/* SPDX-License-Identifier: Apache-2.0
*
* The OpenSearch Contributors require contributions made to
* this file be licensed under the Apache-2.0 license or a
* compatible open source license.
*/
/*
* Modifications Copyright OpenSearch Contributors. See
* GitHub history for details.
*
*  Licensed to Elasticsearch B.V. under one or more contributor
*  license agreements. See the NOTICE file distributed with
*  this work for additional information regarding copyright
*  ownership. Elasticsearch B.V. licenses this file to you under
*  the Apache License, Version 2.0 (the "License"); you may
*  not use this file except in compliance with the License.
*  You may obtain a copy of the License at
*
* 	http://www.apache.org/licenses/LICENSE-2.0
*
*  Unless required by applicable law or agreed to in writing,
*  software distributed under the License is distributed on an
*  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
*  KIND, either express or implied.  See the License for the
*  specific language governing permissions and limitations
*  under the License.
*/

using System;
using System.Collections;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;

namespace OpenSearch.Client
{
	internal class GetEnumerator<TSource> : IEnumerator<TSource>, IObserver<TSource>
	{
		private readonly SemaphoreSlim _gate;
		private readonly ConcurrentQueue<TSource> _queue;
		private TSource _current;
		private bool _disposed;
		private Exception _error;
		private IDisposable _subscription;

		public GetEnumerator()
		{
			_queue = new ConcurrentQueue<TSource>();
			_gate = new SemaphoreSlim(0);
		}

		public TSource Current => _current;

		object IEnumerator.Current => _current;

		public void Dispose()
		{
			_subscription.Dispose();

			_disposed = true;
			_gate.Release();
		}

		public bool MoveNext()
		{
			_gate.Wait();

			if (_disposed)
				throw new ObjectDisposedException("");

			if (_queue.TryDequeue(out _current))
				return true;

			if (_error != null) throw _error;

			_gate.Release(); // In the (rare) case the user calls MoveNext again we shouldn't block!
			return false;
		}

		public void Reset() => throw new NotSupportedException();

		public void OnCompleted()
		{
			_subscription.Dispose();
			_gate.Release();
		}

		public void OnError(Exception error)
		{
			_error = error;
			_subscription.Dispose();
			_gate.Release();
		}

		public virtual void OnNext(TSource value)
		{
			_queue.Enqueue(value);
			_gate.Release();
		}

		private IEnumerator<TSource> Run(IObservable<TSource> source)
		{
			//
			// [OK] Use of unsafe Subscribe: non-pretentious exact mirror with the dual GetEnumerator method.
			//
			_subscription = source.Subscribe /*Unsafe*/(this);
			return this;
		}

		public IEnumerable<TSource> ToEnumerable(IObservable<TSource> source) =>
			new AnonymousEnumerable<TSource>(() => Run(source));

		internal sealed class AnonymousEnumerable<T> : IEnumerable<T>
		{
			private readonly Func<IEnumerator<T>> _getEnumerator;

			public AnonymousEnumerable(Func<IEnumerator<T>> getEnumerator) => _getEnumerator = getEnumerator;

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

			public IEnumerator<T> GetEnumerator() => _getEnumerator();
		}
	}
}