// 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.Collections.Immutable;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Channels;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.PooledObjects;
 
namespace Microsoft.CodeAnalysis.Shared.Extensions
{
    internal static class IAsyncEnumerableExtensions
    {
        internal static class AsyncEnumerable<T>
        {
            public static readonly IAsyncEnumerable<T> Empty = GetEmptyAsync();
 
#pragma warning disable CS1998 // Async method lacks 'await' operators and will run synchronously
            private static async IAsyncEnumerable<T> GetEmptyAsync()
            {
                yield break;
            }
#pragma warning restore CS1998 // Async method lacks 'await' operators and will run synchronously
        }
 
        public static async Task<ImmutableArray<T>> ToImmutableArrayAsync<T>(this IAsyncEnumerable<T> values, CancellationToken cancellationToken)
        {
            using var _ = ArrayBuilder<T>.GetInstance(out var result);
            await foreach (var value in values.WithCancellation(cancellationToken).ConfigureAwait(false))
                result.Add(value);
 
            return result.ToImmutable();
        }
 
        /// <summary>
        /// Takes an array of <see cref="IAsyncEnumerable{T}"/>s and produces a single resultant <see
        /// cref="IAsyncEnumerable{T}"/> with all their values merged together.  Absolutely no ordering guarantee is
        /// provided.  It will be expected that the individual values from distinct enumerables will be interleaved
        /// together.
        /// </summary>
        /// <remarks>This helper is useful when doign parallel processing of work where each job returns an <see
        /// cref="IAsyncEnumerable{T}"/>, but one final stream is desired as the result.</remarks>
        public static IAsyncEnumerable<T> MergeAsync<T>(this ImmutableArray<IAsyncEnumerable<T>> streams, CancellationToken cancellationToken)
        {
            // Code provided by Stephen Toub, but heavily modified after that.
 
            // 1024 chosen as a way to ensure we don't necessarily create a huge unbounded channel, while also making it
            // so that we're unlikely to throttle on any stream unless there is truly a huge amount of results in it.
            var channel = Channel.CreateBounded<T>(1024);
 
            var tasks = new Task[streams.Length];
            for (var i = 0; i < streams.Length; i++)
                tasks[i] = Process(streams[i], channel.Writer, cancellationToken);
 
            // Complete the channel writer with the result of all the tasks.  If nothing failed, t.Exception will be
            // null and this will complete successfully.  If anything failed, the exception will propagate out.
            //
            // Note: passing CancellationToken.None here is intentional/correct.  We must complete all the channels to
            // allow reading to complete as well.
            Task.WhenAll(tasks).CompletesChannel(channel);
 
            return channel.Reader.ReadAllAsync(cancellationToken);
 
            static async Task Process(IAsyncEnumerable<T> stream, ChannelWriter<T> writer, CancellationToken cancellationToken)
            {
                await foreach (var value in stream)
                    await writer.WriteAsync(value, cancellationToken).ConfigureAwait(false);
            }
        }
 
        public static async IAsyncEnumerable<T> ReadAllAsync<T>(
            this ChannelReader<T> reader, [EnumeratorCancellation] CancellationToken cancellationToken)
        {
            while (await reader.WaitToReadAsync(cancellationToken).ConfigureAwait(false))
            {
                while (reader.TryRead(out var item))
                    yield return item;
            }
        }
 
        /// <summary>
        /// Runs after task completes in any fashion (success, cancellation, faulting) and ensures the channel writer is
        /// always completed.  If the task faults then the exception from that task will be used to complete the channel
        /// </summary>
        public static void CompletesChannel<T>(this Task task, Channel<T> channel)
        {
            // Note: using `Complete(task.Exception)` is always fine.  Exception is only produced in the case of
            // faulting. it is null otherwise.
            task.ContinueWith(
                static (task, channel) => ((Channel<T>)channel!).Writer.Complete(task.Exception),
                channel, CancellationToken.None, TaskContinuationOptions.None, TaskScheduler.Default);
        }
    }
}