reactive/Ix.NET/Source/System.Linq.Async/System/Linq/Operators/TakeLast.cs

// 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.Diagnostics;
using System.Threading.Tasks;

namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
        public static IAsyncEnumerable<TSource> TakeLast<TSource>(this IAsyncEnumerable<TSource> source, int count)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));

            if (count <= 0)
            {
                return Empty<TSource>();
            }

            return new TakeLastAsyncIterator<TSource>(source, count);
        }

        private sealed class TakeLastAsyncIterator<TSource> : AsyncIterator<TSource>
        {
            private readonly int _count;
            private readonly IAsyncEnumerable<TSource> _source;

            private IAsyncEnumerator<TSource> _enumerator;
            private bool _isDone;
            private Queue<TSource> _queue;

            public TakeLastAsyncIterator(IAsyncEnumerable<TSource> source, int count)
            {
                Debug.Assert(source != null);

                _source = source;
                _count = count;
            }

            public override AsyncIteratorBase<TSource> Clone()
            {
                return new TakeLastAsyncIterator<TSource>(_source, _count);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    await _enumerator.DisposeAsync().ConfigureAwait(false);
                    _enumerator = null;
                }

                _queue = null; // release the memory

                await base.DisposeAsync().ConfigureAwait(false);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _enumerator = _source.GetAsyncEnumerator(_cancellationToken);
                        _queue = new Queue<TSource>();
                        _isDone = false;

                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;


                    case AsyncIteratorState.Iterating:
                        while (true)
                        {
                            if (!_isDone)
                            {
                                if (await _enumerator.MoveNextAsync().ConfigureAwait(false))
                                {
                                    if (_count > 0)
                                    {
                                        var item = _enumerator.Current;
                                        if (_queue.Count >= _count)
                                        {
                                            _queue.Dequeue();
                                        }
                                        _queue.Enqueue(item);
                                    }
                                }
                                else
                                {
                                    _isDone = true;
                                    // Dispose early here as we can
                                    await _enumerator.DisposeAsync().ConfigureAwait(false);
                                    _enumerator = null;
                                }

                                continue; // loop until queue is drained
                            }

                            if (_queue.Count > 0)
                            {
                                _current = _queue.Dequeue();
                                return true;
                            }

                            break; // while
                        }

                        break; // case
                }

                await DisposeAsync().ConfigureAwait(false);
                return false;
            }
        }
    }
}