using System.Collections.Concurrent;

namespace System.Threading.Tasks;

internal struct TaskCompletionQueue<TCompletion> where TCompletion : class
{
	private TCompletion single;

	private ConcurrentOrderedList<TCompletion> completed;

	public bool HasElements
	{
		get
		{
			if (single == null)
			{
				if (completed != null)
				{
					return completed.Count != 0;
				}
				return false;
			}
			return true;
		}
	}

	public void Add(TCompletion continuation)
	{
		if (single != null || Interlocked.CompareExchange(ref single, continuation, null) != null)
		{
			if (completed == null)
			{
				Interlocked.CompareExchange(ref completed, new ConcurrentOrderedList<TCompletion>(), null);
			}
			completed.TryAdd(continuation);
		}
	}

	public bool Remove(TCompletion continuation)
	{
		TCompletion val = single;
		if (val != null && val == continuation && Interlocked.CompareExchange(ref single, null, continuation) == continuation)
		{
			return true;
		}
		if (completed != null)
		{
			return completed.TryRemove(continuation);
		}
		return false;
	}

	public bool TryGetNextCompletion(out TCompletion continuation)
	{
		continuation = null;
		if (single != null && (continuation = Interlocked.Exchange(ref single, null)) != null)
		{
			return true;
		}
		if (completed != null)
		{
			return completed.TryPop(out continuation);
		}
		return false;
	}
}