reactive/Ix.NET/Source/System.Interactive.Async/Where.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> Where<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (predicate == null)
                throw new ArgumentNullException(nameof(predicate));

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

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

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

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

        public static IAsyncEnumerable<TSource> Where<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int, bool> predicate)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (predicate == null)
                throw new ArgumentNullException(nameof(predicate));

            return CreateEnumerable(
                () =>
                {
                    var e = source.GetEnumerator();
                    var index = 0;

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

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

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