reactive/Ix.NET/Source/System.Interactive.Async/System/Linq/Operators/Max.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.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace System.Linq
{
    public static partial class AsyncEnumerableEx
    {
        public static ValueTask<TSource> MaxAsync<TSource>(this IAsyncEnumerable<TSource> source, IComparer<TSource>? comparer, CancellationToken cancellationToken = default)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));

            return Core(source, comparer, cancellationToken);

            static async ValueTask<TSource> Core(IAsyncEnumerable<TSource> source, IComparer<TSource>? comparer, CancellationToken cancellationToken)
            {
                comparer ??= Comparer<TSource>.Default;

                await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);

                if (!await e.MoveNextAsync())
                    throw Error.NoElements();

                var max = e.Current;

                while (await e.MoveNextAsync())
                {
                    var cur = e.Current;

                    if (comparer.Compare(cur, max) > 0)
                    {
                        max = cur;
                    }
                }

                return max;
            }
        }
    }
}