// 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.Collections.Generic;
using System.Runtime.CompilerServices;

namespace System.Threading.Tasks
{
    public static class AsyncEnumerableExtensions
    {
        public static ConfiguredAsyncEnumerable<T> ConfigureAwait<T>(this IAsyncEnumerable<T> enumerable, bool continueOnCapturedContext)
        {
            if (enumerable == null)
                throw Error.ArgumentNull(nameof(enumerable));

            return new ConfiguredAsyncEnumerable<T>(enumerable, continueOnCapturedContext);
        }

        public static ConfiguredAsyncEnumerable<T>.ConfiguredAsyncEnumerator ConfigureAwait<T>(this IAsyncEnumerator<T> enumerator, bool continueOnCapturedContext)
        {
            if (enumerator == null)
                throw Error.ArgumentNull(nameof(enumerator));

            return new ConfiguredAsyncEnumerable<T>.ConfiguredAsyncEnumerator(enumerator, continueOnCapturedContext);
        }

        // REVIEW: Explicit implementation of the interfaces allows for composition with other "modifier operators" such as WithCancellation.
        //         We expect that the "await foreach" statement will bind to the public struct methods, thus avoiding boxing.

        public readonly struct ConfiguredAsyncEnumerable<T> : IAsyncEnumerable<T>
        {
            private readonly IAsyncEnumerable<T> _enumerable;
            private readonly bool _continueOnCapturedContext;

            internal ConfiguredAsyncEnumerable(IAsyncEnumerable<T> enumerable, bool continueOnCapturedContext)
            {
                _enumerable = enumerable;
                _continueOnCapturedContext = continueOnCapturedContext;
            }

            public ConfiguredAsyncEnumerator GetAsyncEnumerator(CancellationToken cancellationToken)
            {
                cancellationToken.ThrowIfCancellationRequested(); // NB: [LDM-2018-11-28] Equivalent to async iterator behavior.

                return new ConfiguredAsyncEnumerator(_enumerable.GetAsyncEnumerator(cancellationToken), _continueOnCapturedContext);
            }

            IAsyncEnumerator<T> IAsyncEnumerable<T>.GetAsyncEnumerator(CancellationToken cancellationToken) =>
                GetAsyncEnumerator(cancellationToken);

            public readonly struct ConfiguredAsyncEnumerator : IAsyncEnumerator<T>
            {
                private readonly IAsyncEnumerator<T> _enumerator;
                private readonly bool _continueOnCapturedContext;

                internal ConfiguredAsyncEnumerator(IAsyncEnumerator<T> enumerator, bool continueOnCapturedContext)
                {
                    _enumerator = enumerator;
                    _continueOnCapturedContext = continueOnCapturedContext;
                }

                public ConfiguredValueTaskAwaitable<bool> MoveNextAsync() =>
                    _enumerator.MoveNextAsync().ConfigureAwait(_continueOnCapturedContext);

                public T Current => _enumerator.Current;

                public ConfiguredValueTaskAwaitable DisposeAsync() =>
                    _enumerator.DisposeAsync().ConfigureAwait(_continueOnCapturedContext);

                async ValueTask<bool> IAsyncEnumerator<T>.MoveNextAsync() =>
                    await _enumerator.MoveNextAsync().ConfigureAwait(_continueOnCapturedContext);

                async ValueTask IAsyncDisposable.DisposeAsync() =>
                    await _enumerator.DisposeAsync().ConfigureAwait(_continueOnCapturedContext);
            }
        }
    }
}
