#nullable enable

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading;
using Cysharp.Threading.Tasks;

namespace ShrinkEventBus
{
    public delegate UniTask ShrinkAsyncEventHandler<in TEvent>(TEvent eventData,
        CancellationToken cancellationToken) where TEvent : IShrinkEvent;

    internal interface IShrinkEventChannel
    {
        int Count { get; }
        bool RemoveSubscription(long subscriptionId);
        void Clear();
        void DetachSlot(int slot);
    }

    internal static class ShrinkEventChannelSlots<TEvent> where TEvent : IShrinkEvent
    {
        private static readonly object Gate = new();
        private static ShrinkEventChannel<TEvent>?[] _slots =
            Array.Empty<ShrinkEventChannel<TEvent>?>();

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ShrinkEventChannel<TEvent>? Get(int slot)
        {
            var snapshot = _slots;
            return (uint)slot < (uint)snapshot.Length ? snapshot[slot] : null;
        }

        public static void Set(int slot, ShrinkEventChannel<TEvent> channel)
        {
            lock (Gate)
            {
                var current = _slots;
                var length = current.Length;
                if (length <= slot)
                {
                    length = Math.Max(4, length);
                    while (length <= slot)
                        length *= 2;
                }

                var next = new ShrinkEventChannel<TEvent>?[length];
                Array.Copy(current, next, current.Length);
                next[slot] = channel;
                Volatile.Write(ref _slots, next);
            }
        }

        public static void Clear(int slot, ShrinkEventChannel<TEvent> channel)
        {
            lock (Gate)
            {
                var current = _slots;
                if ((uint)slot >= (uint)current.Length || !ReferenceEquals(current[slot], channel))
                    return;
                var next = (ShrinkEventChannel<TEvent>?[])current.Clone();
                next[slot] = null;
                Volatile.Write(ref _slots, next);
            }
        }
    }

    internal sealed class ShrinkEventChannel<TEvent> : IShrinkEventChannel
        where TEvent : IShrinkEvent
    {
        private static readonly bool SupportsCancellation =
            typeof(IShrinkCancelableEvent).IsAssignableFrom(typeof(TEvent));

        private sealed class HandlerEntry
        {
            public long SubscriptionId;
            public long RegistrationOrder;
            public Action<TEvent>? SyncHandler;
            public ShrinkAsyncEventHandler<TEvent>? AsyncHandler;
            public ShrinkSubscribeDescriptor Descriptor;
        }

        private readonly object _gate = new();
        private readonly List<HandlerEntry> _entries = new();
        private HandlerEntry[] _snapshot = Array.Empty<HandlerEntry>();
        private Action<TEvent>? _syncDispatcher;

        public int Count => _snapshot.Length;

        public void Add(long subscriptionId, long registrationOrder, Action<TEvent> handler,
            ShrinkSubscribeDescriptor descriptor)
        {
            if (handler == null)
                throw new ArgumentNullException(nameof(handler));

            lock (_gate)
            {
                _entries.Add(new HandlerEntry
                {
                    SubscriptionId = subscriptionId,
                    RegistrationOrder = registrationOrder,
                    SyncHandler = handler,
                    Descriptor = descriptor
                });
                RebuildSnapshot();
            }
        }

        public void Add(long subscriptionId, long registrationOrder,
            ShrinkAsyncEventHandler<TEvent> handler, ShrinkSubscribeDescriptor descriptor)
        {
            if (handler == null)
                throw new ArgumentNullException(nameof(handler));

            lock (_gate)
            {
                _entries.Add(new HandlerEntry
                {
                    SubscriptionId = subscriptionId,
                    RegistrationOrder = registrationOrder,
                    AsyncHandler = handler,
                    Descriptor = descriptor
                });
                RebuildSnapshot();
            }
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public ShrinkPostResult Post(in TEvent eventData)
        {
            var syncDispatcher = _syncDispatcher;
            if (syncDispatcher != null)
            {
                syncDispatcher(eventData);
                return new ShrinkPostResult(true, true, false);
            }

            var snapshot = _snapshot;
            var handled = false;
            for (var i = 0; i < snapshot.Length; i++)
            {
                var entry = snapshot[i];
                if (IsCanceled(eventData) && !entry.Descriptor.ReceiveCanceled)
                    continue;

                if (entry.SyncHandler != null)
                    entry.SyncHandler(eventData);
                else if (entry.AsyncHandler != null)
                    entry.AsyncHandler(eventData, CancellationToken.None).Forget(ShrinkEventDiagnostics.LogException);
                else
                    continue;
                handled = true;
            }

            return ShrinkPostResult.Completed(handled, IsCanceled(eventData));
        }

        public async UniTask<ShrinkPostResult> PostAsync(TEvent eventData, ShrinkDispatchMode dispatchMode,
            CancellationToken cancellationToken)
        {
            var snapshot = _snapshot;
            var handled = false;
            if (dispatchMode == ShrinkDispatchMode.Parallel)
            {
                var tasks = new List<UniTask>(snapshot.Length);
                for (var i = 0; i < snapshot.Length; i++)
                {
                    cancellationToken.ThrowIfCancellationRequested();
                    var entry = snapshot[i];
                    if (IsCanceled(eventData) && !entry.Descriptor.ReceiveCanceled)
                        continue;

                    if (entry.SyncHandler != null)
                        entry.SyncHandler(eventData);
                    else if (entry.AsyncHandler != null)
                        tasks.Add(entry.AsyncHandler(eventData, cancellationToken));
                    else
                        continue;
                    handled = true;
                }

                if (tasks.Count > 0)
                    await UniTask.WhenAll(tasks);
            }
            else
            {
                for (var i = 0; i < snapshot.Length; i++)
                {
                    cancellationToken.ThrowIfCancellationRequested();
                    var entry = snapshot[i];
                    if (IsCanceled(eventData) && !entry.Descriptor.ReceiveCanceled)
                        continue;

                    if (entry.SyncHandler != null)
                        entry.SyncHandler(eventData);
                    else if (entry.AsyncHandler != null)
                        await entry.AsyncHandler(eventData, cancellationToken);
                    else
                        continue;
                    handled = true;
                }
            }

            return ShrinkPostResult.Completed(handled, IsCanceled(eventData));
        }

        public bool RemoveSubscription(long subscriptionId)
        {
            lock (_gate)
            {
                var removed = _entries.RemoveAll(entry => entry.SubscriptionId == subscriptionId) > 0;
                RebuildSnapshot();
                return removed;
            }
        }

        public void Clear()
        {
            lock (_gate)
            {
                _entries.Clear();
                _snapshot = Array.Empty<HandlerEntry>();
                _syncDispatcher = null;
            }
        }

        public void DetachSlot(int slot) => ShrinkEventChannelSlots<TEvent>.Clear(slot, this);

        private void RebuildSnapshot()
        {
            _entries.Sort(static (left, right) =>
            {
                var leftOrder = left.Descriptor.NumericPriority == 0
                    ? (int)left.Descriptor.Priority * 1000
                    : -left.Descriptor.NumericPriority;
                var rightOrder = right.Descriptor.NumericPriority == 0
                    ? (int)right.Descriptor.Priority * 1000
                    : -right.Descriptor.NumericPriority;
                var priority = leftOrder.CompareTo(rightOrder);
                return priority != 0
                    ? priority
                    : left.RegistrationOrder.CompareTo(right.RegistrationOrder);
            });
            _snapshot = _entries.ToArray();

            Action<TEvent>? dispatcher = null;
            if (!SupportsCancellation)
            {
                for (var i = 0; i < _snapshot.Length; i++)
                {
                    var handler = _snapshot[i].SyncHandler;
                    if (handler == null)
                    {
                        dispatcher = null;
                        break;
                    }
                    dispatcher += handler;
                }
            }
            Volatile.Write(ref _syncDispatcher, dispatcher);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static bool IsCanceled(TEvent eventData)
        {
            if (!SupportsCancellation)
                return false;
            return ((IShrinkCancelableEvent)(object)eventData!).IsCanceled;
        }
    }
}