// ==========================================================================
//  Squidex Headless CMS
// ==========================================================================
//  Copyright (c) Squidex UG (haftungsbeschraenkt)
//  All rights reserved. Licensed under the MIT license.
// ==========================================================================

using System.Threading.Channels;
using Squidex.Infrastructure.Tasks;

namespace Squidex.Infrastructure.EventSourcing.Consume
{
    internal sealed class BatchSubscription : IEventSubscriber<ParsedEvent>, IEventSubscription
    {
        private readonly IEventSubscription eventSubscription;
        private readonly Channel<object> taskQueue;
        private readonly Channel<object> batchQueue;
        private readonly Task handleTask;
        private readonly CancellationTokenSource completed = new CancellationTokenSource();

        public BatchSubscription(
            IEventConsumer eventConsumer,
            IEventSubscriber<ParsedEvents> eventSubscriber,
            EventSubscriptionSource<ParsedEvent> eventSource)
        {
            var batchSize = Math.Max(1, eventConsumer.BatchSize);
            var batchDelay = Math.Max(100, eventConsumer.BatchDelay);

            taskQueue = Channel.CreateBounded<object>(new BoundedChannelOptions(2)
            {
                SingleReader = true,
                SingleWriter = true
            });

            batchQueue = Channel.CreateBounded<object>(new BoundedChannelOptions(batchSize)
            {
                AllowSynchronousContinuations = true,
                SingleReader = true,
                SingleWriter = true
            });

            batchQueue.Batch<ParsedEvent>(taskQueue, batchSize, batchDelay, completed.Token);

            handleTask = Run(eventSubscriber);

            // Run last to subscribe after everything is configured.
            eventSubscription = eventSource(this);
        }

        private async Task Run(IEventSubscriber<ParsedEvents> eventSink)
        {
            try
            {
                var isStopped = false;

                await foreach (var task in taskQueue.Reader.ReadAllAsync(completed.Token))
                {
                    switch (task)
                    {
                        case Exception exception when exception is not OperationCanceledException:
                            {
                                if (!completed.IsCancellationRequested)
                                {
                                    await eventSink.OnErrorAsync(this, exception);
                                }

                                isStopped = true;
                                break;
                            }

                        case List<ParsedEvent> batch:
                            {
                                if (!completed.IsCancellationRequested)
                                {
                                    // Events can be null if the event consumer is not interested in the stored event.
                                    var eventList = batch.Select(x => x.Event).NotNull().ToList();
                                    var eventPosition = batch[^1].Position;

                                    // Use a struct here to save a few allocations.
                                    await eventSink.OnNextAsync(this, new ParsedEvents(eventList, eventPosition));
                                }

                                break;
                            }
                    }

                    if (isStopped)
                    {
                        break;
                    }
                }
            }
            catch (OperationCanceledException)
            {
                return;
            }
            catch (Exception ex)
            {
                if (!completed.IsCancellationRequested)
                {
                    await eventSink.OnErrorAsync(this, ex);
                }
            }
        }

        public void Dispose()
        {
            if (completed.IsCancellationRequested)
            {
                return;
            }

            // It is not necessary to dispose the cancellation token source.
            completed.Cancel();

            // We do not lock here, it is the responsibility of the source subscription to be thread safe.
            eventSubscription.Dispose();
        }

        public async ValueTask CompleteAsync()
        {
            await eventSubscription.CompleteAsync();

            batchQueue.Writer.TryComplete();

            await handleTask;
        }

        public void WakeUp()
        {
            eventSubscription.WakeUp();
        }

        async ValueTask IEventSubscriber<ParsedEvent>.OnErrorAsync(IEventSubscription subscription, Exception exception)
        {
            try
            {
                // Forward the exception from one task only, but bypass the batch.
                await taskQueue.Writer.WriteAsync(exception, completed.Token);
            }
            catch (ChannelClosedException)
            {
                // This exception is acceptable and happens when an exception has been thrown before.
                return;
            }
        }

        async ValueTask IEventSubscriber<ParsedEvent>.OnNextAsync(IEventSubscription subscription, ParsedEvent @event)
        {
            try
            {
                await batchQueue.Writer.WriteAsync(@event, completed.Token);
            }
            catch (ChannelClosedException)
            {
                // This exception is acceptable and happens when an exception has been thrown before.
                return;
            }
        }
    }
}