reactive/Ix.NET/Source/System.Interactive.Async/Reverse.cs

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

namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
        public static IAsyncEnumerable<TSource> Reverse<TSource>(this IAsyncEnumerable<TSource> source)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            return CreateEnumerable(
                () =>
                {
                    var e = source.GetEnumerator();
                    var stack = default(Stack<TSource>);

                    var cts = new CancellationTokenDisposable();
                    var d = Disposable.Create(cts, e);

                    return CreateEnumerator(
                        async ct =>
                        {
                            if (stack == null)
                            {
                                stack = await CreateEnumerable(
                                                () => e)
                                            .Aggregate(new Stack<TSource>(), (s, x) =>
                                                                             {
                                                                                 s.Push(x);
                                                                                 return s;
                                                                             }, cts.Token)
                                            .ConfigureAwait(false);
                                return stack.Count > 0;
                            }
                            stack.Pop();
                            return stack.Count > 0;
                        },
                        () => stack.Peek(),
                        d.Dispose,
                        e
                    );
                });
        }
    }
}