// 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;

namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
        /// <summary>Adds a value to the beginning of the sequence.</summary>
        /// <typeparam name="TSource">The type of the elements of source.</typeparam>
        /// <param name="source">A sequence of values.</param>
        /// <param name="element">The value to prepend to source.</param>
        /// <returns>A new sequence that begins with element.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is <see langword="null"/>.</exception>
        public static IAsyncEnumerable<TSource> Prepend<TSource>(
            this IAsyncEnumerable<TSource> source,
            TSource element)
        {
            ArgumentNullException.ThrowIfNull(source);

            return source is AppendPrependAsyncIterator<TSource> appendable
                ? appendable.Prepend(element)
                : new AppendPrepend1AsyncIterator<TSource>(source, element, appending: false);
        }
    }
}