// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

#if LEGACY_WINRT
using System.ComponentModel;
using Windows.System.Threading;

namespace System.Reactive.Concurrency
{
    /// <summary>
    /// Represents an object that schedules units of work on the Windows Runtime thread pool.
    /// </summary>
    /// <seealso cref="Default">Singleton instance of this type exposed through this static property.</seealso>
    [CLSCompliant(false)]
    public sealed class ThreadPoolScheduler : LocalScheduler, ISchedulerPeriodic
    {
        private static readonly Lazy<ThreadPoolScheduler> LazyDefault = new(static () => new ThreadPoolScheduler());

        /// <summary>
        /// Constructs a ThreadPoolScheduler that schedules units of work on the Windows ThreadPool.
        /// </summary>
        public ThreadPoolScheduler()
        {
        }

        /// <summary>
        /// Constructs a ThreadPoolScheduler that schedules units of work on the Windows ThreadPool with the given priority.
        /// </summary>
        /// <param name="priority">Priority for scheduled units of work.</param>
        public ThreadPoolScheduler(WorkItemPriority priority)
        {
            Priority = priority;
            Options = WorkItemOptions.None;
        }

        /// <summary>
        /// Constructs a ThreadPoolScheduler that schedules units of work on the Windows ThreadPool with the given priority.
        /// </summary>
        /// <param name="priority">Priority for scheduled units of work.</param>
        /// <param name="options">Options that configure how work is scheduled.</param>
        public ThreadPoolScheduler(WorkItemPriority priority, WorkItemOptions options)
        {
            Priority = priority;
            Options = options;
        }

        /// <summary>
        /// Gets the singleton instance of the Windows Runtime thread pool scheduler.
        /// </summary>
        [Obsolete("Use the Instance property", false)]
        [EditorBrowsable(EditorBrowsableState.Never)]
        public static ThreadPoolScheduler Default => LazyDefault.Value;

        /// <summary>
        /// Gets the singleton instance of the Windows Runtime thread pool scheduler.
        /// </summary>
        public static ThreadPoolScheduler Instance => LazyDefault.Value;

        /// <summary>
        /// Gets the priority at which work is scheduled.
        /// </summary>
        public WorkItemPriority Priority { get; }

        /// <summary>
        /// Gets the options that configure how work is scheduled.
        /// </summary>
        public WorkItemOptions Options { get; }

        /// <summary>
        /// Schedules an action to be executed.
        /// </summary>
        /// <typeparam name="TState">The type of the state passed to the scheduled action.</typeparam>
        /// <param name="state">State passed to the action to be executed.</param>
        /// <param name="action">Action to be executed.</param>
        /// <returns>The disposable object used to cancel the scheduled action (best effort).</returns>
        /// <exception cref="ArgumentNullException"><paramref name="action"/> is null.</exception>
        public override IDisposable Schedule<TState>(TState state, Func<IScheduler, TState, IDisposable> action)
        {
            if (action == null)
                throw new ArgumentNullException(nameof(action));

            var userWorkItem = new UserWorkItem<TState>(this, state, action);
            
            var res = ThreadPool.RunAsync(
                iaa => userWorkItem.Run(),
                Priority,
                Options);

            userWorkItem.CancelQueueDisposable = res.AsDisposable();

            return userWorkItem;
        }

        /// <summary>
        /// Schedules an action to be executed after dueTime, using a Windows.System.Threading.ThreadPoolTimer object.
        /// </summary>
        /// <typeparam name="TState">The type of the state passed to the scheduled action.</typeparam>
        /// <param name="state">State passed to the action to be executed.</param>
        /// <param name="action">Action to be executed.</param>
        /// <param name="dueTime">Relative time after which to execute the action.</param>
        /// <returns>The disposable object used to cancel the scheduled action (best effort).</returns>
        /// <exception cref="ArgumentNullException"><paramref name="action"/> is null.</exception>
        public override IDisposable Schedule<TState>(TState state, TimeSpan dueTime, Func<IScheduler, TState, IDisposable> action)
        {
            if (action == null)
                throw new ArgumentNullException(nameof(action));

            var dt = Scheduler.Normalize(dueTime);

            if (dt.Ticks == 0)
            {
                return Schedule(state, action);
            }

            return ScheduleSlow(state, dt, action);
        }

        private IDisposable ScheduleSlow<TState>(TState state, TimeSpan dueTime, Func<IScheduler, TState, IDisposable> action)
        {
            var userWorkItem = new UserWorkItem<TState>(this, state, action);

            var res = ThreadPoolTimer.CreateTimer(
                tpt => userWorkItem.Run(),
                dueTime);

            userWorkItem.CancelQueueDisposable = res.AsDisposable();

            return userWorkItem;
        }

        /// <summary>
        /// Schedules a periodic piece of work, using a Windows.System.Threading.ThreadPoolTimer object.
        /// </summary>
        /// <typeparam name="TState">The type of the state passed to the scheduled action.</typeparam>
        /// <param name="state">Initial state passed to the action upon the first iteration.</param>
        /// <param name="period">Period for running the work periodically.</param>
        /// <param name="action">Action to be executed, potentially updating the state.</param>
        /// <returns>The disposable object used to cancel the scheduled recurring action (best effort).</returns>
        /// <exception cref="ArgumentNullException"><paramref name="action"/> is null.</exception>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="period"/> is less than one millisecond.</exception>
        public IDisposable SchedulePeriodic<TState>(TState state, TimeSpan period, Func<TState, TState> action)
        {
            //
            // The WinRT thread pool is based on the Win32 thread pool and cannot handle
            // sub-1ms resolution. When passing a lower period, we get single-shot
            // timer behavior instead. See MSDN documentation for CreatePeriodicTimer
            // for more information.
            //
            if (period < TimeSpan.FromMilliseconds(1))
                throw new ArgumentOutOfRangeException(nameof(period), Strings_PlatformServices.WINRT_NO_SUB1MS_TIMERS);
            if (action == null)
                throw new ArgumentNullException(nameof(action));

            return new PeriodicallyScheduledWorkItem<TState>(state, period, action);
        }

        private sealed class PeriodicallyScheduledWorkItem<TState> : IDisposable
        {
            private TState _state;
            private Func<TState, TState> _action;

            private readonly ThreadPoolTimer _timer;
            private readonly AsyncLock _gate = new();

            public PeriodicallyScheduledWorkItem(TState state, TimeSpan period, Func<TState, TState> action)
            {
                _state = state;
                _action = action;

                _timer = ThreadPoolTimer.CreatePeriodicTimer(
                    Tick,
                    period);
            }

            private void Tick(ThreadPoolTimer timer)
            {
                _gate.Wait(
                    this,
                    static @this => @this._state = @this._action(@this._state));
            }

            public void Dispose()
            {
                _timer.Cancel();
                _gate.Dispose();
                _action = Stubs<TState>.I;
            }
        }
    }
}
#endif
