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

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

            // optimize these adapters for lists and collections
            if (source is IList<TSource> list)
                return new AsyncIListEnumerableAdapter<TSource>(list);

            if (source is ICollection<TSource> collection)
                return new AsyncICollectionEnumerableAdapter<TSource>(collection);

            return new AsyncEnumerableAdapter<TSource>(source);
        }

        public static IAsyncEnumerable<TSource> ToAsyncEnumerable<TSource>(this Task<TSource> task)
        {
            if (task == null)
                throw Error.ArgumentNull(nameof(task));

            return CreateEnumerable(
                _ =>
                {
                    var called = 0;

                    var value = default(TSource);
                    return CreateEnumerator(
                        async ct =>
                        {
                            if (Interlocked.CompareExchange(ref called, 1, 0) == 0)
                            {
                                value = await task.ConfigureAwait(false);
                                return true;
                            }
                            return false;
                        },
                        () => value,
                        () => TaskExt.CompletedTask);
                });
        }

        public static IAsyncEnumerable<TSource> ToAsyncEnumerable<TSource>(this IObservable<TSource> source)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));

            return CreateEnumerable(
                ct =>
                {
                    var observer = new ToAsyncEnumerableObserver<TSource>();

                    var subscription = source.Subscribe(observer);

                    // REVIEW: Review possible concurrency issues with Dispose calls.

                    var ctr = ct.Register(subscription.Dispose);

                    return CreateEnumerator(
                        tcs =>
                        {
                            var hasValue = false;
                            var hasCompleted = false;
                            var error = default(Exception);

                            lock (observer.SyncRoot)
                            {
                                if (observer.Values.Count > 0)
                                {
                                    hasValue = true;
                                    observer.Current = observer.Values.Dequeue();
                                }
                                else if (observer.HasCompleted)
                                {
                                    hasCompleted = true;
                                }
                                else if (observer.Error != null)
                                {
                                    error = observer.Error;
                                }
                                else
                                {
                                    observer.TaskCompletionSource = tcs;
                                }
                            }

                            if (hasValue)
                            {
                                tcs.TrySetResult(true);
                            }
                            else if (hasCompleted)
                            {
                                tcs.TrySetResult(false);
                            }
                            else if (error != null)
                            {
                                tcs.TrySetException(error);
                            }

                            return new ValueTask<bool>(tcs.Task);
                        },
                        () => observer.Current,
                        () =>
                        {
                            ctr.Dispose();
                            subscription.Dispose();
                            // Should we cancel in-flight operations somehow?
                            return TaskExt.CompletedTask;
                        });
                });
        }

        internal sealed class AsyncEnumerableAdapter<T> : AsyncIterator<T>, IAsyncIListProvider<T>
        {
            private readonly IEnumerable<T> _source;

            private IEnumerator<T> _enumerator;
 
            public AsyncEnumerableAdapter(IEnumerable<T> source)
            {
                Debug.Assert(source != null);

                _source = source;
            }

            public override AsyncIterator<T> Clone()
            {
                return new AsyncEnumerableAdapter<T>(_source);
            }

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

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

            protected override async ValueTask<bool> MoveNextCore(CancellationToken cancellationToken)
            {
                switch (state)
                {
                    case AsyncIteratorState.Allocated:
                        _enumerator = _source.GetEnumerator();
                        state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator.MoveNext())
                        {
                            current = _enumerator.Current;
                            return true;
                        }

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

                return false;
            }

            // These optimizations rely on the Sys.Linq impls from IEnumerable to optimize
            // and short circuit as appropriate
            public Task<T[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                return Task.FromResult(_source.ToArray());
            }

            public Task<List<T>> ToListAsync(CancellationToken cancellationToken)
            {
                return Task.FromResult(_source.ToList());
            }

            public Task<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                return Task.FromResult(_source.Count());
            }
        }

        internal sealed class AsyncIListEnumerableAdapter<T> : AsyncIterator<T>, IAsyncIListProvider<T>, IList<T>
        {
            private readonly IList<T> _source;
            private IEnumerator<T> _enumerator;

            public AsyncIListEnumerableAdapter(IList<T> source)
            {
                Debug.Assert(source != null);

                _source = source;
            }

            public override AsyncIterator<T> Clone()
            {
                return new AsyncIListEnumerableAdapter<T>(_source);
            }

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

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

            protected override async ValueTask<bool> MoveNextCore(CancellationToken cancellationToken)
            {
                switch (state)
                {
                    case AsyncIteratorState.Allocated:
                        _enumerator = _source.GetEnumerator();
                        state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator.MoveNext())
                        {
                            current = _enumerator.Current;
                            return true;
                        }

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

                return false;
            }

            public override IAsyncEnumerable<TResult> Select<TResult>(Func<T, TResult> selector)
            {
                return new SelectIListIterator<T, TResult>(_source, selector);
            }

            // These optimizations rely on the Sys.Linq impls from IEnumerable to optimize
            // and short circuit as appropriate
            public Task<T[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                return Task.FromResult(_source.ToArray());
            }

            public Task<List<T>> ToListAsync(CancellationToken cancellationToken)
            {
                return Task.FromResult(_source.ToList());
            }

            public Task<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                return Task.FromResult(_source.Count);
            }

            IEnumerator<T> IEnumerable<T>.GetEnumerator() => _source.GetEnumerator();

            IEnumerator IEnumerable.GetEnumerator() => _source.GetEnumerator();

            void ICollection<T>.Add(T item) => _source.Add(item);

            void ICollection<T>.Clear() => _source.Clear();

            bool ICollection<T>.Contains(T item) => _source.Contains(item);

            void ICollection<T>.CopyTo(T[] array, int arrayIndex) => _source.CopyTo(array, arrayIndex);

            bool ICollection<T>.Remove(T item) => _source.Remove(item);

            int ICollection<T>.Count => _source.Count;

            bool ICollection<T>.IsReadOnly => _source.IsReadOnly;

            int IList<T>.IndexOf(T item) => _source.IndexOf(item);

            void IList<T>.Insert(int index, T item) => _source.Insert(index, item);

            void IList<T>.RemoveAt(int index) => _source.RemoveAt(index);

            T IList<T>.this[int index]
            {
                get { return _source[index]; }
                set { _source[index] = value; }
            }
        }

        internal sealed class AsyncICollectionEnumerableAdapter<T> : AsyncIterator<T>, IAsyncIListProvider<T>, ICollection<T>
        {
            private readonly ICollection<T> _source;
            private IEnumerator<T> _enumerator;

            public AsyncICollectionEnumerableAdapter(ICollection<T> source)
            {
                Debug.Assert(source != null);

                _source = source;
            }

            public override AsyncIterator<T> Clone()
            {
                return new AsyncICollectionEnumerableAdapter<T>(_source);
            }

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

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

            protected override async ValueTask<bool> MoveNextCore(CancellationToken cancellationToken)
            {
                switch (state)
                {
                    case AsyncIteratorState.Allocated:
                        _enumerator = _source.GetEnumerator();
                        state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator.MoveNext())
                        {
                            current = _enumerator.Current;
                            return true;
                        }

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

                return false;
            }

            // These optimizations rely on the Sys.Linq impls from IEnumerable to optimize
            // and short circuit as appropriate
            public Task<T[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                return Task.FromResult(_source.ToArray());
            }

            public Task<List<T>> ToListAsync(CancellationToken cancellationToken)
            {
                return Task.FromResult(_source.ToList());
            }

            public Task<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                return Task.FromResult(_source.Count);
            }

            IEnumerator<T> IEnumerable<T>.GetEnumerator() => _source.GetEnumerator();

            IEnumerator IEnumerable.GetEnumerator() => _source.GetEnumerator();

            void ICollection<T>.Add(T item) => _source.Add(item);

            void ICollection<T>.Clear() => _source.Clear();

            bool ICollection<T>.Contains(T item) => _source.Contains(item);

            void ICollection<T>.CopyTo(T[] array, int arrayIndex) => _source.CopyTo(array, arrayIndex);

            bool ICollection<T>.Remove(T item) => _source.Remove(item);

            int ICollection<T>.Count => _source.Count;

            bool ICollection<T>.IsReadOnly => _source.IsReadOnly;
        }

        private sealed class ToAsyncEnumerableObserver<T> : IObserver<T>
        {
            public readonly Queue<T> Values;

            public T Current;
            public Exception Error;
            public bool HasCompleted;
            public TaskCompletionSource<bool> TaskCompletionSource;

            public ToAsyncEnumerableObserver()
            {
                Values = new Queue<T>();
            }

            public object SyncRoot
            {
                get { return Values; }
            }

            public void OnCompleted()
            {
                var tcs = default(TaskCompletionSource<bool>);

                lock (SyncRoot)
                {
                    HasCompleted = true;

                    if (TaskCompletionSource != null)
                    {
                        tcs = TaskCompletionSource;
                        TaskCompletionSource = null;
                    }
                }

                tcs?.TrySetResult(false);
            }

            public void OnError(Exception error)
            {
                var tcs = default(TaskCompletionSource<bool>);

                lock (SyncRoot)
                {
                    Error = error;

                    if (TaskCompletionSource != null)
                    {
                        tcs = TaskCompletionSource;
                        TaskCompletionSource = null;
                    }
                }

                tcs?.TrySetException(error);
            }

            public void OnNext(T value)
            {
                var tcs = default(TaskCompletionSource<bool>);

                lock (SyncRoot)
                {
                    if (TaskCompletionSource == null)
                    {
                        Values.Enqueue(value);
                    }
                    else
                    {
                        Current = value;

                        tcs = TaskCompletionSource;
                        TaskCompletionSource = null;
                    }
                }

                tcs?.TrySetResult(true);
            }
        }
    }
}