reactive/Ix.NET/Source/System.Interactive.Async/Concatenate.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;
using System.Threading.Tasks;

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

            return CreateEnumerable(
                () =>
                {
                    var switched = false;
                    var e = first.GetEnumerator();

                    var cts = new CancellationTokenDisposable();
                    var a = new AssignableDisposable
                    {
                        Disposable = e
                    };
                    var d = Disposable.Create(cts, a);

                    var f = default(Func<CancellationToken, Task<bool>>);
                    f = async ct =>
                        {
                            if (await e.MoveNext(ct)
                                       .ConfigureAwait(false))
                            {
                                return true;
                            }
                            if (switched)
                            {
                                return false;
                            }
                            switched = true;

                            e = second.GetEnumerator();
                            a.Disposable = e;

                            return await f(ct)
                                       .ConfigureAwait(false);
                        };

                    return CreateEnumerator(
                        f,
                        () => e.Current,
                        d.Dispose,
                        e
                    );
                });
        }

        public static IAsyncEnumerable<TSource> Concat<TSource>(this IEnumerable<IAsyncEnumerable<TSource>> sources)
        {
            if (sources == null)
                throw new ArgumentNullException(nameof(sources));

            return sources.Concat_();
        }

        public static IAsyncEnumerable<TSource> Concat<TSource>(params IAsyncEnumerable<TSource>[] sources)
        {
            if (sources == null)
                throw new ArgumentNullException(nameof(sources));

            return sources.Concat_();
        }

        private static IAsyncEnumerable<TSource> Concat_<TSource>(this IEnumerable<IAsyncEnumerable<TSource>> sources)
        {
            return CreateEnumerable(
                () =>
                {
                    var se = sources.GetEnumerator();
                    var e = default(IAsyncEnumerator<TSource>);

                    var cts = new CancellationTokenDisposable();
                    var a = new AssignableDisposable();
                    var d = Disposable.Create(cts, se, a);

                    var f = default(Func<CancellationToken, Task<bool>>);
                    f = async ct =>
                        {
                            if (e == null)
                            {
                                var b = false;
                                b = se.MoveNext();
                                if (b)
                                    e = se.Current.GetEnumerator();

                                if (!b)
                                {
                                    return false;
                                }

                                a.Disposable = e;
                            }

                            if (await e.MoveNext(ct)
                                       .ConfigureAwait(false))
                            {
                                return true;
                            }
                            e.Dispose();
                            e = null;

                            return await f(ct)
                                       .ConfigureAwait(false);
                        };

                    return CreateEnumerator(
                        f,
                        () => e.Current,
                        d.Dispose,
                        a
                    );
                });
        }
    }
}