// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the Apache 2.0 License.
// See the LICENSE file in the project root for more information. 

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

namespace System.Reactive
{
    internal sealed class FastImmediateAsyncObserver<T> : IScheduledAsyncObserver<T>
    {
        private readonly IAsyncObserver<T> _observer;

        private readonly AsyncLock _lock = new AsyncLock();
        private readonly Queue<T> _queue = new Queue<T>();

        private bool _hasFaulted = false;
        private bool _busy = false;
        private bool _done;
        private Exception _error;

        public FastImmediateAsyncObserver(IAsyncObserver<T> observer)
        {
            _observer = observer;
        }

        public Task EnsureActive() => EnsureActive(1);

        public async Task EnsureActive(int count)
        {
            var shouldRun = false;

            using (await _lock.LockAsync().ConfigureAwait(false))
            {
                if (!_hasFaulted && !_busy)
                {
                    _busy = true;
                    shouldRun = true;
                }
            }

            if (shouldRun)
            {
                while (true)
                {
                    var values = default(T[]);
                    var error = default(Exception);
                    var done = false;

                    using (await _lock.LockAsync().ConfigureAwait(false))
                    {
                        if (_queue.Count > 0)
                        {
                            values = _queue.ToArray();
                            _queue.Clear();
                        }

                        if (_done)
                        {
                            done = _done;
                            error = _error;
                        }

                        if (values == null && !done)
                        {
                            _busy = false;
                            break;
                        }
                    }

                    try
                    {
                        if (values != null)
                        {
                            foreach (var value in values)
                            {
                                await _observer.OnNextAsync(value).ConfigureAwait(false);
                            }
                        }

                        if (done)
                        {
                            if (error != null)
                            {
                                await _observer.OnErrorAsync(error).ConfigureAwait(false);
                            }
                            else
                            {
                                await _observer.OnCompletedAsync().ConfigureAwait(false);
                            }

                            break;
                        }
                    }
                    catch
                    {
                        using (await _lock.LockAsync().ConfigureAwait(false))
                        {
                            _hasFaulted = true;
                            _queue.Clear();
                        }

                        throw;
                    }
                }
            }
        }

        public async Task OnCompletedAsync()
        {
            using (await _lock.LockAsync().ConfigureAwait(false))
            {
                if (!_hasFaulted)
                {
                    _done = true;
                }
            }
        }

        public async Task OnErrorAsync(Exception error)
        {
            using (await _lock.LockAsync().ConfigureAwait(false))
            {
                if (!_hasFaulted)
                {
                    _done = true;
                    _error = error;
                }
            }
        }

        public async Task OnNextAsync(T value)
        {
            using (await _lock.LockAsync().ConfigureAwait(false))
            {
                if (!_hasFaulted)
                {
                    _queue.Enqueue(value);
                }
            }
        }
    }
}