// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
 
namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
        /// <summary>Determines whether two sequences are equal by comparing their elements.</summary>
        /// <typeparam name="TSource"></typeparam>
        /// <param name="first">An <see cref="IAsyncEnumerable{T}"/> to compare to <paramref name="second"/>.</param>
        /// <param name="second">An <see cref="IAsyncEnumerable{T}"/> to compare to the first sequence.</param>
        /// <param name="comparer">An <see cref="IEqualityComparer{T}"/> to use to compare elements.</param>
        /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
        /// <returns>
        /// true if the two source sequences are of equal length and their corresponding
        /// elements compare equal according to comparer; otherwise, false.
        /// </returns>
        public static ValueTask<bool> SequenceEqualAsync<TSource>(
            this IAsyncEnumerable<TSource> first,
            IAsyncEnumerable<TSource> second,
            IEqualityComparer<TSource>? comparer = null,
            CancellationToken cancellationToken = default)
        {
            ThrowHelper.ThrowIfNull(first);
            ThrowHelper.ThrowIfNull(second);
 
            return Impl(first, second, comparer ?? EqualityComparer<TSource>.Default, cancellationToken);
 
            static async ValueTask<bool> Impl(
                IAsyncEnumerable<TSource> first,
                IAsyncEnumerable<TSource> second,
                IEqualityComparer<TSource> comparer,
                CancellationToken cancellationToken)
            {
                IAsyncEnumerator<TSource> e1 = first.GetAsyncEnumerator(cancellationToken);
                try
                {
                    IAsyncEnumerator<TSource> e2 = second.GetAsyncEnumerator(cancellationToken);
                    try
                    {
                        while (await e1.MoveNextAsync().ConfigureAwait(false))
                        {
                            if (!await e2.MoveNextAsync().ConfigureAwait(false) || !comparer.Equals(e1.Current, e2.Current))
                            {
                                return false;
                            }
                        }
 
                        return !await e2.MoveNextAsync().ConfigureAwait(false);
                    }
                    finally
                    {
                        await e2.DisposeAsync().ConfigureAwait(false);
                    }
                }
                finally
                {
                    await e1.DisposeAsync().ConfigureAwait(false);
                }
            }
        }
    }
}