// -----------------------------------------------------------------------
// <copyright file="AsyncSemaphore.cs" company="Microsoft">
// TODO: Update copyright text.
// </copyright>
// -----------------------------------------------------------------------

using System.Threading.Tasks;

namespace Pithos.Core
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;

    /// <summary>
    /// TODO: Update summary.
    /// </summary>
    public class AsyncSemaphore
    {
        private readonly static Task s_completed = TaskEx.FromResult(true);
        private readonly Queue<TaskCompletionSource<bool>> m_waiters = new Queue<TaskCompletionSource<bool>>();
        private int m_currentCount; 

        public AsyncSemaphore(int initialCount)
        {
            if (initialCount < 0) throw new ArgumentOutOfRangeException("initialCount");
            m_currentCount = initialCount; 
        }
        public Task WaitAsync()
        {

            lock (m_waiters)
            {
                if (m_currentCount > 0)
                {
                    --m_currentCount;
                    return s_completed;
                }
                else
                {
                    var waiter = new TaskCompletionSource<bool>();
                    m_waiters.Enqueue(waiter);
                    return waiter.Task;
                }
            }

        }

        public void Release()
        {
            TaskCompletionSource<bool> toRelease = null;
            lock (m_waiters)
            {
                if (m_waiters.Count > 0)
                    toRelease = m_waiters.Dequeue();
                else
                    ++m_currentCount;
            }
            if (toRelease != null)
                toRelease.SetResult(true);
        }
    }
}