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

namespace UnityMemoryMappedFile
{
    public sealed class AsyncLock
    {
        private readonly System.Threading.SemaphoreSlim m_semaphore
        = new System.Threading.SemaphoreSlim(1, 1);
        private readonly Task<IDisposable> m_releaser;

        public AsyncLock()
        {
            m_releaser = Task.FromResult((IDisposable)new Releaser(this));
        }

        public Task<IDisposable> LockAsync()
        {
            var wait = m_semaphore.WaitAsync();
            return wait.IsCompleted ?
            m_releaser :
            wait.ContinueWith(
            (_, state) => (IDisposable)state,
            m_releaser.Result,
            System.Threading.CancellationToken.None,
            TaskContinuationOptions.ExecuteSynchronously,
            TaskScheduler.Default
            );
        }
        private sealed class Releaser : IDisposable
        {
            private readonly AsyncLock m_toRelease;
            internal Releaser(AsyncLock toRelease) { m_toRelease = toRelease; }
            public void Dispose() { m_toRelease.m_semaphore.Release(); }
        }
    }
}