// 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 AsyncEnumerable
    {
        /// <summary>
        /// Returns the only element of an async-enumerable sequence, or a default value if the async-enumerable sequence is empty; this method reports an exception if there is more than one element in the async-enumerable sequence.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Source async-enumerable sequence.</param>
        /// <param name="cancellationToken">The optional cancellation token to be used for cancelling the sequence at any time.</param>
        /// <returns>Sequence containing the single element in the async-enumerable sequence, or a default value if no such element exists.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        /// <exception cref="InvalidOperationException">(Asynchronous) The source sequence contains more than one element.</exception>
        public static ValueTask<TSource> SingleOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));

            return Core(source, cancellationToken);

            static async ValueTask<TSource> Core(IAsyncEnumerable<TSource> source, CancellationToken cancellationToken)
            {
                if (source is IList<TSource> list)
                {
                    return list.Count switch
                    {
                        0 => default!,
                        1 => list[0],
                        _ => throw Error.MoreThanOneElement(),
                    };
                }

                await using (var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false))
                {
                    if (!await e.MoveNextAsync())
                    {
                        return default!;
                    }

                    var result = e.Current;

                    if (!await e.MoveNextAsync())
                    {
                        return result;
                    }
                }

                throw Error.MoreThanOneElement();
            }
        }

        /// <summary>
        /// Returns the only element of an async-enumerable sequence that matches the predicate, or a default value if no such element exists; this method reports an exception if there is more than one element in the async-enumerable sequence.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Source async-enumerable sequence.</param>
        /// <param name="predicate">A predicate function to evaluate for elements in the source sequence.</param>
        /// <param name="cancellationToken">The optional cancellation token to be used for cancelling the sequence at any time.</param>
        /// <returns>Sequence containing the single element in the async-enumerable sequence that satisfies the condition in the predicate, or a default value if no such element exists.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="predicate"/> is null.</exception>
        /// <exception cref="InvalidOperationException">(Asynchronous) The sequence contains more than one element that satisfies the condition in the predicate.</exception>
        public static ValueTask<TSource> SingleOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (predicate == null)
                throw Error.ArgumentNull(nameof(predicate));

            return Core(source, predicate, cancellationToken);

            static async ValueTask<TSource> Core(IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken)
            {
                await using (var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false))
                {
                    while (await e.MoveNextAsync())
                    {
                        var result = e.Current;

                        if (predicate(result))
                        {
                            while (await e.MoveNextAsync())
                            {
                                if (predicate(e.Current))
                                {
                                    throw Error.MoreThanOneElement();
                                }
                            }

                            return result;
                        }
                    }
                }

                return default!;
            }
        }

        internal static ValueTask<TSource> SingleOrDefaultAwaitAsyncCore<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<bool>> predicate, CancellationToken cancellationToken = default)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (predicate == null)
                throw Error.ArgumentNull(nameof(predicate));

            return Core(source, predicate, cancellationToken);

            static async ValueTask<TSource> Core(IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<bool>> predicate, CancellationToken cancellationToken)
            {
                await using (var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false))
                {
                    while (await e.MoveNextAsync())
                    {
                        var result = e.Current;

                        if (await predicate(result).ConfigureAwait(false))
                        {
                            while (await e.MoveNextAsync())
                            {
                                if (await predicate(e.Current).ConfigureAwait(false))
                                {
                                    throw Error.MoreThanOneElement();
                                }
                            }

                            return result;
                        }
                    }
                }

                return default!;
            }
        }

#if !NO_DEEP_CANCELLATION
        internal static ValueTask<TSource> SingleOrDefaultAwaitWithCancellationAsyncCore<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (predicate == null)
                throw Error.ArgumentNull(nameof(predicate));

            return Core(source, predicate, cancellationToken);

            static async ValueTask<TSource> Core(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken)
            {
                await using (var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false))
                {
                    while (await e.MoveNextAsync())
                    {
                        var result = e.Current;

                        if (await predicate(result, cancellationToken).ConfigureAwait(false))
                        {
                            while (await e.MoveNextAsync())
                            {
                                if (await predicate(e.Current, cancellationToken).ConfigureAwait(false))
                                {
                                    throw Error.MoreThanOneElement();
                                }
                            }

                            return result;
                        }
                    }
                }

                return default!;
            }
        }
#endif
    }
}