// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT 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 AsyncEnumerable
    {
#if INCLUDE_SYSTEM_LINQ_ASYNCENUMERABLE_DUPLICATES
        // The next two methods are replaced by a single method in System.Linq.AsyncEnumerable:
        // https://learn.microsoft.com/en-us/dotnet/api/system.linq.asyncenumerable.groupby?view=net-9.0-pp#system-linq-asyncenumerable-groupby-2(system-collections-generic-iasyncenumerable((-0))-system-func((-0-1))-system-collections-generic-iequalitycomparer((-1)))
        // It has a different signature from both:
        //  Returns an IAsyncEnumerable<IGrouping<TKey, TElement>>, which is not the same as IAsyncGrouping<TKey, TElement>
        //  Supplies a default value of null for the comparer
        // That second difference is why there's only the one overload.
        // The first difference seems large: IAsyncGrouping returns each group as an IAsyncEnumerable. In practice,
        // the grouping operators enumerate the source to completion before returning anything so in practice this
        // async capability is not used, and just complicates things for the consumer.

        /// <summary>
        /// Groups the elements of an async-enumerable sequence according to a specified key selector function.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">A function to extract the key for each element.</param>
        /// <returns>A sequence of async-enumerable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> is null.</exception>
        public static IAsyncEnumerable<IAsyncGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector) =>
            new GroupedAsyncEnumerable<TSource, TKey>(source, keySelector, comparer: null);

        /// <summary>
        /// Groups the elements of an async-enumerable sequence according to a specified key selector function and comparer.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">A function to extract the key for each element.</param>
        /// <param name="comparer">An equality comparer to compare keys with.</param>
        /// <returns>A sequence of async-enumerable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="comparer"/> is null.</exception>
        public static IAsyncEnumerable<IAsyncGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedAsyncEnumerable<TSource, TKey>(source, keySelector, comparer);
#endif // INCLUDE_SYSTEM_LINQ_ASYNCENUMERABLE_DUPLICATES

        /// <summary>
        /// Groups the elements of an async-enumerable sequence according to a specified key selector function.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">An asynchronous function to extract the key for each element.</param>
        /// <returns>A sequence of async-enumerable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> is <see langword="null"/>.</exception>
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwait functionality now exists as overloads of GroupBy. You will need to modify your callback to take an additional CancellationToken argument.")]
        private static IAsyncEnumerable<IAsyncGrouping<TKey, TSource>> GroupByAwaitCore<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector) =>
            new GroupedAsyncEnumerableWithTask<TSource, TKey>(source, keySelector, comparer: null);

        /// <summary>
        /// Groups the elements of an async-enumerable sequence according to a specified key selector function and comparer.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">An asynchronous function to extract the key for each element.</param>
        /// <param name="comparer">An equality comparer to compare keys with.</param>
        /// <returns>A sequence of async-enumerable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="comparer"/> is <see langword="null"/>.</exception>
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwait functionality now exists as overloads of GroupBy. You will need to modify your callback to take an additional CancellationToken argument.")]
        private static IAsyncEnumerable<IAsyncGrouping<TKey, TSource>> GroupByAwaitCore<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedAsyncEnumerableWithTask<TSource, TKey>(source, keySelector, comparer);

#if !NO_DEEP_CANCELLATION
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwaitWithCancellationAsync functionality now exists as overloads of GroupBy.")]
        private static IAsyncEnumerable<IAsyncGrouping<TKey, TSource>> GroupByAwaitWithCancellationCore<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector) =>
            new GroupedAsyncEnumerableWithTaskAndCancellation<TSource, TKey>(source, keySelector, comparer: null);

        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwaitWithCancellationAsync functionality now exists as overloads of GroupBy.")]
        private static IAsyncEnumerable<IAsyncGrouping<TKey, TSource>> GroupByAwaitWithCancellationCore<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedAsyncEnumerableWithTaskAndCancellation<TSource, TKey>(source, keySelector, comparer);
#endif
#if INCLUDE_SYSTEM_LINQ_ASYNCENUMERABLE_DUPLICATES
        // The next two methods are replaced by a single method in System.Linq.AsyncEnumerable:
        // https://learn.microsoft.com/en-us/dotnet/api/system.linq.asyncenumerable.groupby?view=net-9.0-pp#system-linq-asyncenumerable-groupby-3(system-collections-generic-iasyncenumerable((-0))-system-func((-0-1))-system-func((-0-2))-system-collections-generic-iequalitycomparer((-1)))
        // It has a different signature from both:
        //  Returns an IAsyncEnumerable<IGrouping<TKey, TElement>>, which is not the same as IAsyncGrouping<TKey, TElement>
        //  Supplies a default value of null for the comparer
        // That second difference is why there's only the one overload.
        // The first difference seems large: IAsyncGrouping returns each group as an IAsyncEnumerable. In practice,
        // the grouping operators enumerate the source to completion before returning anything so in practice this
        // async capability is not used, and just complicates things for the consumer.

        /// <summary>
        /// Groups the elements of an async-enumerable sequence and selects the resulting elements by using a specified function.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <typeparam name="TElement">The type of the elements within the groups computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">A function to extract the key for each element.</param>
        /// <param name="elementSelector">A function to map each source element to an element in an async-enumerable group.</param>
        /// <returns>A sequence of async-enumerable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="elementSelector"/> is null.</exception>
        public static IAsyncEnumerable<IAsyncGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector) =>
            new GroupedAsyncEnumerable<TSource, TKey, TElement>(source, keySelector, elementSelector, comparer: null);

        /// <summary>
        /// Groups the elements of an async-enumerable sequence according to a specified key selector function and comparer and selects the resulting elements by using a specified function.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <typeparam name="TElement">The type of the elements within the groups computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">A function to extract the key for each element.</param>
        /// <param name="elementSelector">A function to map each source element to an element in an async-enumerable group.</param>
        /// <param name="comparer">An equality comparer to compare keys with.</param>
        /// <returns>A sequence of async-enumerable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="elementSelector"/> or <paramref name="comparer"/> is null.</exception>
        public static IAsyncEnumerable<IAsyncGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedAsyncEnumerable<TSource, TKey, TElement>(source, keySelector, elementSelector, comparer);
#endif // INCLUDE_SYSTEM_LINQ_ASYNCENUMERABLE_DUPLICATES

        /// <summary>
        /// Groups the elements of an async-enumerable sequence and selects the resulting elements by using a specified function.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <typeparam name="TElement">The type of the elements within the groups computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">An asynchronous function to extract the key for each element.</param>
        /// <param name="elementSelector">An asynchronous function to map each source element to an element in an async-enumerable group.</param>
        /// <returns>A sequence of async-enumerable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="elementSelector"/> is <see langword="null"/>.</exception>
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwait functionality now exists as overloads of GroupBy. You will need to modify your callback to take an additional CancellationToken argument.")]
        private static IAsyncEnumerable<IAsyncGrouping<TKey, TElement>> GroupByAwaitCore<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, Func<TSource, ValueTask<TElement>> elementSelector) =>
            new GroupedAsyncEnumerableWithTask<TSource, TKey, TElement>(source, keySelector, elementSelector, comparer: null);

        /// <summary>
        /// Groups the elements of an async-enumerable sequence and selects the resulting elements by using a specified function.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <typeparam name="TElement">The type of the elements within the groups computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">An asynchronous function to extract the key for each element.</param>
        /// <param name="elementSelector">An asynchronous function to map each source element to an element in an async-enumerable group.</param>
        /// <param name="comparer">An equality comparer to use to compare keys.</param>
        /// <returns>A sequence of async-enumerable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="elementSelector"/> or <paramref name="comparer"/> is <see langword="null"/>.</exception>
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwait functionality now exists as overloads of GroupBy. You will need to modify your callback to take an additional CancellationToken argument.")]
        private static IAsyncEnumerable<IAsyncGrouping<TKey, TElement>> GroupByAwaitCore<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, Func<TSource, ValueTask<TElement>> elementSelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedAsyncEnumerableWithTask<TSource, TKey, TElement>(source, keySelector, elementSelector, comparer);

#if !NO_DEEP_CANCELLATION
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwaitWithCancellationAsync functionality now exists as overloads of GroupBy.")]
        private static IAsyncEnumerable<IAsyncGrouping<TKey, TElement>> GroupByAwaitWithCancellationCore<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TSource, CancellationToken, ValueTask<TElement>> elementSelector) =>
            new GroupedAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TElement>(source, keySelector, elementSelector, comparer: null);

        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwaitWithCancellationAsync functionality now exists as overloads of GroupBy.")]
        private static IAsyncEnumerable<IAsyncGrouping<TKey, TElement>> GroupByAwaitWithCancellationCore<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TSource, CancellationToken, ValueTask<TElement>> elementSelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TElement>(source, keySelector, elementSelector, comparer);
#endif

#if INCLUDE_SYSTEM_LINQ_ASYNCENUMERABLE_DUPLICATES
        // https://learn.microsoft.com/en-us/dotnet/api/system.linq.asyncenumerable.groupby?view=net-9.0-pp#system-linq-asyncenumerable-groupby-3(system-collections-generic-iasyncenumerable((-0))-system-func((-0-1))-system-func((-1-system-collections-generic-ienumerable((-0))-2))-system-collections-generic-iequalitycomparer((-1)))
        public static IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IAsyncEnumerable<TSource>, TResult> resultSelector) =>
            new GroupedResultAsyncEnumerable<TSource, TKey, TResult>(source, keySelector, resultSelector, comparer: null);

        public static IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IAsyncEnumerable<TSource>, TResult> resultSelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedResultAsyncEnumerable<TSource, TKey, TResult>(source, keySelector, resultSelector, comparer);
#endif // INCLUDE_SYSTEM_LINQ_ASYNCENUMERABLE_DUPLICATES

        /// <summary>
        /// Groups the elements of an async-enumerable sequence according to a specified key selector function, and then applies a result selector function to each group.
        /// </summary>
        /// <typeparam name="TSource">Type of element in the source sequence.</typeparam>
        /// <typeparam name="TKey">Type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <typeparam name="TResult">The result type returned by the result selector function.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">An asynchronous function to extract the key for each element.</param>
        /// <param name="resultSelector">An asynchronous function to transform each group into the result type.</param>
        /// <returns>An async-enumerable sequence of results obtained by invoking and awaiting the result-selector function on each group.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="resultSelector"/> is <see langword="null"/>.</exception>
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwait functionality now exists as overloads of GroupBy. You will need to modify your callback to take an additional CancellationToken argument.")]
        private static IAsyncEnumerable<TResult> GroupByAwaitCore<TSource, TKey, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, Func<TKey, IAsyncEnumerable<TSource>, ValueTask<TResult>> resultSelector) =>
            new GroupedResultAsyncEnumerableWithTask<TSource, TKey, TResult>(source, keySelector, resultSelector, comparer: null);

        /// <summary>
        /// Groups the elements of an async-enumerable sequence according to a specified key selector function, and then applies a result selector function to each group.
        /// </summary>
        /// <typeparam name="TSource">Type of element in the source sequence.</typeparam>
        /// <typeparam name="TKey">Type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <typeparam name="TResult">The result type returned by the result selector function.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">An asynchronous function to extract the key for each element.</param>
        /// <param name="resultSelector">An asynchronous function to transform each group into the result type.</param>
        /// <param name="comparer">An equality comparer to use to compare keys.</param>
        /// <returns>An async-enumerable sequence of results obtained by invoking and awaiting the result-selector function on each group.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="resultSelector"/> or <paramref name="comparer"/> is <see langword="null"/>.</exception>
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwait functionality now exists as overloads of GroupBy. You will need to modify your callback to take an additional CancellationToken argument.")]
        private static IAsyncEnumerable<TResult> GroupByAwaitCore<TSource, TKey, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, Func<TKey, IAsyncEnumerable<TSource>, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedResultAsyncEnumerableWithTask<TSource, TKey, TResult>(source, keySelector, resultSelector, comparer);

#if !NO_DEEP_CANCELLATION
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwaitWithCancellationAsync functionality now exists as overloads of GroupBy.")]
        private static IAsyncEnumerable<TResult> GroupByAwaitWithCancellationCore<TSource, TKey, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TKey, IAsyncEnumerable<TSource>, CancellationToken, ValueTask<TResult>> resultSelector) =>
            new GroupedResultAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TResult>(source, keySelector, resultSelector, comparer: null);

        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwaitWithCancellationAsync functionality now exists as overloads of GroupBy.")]
        private static IAsyncEnumerable<TResult> GroupByAwaitWithCancellationCore<TSource, TKey, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TKey, IAsyncEnumerable<TSource>, CancellationToken, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedResultAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TResult>(source, keySelector, resultSelector, comparer);
#endif

#if INCLUDE_SYSTEM_LINQ_ASYNCENUMERABLE_DUPLICATES
        // This covers the next two
        // https://learn.microsoft.com/en-us/dotnet/api/system.linq.asyncenumerable.groupby?view=net-9.0-pp#system-linq-asyncenumerable-groupby-4(system-collections-generic-iasyncenumerable((-0))-system-func((-0-system-threading-cancellationtoken-system-threading-tasks-valuetask((-1))))-system-func((-0-system-threading-cancellationtoken-system-threading-tasks-valuetask((-2))))-system-func((-1-system-collections-generic-ienumerable((-2))-system-threading-cancellationtoken-system-threading-tasks-valuetask((-3))))-system-collections-generic-iequalitycomparer((-1)))

        public static IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IAsyncEnumerable<TElement>, TResult> resultSelector) =>
            new GroupedResultAsyncEnumerable<TSource, TKey, TElement, TResult>(source, keySelector, elementSelector, resultSelector, comparer: null);

        public static IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IAsyncEnumerable<TElement>, TResult> resultSelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedResultAsyncEnumerable<TSource, TKey, TElement, TResult>(source, keySelector, elementSelector, resultSelector, comparer);
#endif // INCLUDE_SYSTEM_LINQ_ASYNCENUMERABLE_DUPLICATES

        /// <summary>
        /// Groups the elements of an async-enumerable sequence according to a specified key-selector function, applies an element selector to each element of each group, then applies a result selector to each transformed group.
        /// </summary>
        /// <typeparam name="TSource">The type of element in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <typeparam name="TElement">The type of element computed by the element selector.</typeparam>
        /// <typeparam name="TResult">The type of the final result, computed by applying the result selector to each transformed group of elements.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">An asynchronous function to extract the key for each element.</param>
        /// <param name="elementSelector">An asynchronous function to apply to each element of each group. </param>
        /// <param name="resultSelector">An asynchronous function to transform each group into the result type.</param>
        /// <returns>An async-enumerable sequence of results obtained by invoking the result selector function on each group and awaiting the result.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="elementSelector"/> or <paramref name="resultSelector"/> is <see langword="null"/>.</exception>
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwait functionality now exists as overloads of GroupBy. You will need to modify your callback to take an additional CancellationToken argument.")]
        private static IAsyncEnumerable<TResult> GroupByAwaitCore<TSource, TKey, TElement, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, Func<TSource, ValueTask<TElement>> elementSelector, Func<TKey, IAsyncEnumerable<TElement>, ValueTask<TResult>> resultSelector) =>
            new GroupedResultAsyncEnumerableWithTask<TSource, TKey, TElement, TResult>(source, keySelector, elementSelector, resultSelector, comparer: null);

        /// <summary>
        /// Groups the elements of an async-enumerable sequence according to a specified key-selector function, applies an element selector to each element of each group, then applies a result selector to each transformed group.
        /// </summary>
        /// <typeparam name="TSource">The type of element in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the grouping key computed for each element in the source sequence.</typeparam>
        /// <typeparam name="TElement">The type of element computed by the element selector.</typeparam>
        /// <typeparam name="TResult">The type of the final result, computed by applying the result selector to each transformed group of elements.</typeparam>
        /// <param name="source">An async-enumerable sequence whose elements to group.</param>
        /// <param name="keySelector">An asynchronous function to extract the key for each element.</param>
        /// <param name="elementSelector">An asynchronous function to apply to each element of each group. </param>
        /// <param name="resultSelector">An asynchronous function to transform each group into the result type.</param>
        /// <param name="comparer">An equality comparer to use to compare keys.</param>
        /// <returns>An async-enumerable sequence of results obtained by invoking the result selector function on each group and awaiting the result.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="elementSelector"/> or <paramref name="resultSelector"/> or <paramref name="comparer"/> is <see langword="null"/>.</exception>
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwait functionality now exists as overloads of GroupBy. You will need to modify your callback to take an additional CancellationToken argument.")]
        private static IAsyncEnumerable<TResult> GroupByAwaitCore<TSource, TKey, TElement, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, Func<TSource, ValueTask<TElement>> elementSelector, Func<TKey, IAsyncEnumerable<TElement>, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedResultAsyncEnumerableWithTask<TSource, TKey, TElement, TResult>(source, keySelector, elementSelector, resultSelector, comparer);

#if !NO_DEEP_CANCELLATION
        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwaitWithCancellationAsync functionality now exists as overloads of GroupBy.")]
        private static IAsyncEnumerable<TResult> GroupByAwaitWithCancellationCore<TSource, TKey, TElement, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TSource, CancellationToken, ValueTask<TElement>> elementSelector, Func<TKey, IAsyncEnumerable<TElement>, CancellationToken, ValueTask<TResult>> resultSelector) =>
            new GroupedResultAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TElement, TResult>(source, keySelector, elementSelector, resultSelector, comparer: null);

        [GenerateAsyncOverload]
        [Obsolete("Use GroupBy. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the GroupByAwaitWithCancellationAsync functionality now exists as overloads of GroupBy.")]
        private static IAsyncEnumerable<TResult> GroupByAwaitWithCancellationCore<TSource, TKey, TElement, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TSource, CancellationToken, ValueTask<TElement>> elementSelector, Func<TKey, IAsyncEnumerable<TElement>, CancellationToken, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer) =>
            new GroupedResultAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TElement, TResult>(source, keySelector, elementSelector, resultSelector, comparer);
#endif

        private sealed class GroupedResultAsyncEnumerable<TSource, TKey, TResult> : AsyncIterator<TResult>, IAsyncIListProvider<TResult>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, TKey> _keySelector;
            private readonly Func<TKey, IAsyncEnumerable<TSource>, TResult> _resultSelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.Lookup<TKey, TSource>? _lookup;
            private IEnumerator<TResult>? _enumerator;

            public GroupedResultAsyncEnumerable(IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IAsyncEnumerable<TSource>, TResult> resultSelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _resultSelector = resultSelector ?? throw Error.ArgumentNull(nameof(resultSelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new GroupedResultAsyncEnumerable<TSource, TKey, TResult>(_source, _keySelector, _resultSelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.Lookup<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.ApplyResultSelector(_resultSelector).GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = _enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<TResult[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.Lookup<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return l.ToArray(_resultSelector);
            }

            public async ValueTask<List<TResult>> ToListAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.Lookup<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return l.ToList(_resultSelector);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.Lookup<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }

        private sealed class GroupedResultAsyncEnumerableWithTask<TSource, TKey, TResult> : AsyncIterator<TResult>, IAsyncIListProvider<TResult>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, ValueTask<TKey>> _keySelector;
            private readonly Func<TKey, IAsyncEnumerable<TSource>, ValueTask<TResult>> _resultSelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.LookupWithTask<TKey, TSource>? _lookup;
            private IAsyncEnumerator<TResult>? _enumerator;

            public GroupedResultAsyncEnumerableWithTask(IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, Func<TKey, IAsyncEnumerable<TSource>, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _resultSelector = resultSelector ?? throw Error.ArgumentNull(nameof(resultSelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new GroupedResultAsyncEnumerableWithTask<TSource, TKey, TResult>(_source, _keySelector, _resultSelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    await _enumerator.DisposeAsync().ConfigureAwait(false);
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.SelectAwaitCore(async g => await _resultSelector(g.Key, g).ConfigureAwait(false)).GetAsyncEnumerator(_cancellationToken); // REVIEW: Introduce another ApplyResultSelector?
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (await _enumerator!.MoveNextAsync().ConfigureAwait(false))
                        {
                            _current = _enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<TResult[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToArray(_resultSelector).ConfigureAwait(false);
            }

            public async ValueTask<List<TResult>> ToListAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToList(_resultSelector).ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }

#if !NO_DEEP_CANCELLATION
        private sealed class GroupedResultAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TResult> : AsyncIterator<TResult>, IAsyncIListProvider<TResult>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, CancellationToken, ValueTask<TKey>> _keySelector;
            private readonly Func<TKey, IAsyncEnumerable<TSource>, CancellationToken, ValueTask<TResult>> _resultSelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.LookupWithTask<TKey, TSource>? _lookup;
            private IAsyncEnumerator<TResult>? _enumerator;

            public GroupedResultAsyncEnumerableWithTaskAndCancellation(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TKey, IAsyncEnumerable<TSource>, CancellationToken, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _resultSelector = resultSelector ?? throw Error.ArgumentNull(nameof(resultSelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new GroupedResultAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TResult>(_source, _keySelector, _resultSelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    await _enumerator.DisposeAsync().ConfigureAwait(false);
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.SelectAwaitCore(async g => await _resultSelector(g.Key, g, _cancellationToken).ConfigureAwait(false)).GetAsyncEnumerator(_cancellationToken); // REVIEW: Introduce another ApplyResultSelector?
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (await _enumerator!.MoveNextAsync().ConfigureAwait(false))
                        {
                            _current = _enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<TResult[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToArray(_resultSelector, cancellationToken).ConfigureAwait(false);
            }

            public async ValueTask<List<TResult>> ToListAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToList(_resultSelector, cancellationToken).ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }
#endif

        private sealed class GroupedResultAsyncEnumerable<TSource, TKey, TElement, TResult> : AsyncIterator<TResult>, IAsyncIListProvider<TResult>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, TKey> _keySelector;
            private readonly Func<TSource, TElement> _elementSelector;
            private readonly Func<TKey, IAsyncEnumerable<TElement>, TResult> _resultSelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.Lookup<TKey, TElement>? _lookup;
            private IEnumerator<TResult>? _enumerator;

            public GroupedResultAsyncEnumerable(IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IAsyncEnumerable<TElement>, TResult> resultSelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _elementSelector = elementSelector ?? throw Error.ArgumentNull(nameof(elementSelector));
                _resultSelector = resultSelector ?? throw Error.ArgumentNull(nameof(resultSelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new GroupedResultAsyncEnumerable<TSource, TKey, TElement, TResult>(_source, _keySelector, _elementSelector, _resultSelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.Lookup<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.ApplyResultSelector(_resultSelector).GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = _enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<TResult[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.Lookup<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return l.ToArray(_resultSelector);
            }

            public async ValueTask<List<TResult>> ToListAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.Lookup<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return l.ToList(_resultSelector);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.Lookup<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }

        private sealed class GroupedResultAsyncEnumerableWithTask<TSource, TKey, TElement, TResult> : AsyncIterator<TResult>, IAsyncIListProvider<TResult>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, ValueTask<TKey>> _keySelector;
            private readonly Func<TSource, ValueTask<TElement>> _elementSelector;
            private readonly Func<TKey, IAsyncEnumerable<TElement>, ValueTask<TResult>> _resultSelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.LookupWithTask<TKey, TElement>? _lookup;
            private IAsyncEnumerator<TResult>? _enumerator;

            public GroupedResultAsyncEnumerableWithTask(IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, Func<TSource, ValueTask<TElement>> elementSelector, Func<TKey, IAsyncEnumerable<TElement>, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _elementSelector = elementSelector ?? throw Error.ArgumentNull(nameof(elementSelector));
                _resultSelector = resultSelector ?? throw Error.ArgumentNull(nameof(resultSelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new GroupedResultAsyncEnumerableWithTask<TSource, TKey, TElement, TResult>(_source, _keySelector, _elementSelector, _resultSelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    await _enumerator.DisposeAsync().ConfigureAwait(false);
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.SelectAwaitCore(async g => await _resultSelector(g.Key, g).ConfigureAwait(false)).GetAsyncEnumerator(_cancellationToken); // REVIEW: Introduce another ApplyResultSelector?
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (await _enumerator!.MoveNextAsync().ConfigureAwait(false))
                        {
                            _current = _enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<TResult[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToArray(_resultSelector).ConfigureAwait(false);
            }

            public async ValueTask<List<TResult>> ToListAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToList(_resultSelector).ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }

#if !NO_DEEP_CANCELLATION
        private sealed class GroupedResultAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TElement, TResult> : AsyncIterator<TResult>, IAsyncIListProvider<TResult>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, CancellationToken, ValueTask<TKey>> _keySelector;
            private readonly Func<TSource, CancellationToken, ValueTask<TElement>> _elementSelector;
            private readonly Func<TKey, IAsyncEnumerable<TElement>, CancellationToken, ValueTask<TResult>> _resultSelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.LookupWithTask<TKey, TElement>? _lookup;
            private IAsyncEnumerator<TResult>? _enumerator;

            public GroupedResultAsyncEnumerableWithTaskAndCancellation(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TSource, CancellationToken, ValueTask<TElement>> elementSelector, Func<TKey, IAsyncEnumerable<TElement>, CancellationToken, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _elementSelector = elementSelector ?? throw Error.ArgumentNull(nameof(elementSelector));
                _resultSelector = resultSelector ?? throw Error.ArgumentNull(nameof(resultSelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new GroupedResultAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TElement, TResult>(_source, _keySelector, _elementSelector, _resultSelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    await _enumerator.DisposeAsync().ConfigureAwait(false);
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.SelectAwaitCore(async g => await _resultSelector(g.Key, g, _cancellationToken).ConfigureAwait(false)).GetAsyncEnumerator(_cancellationToken); // REVIEW: Introduce another ApplyResultSelector?
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (await _enumerator!.MoveNextAsync().ConfigureAwait(false))
                        {
                            _current = _enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<TResult[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToArray(_resultSelector, cancellationToken).ConfigureAwait(false);
            }

            public async ValueTask<List<TResult>> ToListAsync(CancellationToken cancellationToken)
            {
                var l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToList(_resultSelector, cancellationToken).ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }
#endif

        private sealed class GroupedAsyncEnumerable<TSource, TKey, TElement> : AsyncIterator<IAsyncGrouping<TKey, TElement>>, IAsyncIListProvider<IAsyncGrouping<TKey, TElement>>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, TKey> _keySelector;
            private readonly Func<TSource, TElement> _elementSelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.Lookup<TKey, TElement>? _lookup;
            private IEnumerator<IGrouping<TKey, TElement>>? _enumerator;

            public GroupedAsyncEnumerable(IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _elementSelector = elementSelector ?? throw Error.ArgumentNull(nameof(elementSelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<IAsyncGrouping<TKey, TElement>> Clone()
            {
                return new GroupedAsyncEnumerable<TSource, TKey, TElement>(_source, _keySelector, _elementSelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.Lookup<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = (IAsyncGrouping<TKey, TElement>)_enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }
            
            public async ValueTask<IAsyncGrouping<TKey, TElement>[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TElement>> l = await Internal.Lookup<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToArrayAsync(cancellationToken).ConfigureAwait(false);
            }

            public async ValueTask<List<IAsyncGrouping<TKey, TElement>>> ToListAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TElement>> l = await Internal.Lookup<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToListAsync(cancellationToken).ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.Lookup<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }

        private sealed class GroupedAsyncEnumerableWithTask<TSource, TKey, TElement> : AsyncIterator<IAsyncGrouping<TKey, TElement>>, IAsyncIListProvider<IAsyncGrouping<TKey, TElement>>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, ValueTask<TKey>> _keySelector;
            private readonly Func<TSource, ValueTask<TElement>> _elementSelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.LookupWithTask<TKey, TElement>? _lookup;
            private IEnumerator<IGrouping<TKey, TElement>>? _enumerator;

            public GroupedAsyncEnumerableWithTask(IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, Func<TSource, ValueTask<TElement>> elementSelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _elementSelector = elementSelector ?? throw Error.ArgumentNull(nameof(elementSelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<IAsyncGrouping<TKey, TElement>> Clone()
            {
                return new GroupedAsyncEnumerableWithTask<TSource, TKey, TElement>(_source, _keySelector, _elementSelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = (IAsyncGrouping<TKey, TElement>)_enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<IAsyncGrouping<TKey, TElement>[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TElement>> l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToArrayAsync(cancellationToken).ConfigureAwait(false);
            }

            public async ValueTask<List<IAsyncGrouping<TKey, TElement>>> ToListAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TElement>> l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToListAsync(cancellationToken).ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }

#if !NO_DEEP_CANCELLATION
        private sealed class GroupedAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TElement> : AsyncIterator<IAsyncGrouping<TKey, TElement>>, IAsyncIListProvider<IAsyncGrouping<TKey, TElement>>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, CancellationToken, ValueTask<TKey>> _keySelector;
            private readonly Func<TSource, CancellationToken, ValueTask<TElement>> _elementSelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.LookupWithTask<TKey, TElement>? _lookup;
            private IEnumerator<IGrouping<TKey, TElement>>? _enumerator;

            public GroupedAsyncEnumerableWithTaskAndCancellation(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TSource, CancellationToken, ValueTask<TElement>> elementSelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _elementSelector = elementSelector ?? throw Error.ArgumentNull(nameof(elementSelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<IAsyncGrouping<TKey, TElement>> Clone()
            {
                return new GroupedAsyncEnumerableWithTaskAndCancellation<TSource, TKey, TElement>(_source, _keySelector, _elementSelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = (IAsyncGrouping<TKey, TElement>)_enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<IAsyncGrouping<TKey, TElement>[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TElement>> l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToArrayAsync(cancellationToken).ConfigureAwait(false);
            }

            public async ValueTask<List<IAsyncGrouping<TKey, TElement>>> ToListAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TElement>> l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToListAsync(cancellationToken).ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.LookupWithTask<TKey, TElement>.CreateAsync(_source, _keySelector, _elementSelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }
#endif

        private sealed class GroupedAsyncEnumerable<TSource, TKey> : AsyncIterator<IAsyncGrouping<TKey, TSource>>, IAsyncIListProvider<IAsyncGrouping<TKey, TSource>>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, TKey> _keySelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.Lookup<TKey, TSource>? _lookup;
            private IEnumerator<IGrouping<TKey, TSource>>? _enumerator;

            public GroupedAsyncEnumerable(IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<IAsyncGrouping<TKey, TSource>> Clone()
            {
                return new GroupedAsyncEnumerable<TSource, TKey>(_source, _keySelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.Lookup<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = (IAsyncGrouping<TKey, TSource>)_enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<IAsyncGrouping<TKey, TSource>[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TSource>> l = await Internal.Lookup<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToArrayAsync(cancellationToken).ConfigureAwait(false);
            }

            public async ValueTask<List<IAsyncGrouping<TKey, TSource>>> ToListAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TSource>> l = await Internal.Lookup<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToListAsync(cancellationToken).ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.Lookup<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }

        private sealed class GroupedAsyncEnumerableWithTask<TSource, TKey> : AsyncIterator<IAsyncGrouping<TKey, TSource>>, IAsyncIListProvider<IAsyncGrouping<TKey, TSource>>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, ValueTask<TKey>> _keySelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.LookupWithTask<TKey, TSource>? _lookup;
            private IEnumerator<IGrouping<TKey, TSource>>? _enumerator;

            public GroupedAsyncEnumerableWithTask(IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<IAsyncGrouping<TKey, TSource>> Clone()
            {
                return new GroupedAsyncEnumerableWithTask<TSource, TKey>(_source, _keySelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = (IAsyncGrouping<TKey, TSource>)_enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<IAsyncGrouping<TKey, TSource>[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TSource>> l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToArrayAsync(cancellationToken).ConfigureAwait(false);
            }

            public async ValueTask<List<IAsyncGrouping<TKey, TSource>>> ToListAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TSource>> l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToListAsync(cancellationToken).ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }

#if !NO_DEEP_CANCELLATION
        private sealed class GroupedAsyncEnumerableWithTaskAndCancellation<TSource, TKey> : AsyncIterator<IAsyncGrouping<TKey, TSource>>, IAsyncIListProvider<IAsyncGrouping<TKey, TSource>>
        {
            private readonly IAsyncEnumerable<TSource> _source;
            private readonly Func<TSource, CancellationToken, ValueTask<TKey>> _keySelector;
            private readonly IEqualityComparer<TKey>? _comparer;

            private Internal.LookupWithTask<TKey, TSource>? _lookup;
            private IEnumerator<IGrouping<TKey, TSource>>? _enumerator;

            public GroupedAsyncEnumerableWithTaskAndCancellation(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer)
            {
                _source = source ?? throw Error.ArgumentNull(nameof(source));
                _keySelector = keySelector ?? throw Error.ArgumentNull(nameof(keySelector));
                _comparer = comparer;
            }

            public override AsyncIteratorBase<IAsyncGrouping<TKey, TSource>> Clone()
            {
                return new GroupedAsyncEnumerableWithTaskAndCancellation<TSource, TKey>(_source, _keySelector, _comparer);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                    _lookup = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _lookup = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, _cancellationToken).ConfigureAwait(false);
                        _enumerator = _lookup.GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = (IAsyncGrouping<TKey, TSource>)_enumerator.Current;
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }

            public async ValueTask<IAsyncGrouping<TKey, TSource>[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TSource>> l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToArrayAsync(cancellationToken).ConfigureAwait(false);
            }

            public async ValueTask<List<IAsyncGrouping<TKey, TSource>>> ToListAsync(CancellationToken cancellationToken)
            {
                IAsyncIListProvider<IAsyncGrouping<TKey, TSource>> l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);
                return await l.ToListAsync(cancellationToken).ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    var l = await Internal.LookupWithTask<TKey, TSource>.CreateAsync(_source, _keySelector, _comparer, cancellationToken).ConfigureAwait(false);

                    return l.Count;
                }
            }
        }
#endif
    }
}