using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace SystemX.Net.Platform.Common.Event
{
    public class PauseTokenSource
    {
        private volatile TaskCompletionSource<bool> m_paused;
        internal static readonly Task s_completedTask = Task.FromResult(true);

        public void Pause()
        {
            Interlocked.CompareExchange(
                ref m_paused, new TaskCompletionSource<bool>(), null);
        }

        public void Resume()
        {
            while (true)
            {
                var tcs = m_paused;
                if (tcs == null) return;
                if (Interlocked.CompareExchange(ref m_paused, null, tcs) == tcs)
                {
                    tcs.SetResult(true);
                    break;
                }
            }
        }

        public bool IsPaused
        {
            get { return m_paused != null; }            
        }

        public PauseToken Token { get { return new PauseToken(this); } }

        internal Task WaitWhilePausedAsync()
        {
            var cur = m_paused;
            return cur != null ? cur.Task : s_completedTask;
        }
    }

    public struct PauseToken
    {
        private readonly PauseTokenSource m_source;
        internal PauseToken(PauseTokenSource source) { m_source = source; }

        public bool IsPaused { get { return m_source != null && m_source.IsPaused; } }

        public Task WaitWhilePausedAsync()
        {
            return IsPaused ?
                m_source.WaitWhilePausedAsync() :
                PauseTokenSource.s_completedTask;
        }
    }
}