﻿/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace DotPulsar.Internal
{
    using Abstractions;
    using Exceptions;
    using System;
    using System.Collections.Generic;
    using System.Threading;
    using System.Threading.Tasks;

    public sealed class AsyncQueue<T> : IEnqueue<T>, IDequeue<T>, IDisposable
    {
        private readonly object _lock;
        private readonly Queue<T> _queue;
        private readonly LinkedList<CancelableCompletionSource<T>> _pendingDequeues;
        private int _isDisposed;

        public AsyncQueue()
        {
            _lock = new object();
            _queue = new Queue<T>();
            _pendingDequeues = new LinkedList<CancelableCompletionSource<T>>();
        }

        public void Enqueue(T item)
        {
            lock (_lock)
            {
                ThrowIfDisposed();

                if (_pendingDequeues.Count > 0)
                {
                    var tcs = _pendingDequeues.First;
                    _pendingDequeues.RemoveFirst();
                    tcs.Value.SetResult(item);
                    tcs.Value.Dispose();
                }
                else
                    _queue.Enqueue(item);
            }
        }

        public ValueTask<T> Dequeue(CancellationToken cancellationToken = default)
        {
            LinkedListNode<CancelableCompletionSource<T>>? node = null;

            lock (_lock)
            {
                ThrowIfDisposed();

                if (_queue.Count > 0)
                    return new ValueTask<T>(_queue.Dequeue());

                node = _pendingDequeues.AddLast(new CancelableCompletionSource<T>());
            }

            node.Value.SetupCancellation(() => Cancel(node), cancellationToken);
            return new ValueTask<T>(node.Value.Task);
        }

        public void Dispose()
        {
            lock (_lock)
            {
                if (Interlocked.Exchange(ref _isDisposed, 1) != 0)
                    return;

                foreach (var pendingDequeue in _pendingDequeues)
                    pendingDequeue.Dispose();

                _pendingDequeues.Clear();
                _queue.Clear();
            }
        }

        private void Cancel(LinkedListNode<CancelableCompletionSource<T>> node)
        {
            lock (_lock)
            {
                try
                {
                    node.Value.Dispose();
                    _pendingDequeues.Remove(node);
                }
                catch
                {
                    // ignored
                }
            }
        }

        private void ThrowIfDisposed()
        {
            if (_isDisposed != 0)
                throw new AsyncQueueDisposedException();
        }
    }
}