Avalonia/src/Avalonia.Base/Utilities/SingleOrQueue.cs

using System;
using System.Collections.Generic;
using System.Text;

namespace Avalonia.Utilities
{
    /// <summary>
    /// FIFO Queue optimized for holding zero or one items.
    /// </summary>
    /// <typeparam name="T">The type of items held in the queue.</typeparam>
    public class SingleOrQueue<T>
    {
        private T _head;
        private Queue<T> _tail;

        private Queue<T> Tail => _tail ?? (_tail = new Queue<T>());

        private bool HasTail => _tail != null;

        public bool Empty { get; private set; } = true;

        public void Enqueue(T value)
        {
            if (Empty)
            {
                _head = value;
            }
            else
            {
                Tail.Enqueue(value);
            }

            Empty = false;
        }

        public T Dequeue()
        {
            if (Empty)
            {
                throw new InvalidOperationException("Cannot dequeue from an empty queue!");
            }

            var result = _head;

            if (HasTail && Tail.Count != 0)
            {
                _head = Tail.Dequeue();
            }
            else
            {
                _head = default;
                Empty = true;
            }

            return result;
        }
    }
}