// 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> DefaultIfEmpty<TSource>(this IAsyncEnumerable<TSource> source, TSource defaultValue)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            return CreateEnumerable(
                () =>
                {
                    var done = false;
                    var hasElements = false;
                    var e = source.GetEnumerator();
                    var current = default(TSource);

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

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

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

        public static IAsyncEnumerable<TSource> DefaultIfEmpty<TSource>(this IAsyncEnumerable<TSource> source)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            return source.DefaultIfEmpty(default(TSource));
        }
    }
}