, we immediately update the ack so that the next state will not be huge.
                if (stateUpdateMessage.ShouldSendReliably())
                {
                    // TODO: remove this lock by having a single state object per session that contains all per-session state needed.
                    lock (_ackedStates)
                    {
                        _ackedStates[channel.ConnectionId] = _gameTiming.CurTick;
                    }
                }

                _networkManager.ServerSendMessage(stateUpdateMessage, channel);
            }

            if(_pvs.CullingEnabled)
                _pvs.ReturnToPool(playerChunks);
            _pvs.Cleanup(_playerManager.ServerSessions);
            var oldestAck = new GameTick(oldestAckValue);

            // keep the deletion history buffers clean
            if (oldestAck > _lastOldestAck)
            {
                _lastOldestAck = oldestAck;
                _entityManager.CullDeletionHistory(oldestAck);
                _pvs.CullDeletionHistory(oldestAck);
                _mapManager.CullDeletionHistory(oldestAck);
            }
        }
    }
}