#nullable enable

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

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

    public interface IShrinkEventBus : IDisposable
    {
        ShrinkBusKey Key { get; }
        ShrinkBusOptions Options { get; }
        ShrinkPostResult Post<TEvent>(in TEvent eventData) where TEvent : IShrinkEvent;
        ValueTask<ShrinkPostResult> PostAsync<TEvent>(TEvent eventData,
            CancellationToken cancellationToken = default) where TEvent : IShrinkEvent;
        IDisposable Attach(object target);
    }

    public interface IShrinkBusResolver
    {
        IShrinkEventBus GetBus(ShrinkBusKey key);
        bool TryGetBus(ShrinkBusKey key, out IShrinkEventBus bus);
    }

    public interface IShrinkGeneratedSubscriber
    {
        IDisposable AttachGenerated(IShrinkBusResolver resolver, ShrinkBusKey? defaultBus = null);
    }

    public sealed class ShrinkEventBinding : IDisposable
    {
        private readonly List<IDisposable> _items = new List<IDisposable>();
        private bool _disposed;

        public void Add(IDisposable item)
        {
            if (item == null)
                throw new ArgumentNullException(nameof(item));
            if (_disposed)
            {
                item.Dispose();
                throw new ObjectDisposedException(nameof(ShrinkEventBinding));
            }
            _items.Add(item);
        }

        public void Dispose()
        {
            if (_disposed)
                return;
            _disposed = true;
            for (var i = _items.Count - 1; i >= 0; i--)
                _items[i].Dispose();
            _items.Clear();
        }
    }

    public static class ShrinkStaticBindingRegistry
    {
        internal sealed class Entry
        {
            public long Id;
            public ShrinkBusKey Key;
            public Func<IShrinkBusResolver, IDisposable> Factory = null!;
        }

        private static readonly object Gate = new object();
        private static readonly List<Entry> Entries = new List<Entry>();
        private static readonly List<WeakReference<ShrinkEventBusHost>> Hosts =
            new List<WeakReference<ShrinkEventBusHost>>();
        private static long _nextId;

        public static void Register(ShrinkBusKey key,
            Func<IShrinkBusResolver, IDisposable> factory)
        {
            if (factory == null)
                throw new ArgumentNullException(nameof(factory));

            ShrinkEventBusHost[] hosts;
            Entry entry;
            lock (Gate)
            {
                entry = new Entry
                {
                    Id = Interlocked.Increment(ref _nextId),
                    Key = key,
                    Factory = factory
                };
                Entries.Add(entry);
                hosts = LiveHostsLocked();
            }

            for (var i = 0; i < hosts.Length; i++)
                hosts[i].TryAttachStatic(entry);
        }

        internal static void TrackHost(ShrinkEventBusHost host)
        {
            lock (Gate)
            {
                LiveHostsLocked();
                Hosts.Add(new WeakReference<ShrinkEventBusHost>(host));
            }
        }

        internal static Entry[] GetEntries(ShrinkBusKey key)
        {
            lock (Gate)
                return Entries.FindAll(item => item.Key == key).ToArray();
        }

        private static ShrinkEventBusHost[] LiveHostsLocked()
        {
            var result = new List<ShrinkEventBusHost>(Hosts.Count);
            for (var i = Hosts.Count - 1; i >= 0; i--)
            {
                if (Hosts[i].TryGetTarget(out var host))
                    result.Add(host);
                else
                    Hosts.RemoveAt(i);
            }
            return result.ToArray();
        }
    }

    public sealed class ShrinkEventBusHost : IShrinkBusResolver, IDisposable
    {
        private readonly ConcurrentDictionary<ShrinkBusKey, IShrinkEventBus> _buses =
            new ConcurrentDictionary<ShrinkBusKey, IShrinkEventBus>();
        private readonly object _staticGate = new object();
        private readonly Dictionary<long, IDisposable> _staticBindings =
            new Dictionary<long, IDisposable>();
        private bool _disposed;

        public ShrinkEventBusHost()
        {
            ShrinkStaticBindingRegistry.TrackHost(this);
        }

        public IShrinkEventBus CreateBus(ShrinkBusKey key, ShrinkBusOptions options)
        {
            if (_disposed)
                throw new ObjectDisposedException(nameof(ShrinkEventBusHost));
            var bus = new DotNetShrinkEventBus(key, options);
            if (!_buses.TryAdd(key, bus))
            {
                bus.Dispose();
                throw new InvalidOperationException($"Bus '{key}' is already registered.");
            }
            var entries = ShrinkStaticBindingRegistry.GetEntries(key);
            for (var i = 0; i < entries.Length; i++)
                TryAttachStatic(entries[i]);
            return bus;
        }

        public IShrinkEventBus GetBus(ShrinkBusKey key)
        {
            if (_buses.TryGetValue(key, out var bus))
                return bus;
            throw new KeyNotFoundException($"Bus '{key}' is not registered.");
        }

        public bool TryGetBus(ShrinkBusKey key, out IShrinkEventBus bus) =>
            _buses.TryGetValue(key, out bus!);

        public IDisposable Attach(object target, ShrinkBusKey? defaultBus = null)
        {
            if (target is not IShrinkGeneratedSubscriber generated)
                throw new InvalidOperationException(
                    $"Type {target?.GetType().FullName ?? "<null>"} has no generated event binding.");
            return generated.AttachGenerated(this, defaultBus);
        }

        public void Dispose()
        {
            if (_disposed)
                return;
            _disposed = true;
            lock (_staticGate)
            {
                foreach (var binding in _staticBindings.Values)
                    binding.Dispose();
                _staticBindings.Clear();
            }
            foreach (var bus in _buses.Values)
                bus.Dispose();
            _buses.Clear();
        }

        internal void TryAttachStatic(ShrinkStaticBindingRegistry.Entry entry)
        {
            lock (_staticGate)
            {
                if (_disposed || _staticBindings.ContainsKey(entry.Id) || !_buses.ContainsKey(entry.Key))
                    return;
                _staticBindings.Add(entry.Id, entry.Factory(this));
            }
        }
    }

    public static class ShrinkGeneratedBinding
    {
        public static IDisposable Subscribe<TEvent>(IShrinkBusResolver resolver, ShrinkBusKey? defaultBus,
            string? configuredBus, object? owner, Action<TEvent> handler, ShrinkEventPriority priority,
            int numericPriority, bool receiveCanceled) where TEvent : IShrinkEvent
        {
            return Resolve(resolver, defaultBus, configuredBus).Subscribe(
                owner, handler, priority, numericPriority, receiveCanceled);
        }

        public static IDisposable SubscribeAsync<TEvent>(IShrinkBusResolver resolver, ShrinkBusKey? defaultBus,
            string? configuredBus, object? owner, ShrinkAsyncEventHandler<TEvent> handler,
            ShrinkEventPriority priority, int numericPriority, bool receiveCanceled) where TEvent : IShrinkEvent
        {
            return Resolve(resolver, defaultBus, configuredBus).SubscribeAsync(
                owner, handler, priority, numericPriority, receiveCanceled);
        }

        private static DotNetShrinkEventBus Resolve(IShrinkBusResolver resolver, ShrinkBusKey? defaultBus,
            string? configuredBus)
        {
            var key = string.IsNullOrWhiteSpace(configuredBus)
                ? defaultBus ?? ShrinkBusKey.Game
                : ShrinkBusKey.Parse(configuredBus);
            return resolver.GetBus(key) as DotNetShrinkEventBus
                ?? throw new InvalidOperationException("Generated bindings require the built-in bus implementation.");
        }
    }

    internal sealed class DotNetShrinkEventBus : IShrinkEventBus
    {
        private readonly ConcurrentDictionary<Type, IEventChannel> _channels =
            new ConcurrentDictionary<Type, IEventChannel>();
        private readonly IDotNetScheduler _scheduler;
        private long _nextSubscriptionId;
        private bool _disposed;

        public DotNetShrinkEventBus(ShrinkBusKey key, ShrinkBusOptions options)
        {
            Key = key;
            Options = (options ?? throw new ArgumentNullException(nameof(options))).CloneValidated();
            _scheduler = DotNetSchedulerFactory.Create(key.Id, Options);
        }

        public ShrinkBusKey Key { get; }
        public ShrinkBusOptions Options { get; }

        public ShrinkPostResult Post<TEvent>(in TEvent eventData) where TEvent : IShrinkEvent
        {
            if (_disposed)
                return ShrinkPostResult.Rejected(ShrinkPostFailure.BusStopped);
            var copied = eventData;
            if (_scheduler.IsOnSchedulerThread)
                return Channel<TEvent>().Post(copied);

            return _scheduler.TryPost(() => Channel<TEvent>().Post(copied))
                ? new ShrinkPostResult(true, false, false)
                : ShrinkPostResult.Rejected(ShrinkPostFailure.QueueFull);
        }

        public async ValueTask<ShrinkPostResult> PostAsync<TEvent>(TEvent eventData,
            CancellationToken cancellationToken = default) where TEvent : IShrinkEvent
        {
            if (_disposed)
                return ShrinkPostResult.Rejected(ShrinkPostFailure.BusStopped);
            ShrinkPostResult result = default;
            try
            {
                await _scheduler.PostAsync(async () =>
                {
                    result = await Channel<TEvent>().PostAsync(eventData, Options.DispatchMode, cancellationToken)
                        .ConfigureAwait(false);
                }, cancellationToken).ConfigureAwait(false);
                return result;
            }
            catch (QueueFullException)
            {
                return ShrinkPostResult.Rejected(ShrinkPostFailure.QueueFull);
            }
            catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
            {
                return new ShrinkPostResult(true, false, false, ShrinkPostFailure.Canceled);
            }
        }

        public IDisposable Attach(object target)
        {
            if (target is not IShrinkGeneratedSubscriber generated)
                throw new InvalidOperationException(
                    $"Type {target?.GetType().FullName ?? "<null>"} has no generated event binding.");
            return generated.AttachGenerated(new SingleBusResolver(this), Key);
        }

        public IDisposable Subscribe<TEvent>(object? owner, Action<TEvent> handler,
            ShrinkEventPriority priority, int numericPriority, bool receiveCanceled) where TEvent : IShrinkEvent
        {
            return Channel<TEvent>().Add(Interlocked.Increment(ref _nextSubscriptionId), owner,
                handler, null, priority, numericPriority, receiveCanceled);
        }

        public IDisposable SubscribeAsync<TEvent>(object? owner, ShrinkAsyncEventHandler<TEvent> handler,
            ShrinkEventPriority priority, int numericPriority, bool receiveCanceled) where TEvent : IShrinkEvent
        {
            return Channel<TEvent>().Add(Interlocked.Increment(ref _nextSubscriptionId), owner,
                null, handler, priority, numericPriority, receiveCanceled);
        }

        public void Dispose()
        {
            if (_disposed)
                return;
            _disposed = true;
            _scheduler.Dispose();
            foreach (var channel in _channels.Values)
                channel.Clear();
            _channels.Clear();
        }

        private EventChannel<TEvent> Channel<TEvent>() where TEvent : IShrinkEvent =>
            (EventChannel<TEvent>)_channels.GetOrAdd(typeof(TEvent), _ => new EventChannel<TEvent>());

        private sealed class SingleBusResolver : IShrinkBusResolver
        {
            private readonly IShrinkEventBus _bus;
            public SingleBusResolver(IShrinkEventBus bus) => _bus = bus;
            public IShrinkEventBus GetBus(ShrinkBusKey key) => key == _bus.Key
                ? _bus
                : throw new KeyNotFoundException($"Bus '{key}' is not available.");
            public bool TryGetBus(ShrinkBusKey key, out IShrinkEventBus bus)
            {
                bus = _bus;
                return key == _bus.Key;
            }
        }
    }

    internal interface IEventChannel
    {
        void Clear();
    }

    internal sealed class EventChannel<TEvent> : IEventChannel where TEvent : IShrinkEvent
    {
        private sealed class Entry
        {
            public long Id = 0;
            public object? Owner;
            public Action<TEvent>? Sync;
            public ShrinkAsyncEventHandler<TEvent>? Async;
            public ShrinkEventPriority Priority;
            public int NumericPriority;
            public bool ReceiveCanceled;
        }

        private readonly object _gate = new object();
        private readonly List<Entry> _entries = new List<Entry>();
        private Entry[] _snapshot = Array.Empty<Entry>();

        public IDisposable Add(long id, object? owner, Action<TEvent>? sync,
            ShrinkAsyncEventHandler<TEvent>? asyncHandler, ShrinkEventPriority priority,
            int numericPriority, bool receiveCanceled)
        {
            lock (_gate)
            {
                _entries.Add(new Entry
                {
                    Id = id,
                    Owner = owner,
                    Sync = sync,
                    Async = asyncHandler,
                    Priority = priority,
                    NumericPriority = numericPriority,
                    ReceiveCanceled = receiveCanceled
                });
                Rebuild();
            }
            return new Subscription(this, id);
        }

        public ShrinkPostResult Post(TEvent eventData)
        {
            var handled = false;
            var snapshot = _snapshot;
            for (var i = 0; i < snapshot.Length; i++)
            {
                var entry = snapshot[i];
                if (IsCanceled(eventData) && !entry.ReceiveCanceled)
                    continue;
                if (entry.Sync != null)
                    entry.Sync(eventData);
                else if (entry.Async != null)
                    _ = entry.Async(eventData, CancellationToken.None).AsTask();
                else
                    continue;
                handled = true;
            }
            return ShrinkPostResult.Completed(handled, IsCanceled(eventData));
        }

        public async ValueTask<ShrinkPostResult> PostAsync(TEvent eventData, ShrinkDispatchMode mode,
            CancellationToken cancellationToken)
        {
            var handled = false;
            var snapshot = _snapshot;
            if (mode == ShrinkDispatchMode.Parallel)
            {
                var tasks = new List<Task>();
                for (var i = 0; i < snapshot.Length; i++)
                {
                    var entry = snapshot[i];
                    if (IsCanceled(eventData) && !entry.ReceiveCanceled)
                        continue;
                    if (entry.Sync != null)
                        entry.Sync(eventData);
                    else if (entry.Async != null)
                        tasks.Add(entry.Async(eventData, cancellationToken).AsTask());
                    else
                        continue;
                    handled = true;
                }
                if (tasks.Count > 0)
                    await Task.WhenAll(tasks).ConfigureAwait(false);
            }
            else
            {
                for (var i = 0; i < snapshot.Length; i++)
                {
                    cancellationToken.ThrowIfCancellationRequested();
                    var entry = snapshot[i];
                    if (IsCanceled(eventData) && !entry.ReceiveCanceled)
                        continue;
                    if (entry.Sync != null)
                        entry.Sync(eventData);
                    else if (entry.Async != null)
                        await entry.Async(eventData, cancellationToken).ConfigureAwait(false);
                    else
                        continue;
                    handled = true;
                }
            }
            return ShrinkPostResult.Completed(handled, IsCanceled(eventData));
        }

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

        private void Remove(long id)
        {
            lock (_gate)
            {
                _entries.RemoveAll(item => item.Id == id);
                Rebuild();
            }
        }

        private void Rebuild()
        {
            _entries.Sort((left, right) =>
            {
                var leftValue = left.NumericPriority == 0 ? (int)left.Priority * 1000 : -left.NumericPriority;
                var rightValue = right.NumericPriority == 0 ? (int)right.Priority * 1000 : -right.NumericPriority;
                var result = leftValue.CompareTo(rightValue);
                return result != 0 ? result : left.Id.CompareTo(right.Id);
            });
            _snapshot = _entries.ToArray();
        }

        private static bool IsCanceled(TEvent value) =>
            value is IShrinkCancelableEvent cancelable && cancelable.IsCanceled;

        private sealed class Subscription : IDisposable
        {
            private EventChannel<TEvent>? _owner;
            private readonly long _id;
            public Subscription(EventChannel<TEvent> owner, long id)
            {
                _owner = owner;
                _id = id;
            }
            public void Dispose() => Interlocked.Exchange(ref _owner, null)?.Remove(_id);
        }
    }
}