reactive/Ix.NET/Source/System.Interactive.Async/Except.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> Except<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 first.Except(second, EqualityComparer<TSource>.Default);
        }

        public static IAsyncEnumerable<TSource> Except<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
        {
            if (first == null)
                throw new ArgumentNullException(nameof(first));
            if (second == null)
                throw new ArgumentNullException(nameof(second));
            if (comparer == null)
                throw new ArgumentNullException(nameof(comparer));

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

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

                    var mapTask = default(Task<Dictionary<TSource, TSource>>);
                    var getMapTask = new Func<CancellationToken, Task<Dictionary<TSource, TSource>>>(
                        ct => mapTask ?? (mapTask = second.ToDictionary(x => x, comparer, ct)));

                    var f = default(Func<CancellationToken, Task<bool>>);
                    f = async ct =>
                        {
                            if (await e.MoveNext(ct)
                                       .Zip(getMapTask(ct), (b, _) => b)
                                       .ConfigureAwait(false))
                            {
                                if (!mapTask.Result.ContainsKey(e.Current))
                                    return true;
                                return await f(ct)
                                           .ConfigureAwait(false);
                            }
                            return false;
                        };

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