// 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 AsyncEnumerableEx
    {
        /// <summary>
        /// Expands (breadth first) the async-enumerable sequence by recursively applying a selector function to generate more sequences at each recursion level.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="source">Source async-enumerable sequence.</param>
        /// <param name="selector">Selector function to retrieve the next sequence to expand.</param>
        /// <returns>Sequence with results from the recursive expansion of the source sequence.</returns>
        public static IAsyncEnumerable<TSource> Expand<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, IAsyncEnumerable<TSource>> selector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (selector == null)
                throw Error.ArgumentNull(nameof(selector));

            return Core(source, selector);

            static async IAsyncEnumerable<TSource> Core(IAsyncEnumerable<TSource> source, Func<TSource, IAsyncEnumerable<TSource>> selector, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
            {
                var queue = new Queue<IAsyncEnumerable<TSource>>();

                queue.Enqueue(source);

                while (queue.Count > 0)
                {
                    await foreach (var item in queue.Dequeue().WithCancellation(cancellationToken).ConfigureAwait(false))
                    {
                        queue.Enqueue(selector(item));

                        yield return item;
                    }
                }
            }
        }

        /// <summary>
        /// Expands (breadth first) the async-enumerable sequence by recursively applying an asynchronous selector function to generate more sequences at each recursion level.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="source">Source async-enumerable sequence.</param>
        /// <param name="selector">Asynchronous selector function to retrieve the next sequence to expand.</param>
        /// <returns>Sequence with results from the recursive expansion of the source sequence.</returns>
        public static IAsyncEnumerable<TSource> Expand<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<IAsyncEnumerable<TSource>>> selector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (selector == null)
                throw Error.ArgumentNull(nameof(selector));

            return Core(source, selector);

            static async IAsyncEnumerable<TSource> Core(IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<IAsyncEnumerable<TSource>>> selector, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
            {
                var queue = new Queue<IAsyncEnumerable<TSource>>();

                queue.Enqueue(source);

                while (queue.Count > 0)
                {
                    await foreach (var item in queue.Dequeue().WithCancellation(cancellationToken).ConfigureAwait(false))
                    {
                        queue.Enqueue(await selector(item).ConfigureAwait(false));

                        yield return item;
                    }
                }
            }
        }

#if !NO_DEEP_CANCELLATION
        /// <summary>
        /// Expands (breadth first) the async-enumerable sequence by recursively applying an asynchronous (cancellable) selector function to generate more sequences at each recursion level.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="source">Source async-enumerable sequence.</param>
        /// <param name="selector">Asynchronous (cancellable) selector function to retrieve the next sequence to expand.</param>
        /// <returns>Sequence with results from the recursive expansion of the source sequence.</returns>
        public static IAsyncEnumerable<TSource> Expand<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<IAsyncEnumerable<TSource>>> selector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (selector == null)
                throw Error.ArgumentNull(nameof(selector));

            return Core(source, selector);

            static async IAsyncEnumerable<TSource> Core(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<IAsyncEnumerable<TSource>>> selector, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
            {
                var queue = new Queue<IAsyncEnumerable<TSource>>();

                queue.Enqueue(source);

                while (queue.Count > 0)
                {
                    await foreach (var item in queue.Dequeue().WithCancellation(cancellationToken).ConfigureAwait(false))
                    {
                        queue.Enqueue(await selector(item, cancellationToken).ConfigureAwait(false));

                        yield return item;
                    }
                }
            }
        }
#endif
    }
}