// 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. 

using System.Collections.Generic;
using System.Globalization;

namespace System.Reactive.Concurrency
{
    /// <summary>
    /// Base class for virtual time schedulers.
    /// </summary>
    /// <typeparam name="TAbsolute">Absolute time representation type.</typeparam>
    /// <typeparam name="TRelative">Relative time representation type.</typeparam>
    public abstract class VirtualTimeSchedulerBase<TAbsolute, TRelative> : IScheduler, IServiceProvider, IStopwatchProvider
        where TAbsolute : IComparable<TAbsolute>
    {
        /// <summary>
        /// Creates a new virtual time scheduler with the default value of TAbsolute as the initial clock value.
        /// </summary>
        protected VirtualTimeSchedulerBase()
            : this(default!, Comparer<TAbsolute>.Default)
        {
            //
            // NB: We allow a default value for TAbsolute here, which typically is a struct. For compat reasons, we can't
            //     add a generic constraint (either struct or, better, new()), and maybe a derived class has handled null
            //     in all abstract methods.
            //
        }

        /// <summary>
        /// Creates a new virtual time scheduler with the specified initial clock value and absolute time comparer.
        /// </summary>
        /// <param name="initialClock">Initial value for the clock.</param>
        /// <param name="comparer">Comparer to determine causality of events based on absolute time.</param>
        /// <exception cref="ArgumentNullException"><paramref name="comparer"/> is <c>null</c>.</exception>
        protected VirtualTimeSchedulerBase(TAbsolute initialClock, IComparer<TAbsolute> comparer)
        {
            Clock = initialClock;
            Comparer = comparer ?? throw new ArgumentNullException(nameof(comparer));
        }

        /// <summary>
        /// Adds a relative time value to an absolute time value.
        /// </summary>
        /// <param name="absolute">Absolute time value.</param>
        /// <param name="relative">Relative time value to add.</param>
        /// <returns>The resulting absolute time sum value.</returns>
        protected abstract TAbsolute Add(TAbsolute absolute, TRelative relative);

        /// <summary>
        /// Converts the absolute time value to a DateTimeOffset value.
        /// </summary>
        /// <param name="absolute">Absolute time value to convert.</param>
        /// <returns>The corresponding DateTimeOffset value.</returns>
        protected abstract DateTimeOffset ToDateTimeOffset(TAbsolute absolute);

        /// <summary>
        /// Converts the TimeSpan value to a relative time value.
        /// </summary>
        /// <param name="timeSpan">TimeSpan value to convert.</param>
        /// <returns>The corresponding relative time value.</returns>
        protected abstract TRelative ToRelative(TimeSpan timeSpan);

        /// <summary>
        /// Gets whether the scheduler is enabled to run work.
        /// </summary>
        public bool IsEnabled { get; private set; }

        /// <summary>
        /// Gets the comparer used to compare absolute time values.
        /// </summary>
        protected IComparer<TAbsolute> Comparer { get; }

        /// <summary>
        /// Schedules an action to be executed at dueTime.
        /// </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="dueTime">Absolute time at which to execute the action.</param>
        /// <param name="action">Action to be executed.</param>
        /// <returns>The disposable object used to cancel the scheduled action (best effort).</returns>
        public abstract IDisposable ScheduleAbsolute<TState>(TState state, TAbsolute dueTime, Func<IScheduler, TState, IDisposable> action);

        /// <summary>
        /// Schedules an action to be executed at dueTime.
        /// </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="dueTime">Relative time after which to execute the action.</param>
        /// <param name="action">Action to be executed.</param>
        /// <returns>The disposable object used to cancel the scheduled action (best effort).</returns>
        public IDisposable ScheduleRelative<TState>(TState state, TRelative dueTime, Func<IScheduler, TState, IDisposable> action)
        {
            if (action == null)
            {
                throw new ArgumentNullException(nameof(action));
            }

            var runAt = Add(Clock, dueTime);

            return ScheduleAbsolute(state, runAt, action);
        }

        /// <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 <c>null</c>.</exception>
        public IDisposable Schedule<TState>(TState state, Func<IScheduler, TState, IDisposable> action)
        {
            if (action == null)
            {
                throw new ArgumentNullException(nameof(action));
            }

            return ScheduleAbsolute(state, Clock, action);
        }

        /// <summary>
        /// Schedules an action to be executed after dueTime.
        /// </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="dueTime">Relative time after which to execute the action.</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 <c>null</c>.</exception>
        public IDisposable Schedule<TState>(TState state, TimeSpan dueTime, Func<IScheduler, TState, IDisposable> action)
        {
            if (action == null)
            {
                throw new ArgumentNullException(nameof(action));
            }

            return ScheduleRelative(state, ToRelative(dueTime), action);
        }

        /// <summary>
        /// Schedules an action to be executed at dueTime.
        /// </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="dueTime">Absolute time at which to execute the action.</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 <c>null</c>.</exception>
        public IDisposable Schedule<TState>(TState state, DateTimeOffset dueTime, Func<IScheduler, TState, IDisposable> action)
        {
            if (action == null)
            {
                throw new ArgumentNullException(nameof(action));
            }

            return ScheduleRelative(state, ToRelative(dueTime - Now), action);
        }

        /// <summary>
        /// Starts the virtual time scheduler.
        /// </summary>
        public void Start()
        {
            if (!IsEnabled)
            {
                IsEnabled = true;
                do
                {
                    var next = GetNext();
                    if (next != null)
                    {
                        if (Comparer.Compare(next.DueTime, Clock) > 0)
                        {
                            Clock = next.DueTime;
                        }

                        next.Invoke();
                    }
                    else
                    {
                        IsEnabled = false;
                    }
                } while (IsEnabled);
            }
        }

        /// <summary>
        /// Stops the virtual time scheduler.
        /// </summary>
        public void Stop()
        {
            IsEnabled = false;
        }

        /// <summary>
        /// Advances the scheduler's clock to the specified time, running all work till that point.
        /// </summary>
        /// <param name="time">Absolute time to advance the scheduler's clock to.</param>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="time"/> is in the past.</exception>
        /// <exception cref="InvalidOperationException">The scheduler is already running. VirtualTimeScheduler doesn't support running nested work dispatch loops. To simulate time slippage while running work on the scheduler, use <see cref="Sleep"/>.</exception>
        public void AdvanceTo(TAbsolute time)
        {
            var dueToClock = Comparer.Compare(time, Clock);
            if (dueToClock < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(time));
            }

            if (dueToClock == 0)
            {
                return;
            }

            if (!IsEnabled)
            {
                IsEnabled = true;
                do
                {
                    var next = GetNext();
                    if (next != null && Comparer.Compare(next.DueTime, time) <= 0)
                    {
                        if (Comparer.Compare(next.DueTime, Clock) > 0)
                        {
                            Clock = next.DueTime;
                        }

                        next.Invoke();
                    }
                    else
                    {
                        IsEnabled = false;
                    }
                } while (IsEnabled);

                Clock = time;
            }
            else
            {
                throw new InvalidOperationException(string.Format(CultureInfo.CurrentCulture, Strings_Linq.CANT_ADVANCE_WHILE_RUNNING, nameof(AdvanceTo)));
            }
        }

        /// <summary>
        /// Advances the scheduler's clock by the specified relative time, running all work scheduled for that timespan.
        /// </summary>
        /// <param name="time">Relative time to advance the scheduler's clock by.</param>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="time"/> is negative.</exception>
        /// <exception cref="InvalidOperationException">The scheduler is already running. VirtualTimeScheduler doesn't support running nested work dispatch loops. To simulate time slippage while running work on the scheduler, use <see cref="Sleep"/>.</exception>
        public void AdvanceBy(TRelative time)
        {
            var dt = Add(Clock, time);

            var dueToClock = Comparer.Compare(dt, Clock);
            if (dueToClock < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(time));
            }

            if (dueToClock == 0)
            {
                return;
            }

            if (!IsEnabled)
            {
                AdvanceTo(dt);
            }
            else
            {
                throw new InvalidOperationException(string.Format(CultureInfo.CurrentCulture, Strings_Linq.CANT_ADVANCE_WHILE_RUNNING, nameof(AdvanceBy)));
            }
        }

        /// <summary>
        /// Advances the scheduler's clock by the specified relative time.
        /// </summary>
        /// <param name="time">Relative time to advance the scheduler's clock by.</param>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="time"/> is negative.</exception>
        public void Sleep(TRelative time)
        {
            var dt = Add(Clock, time);

            var dueToClock = Comparer.Compare(dt, Clock);
            if (dueToClock < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(time));
            }

            Clock = dt;
        }

        /// <summary>
        /// Gets the scheduler's absolute time clock value.
        /// </summary>
        public TAbsolute Clock
        {
            get;
            protected set;
        }

        /// <summary>
        /// Gets the scheduler's notion of current time.
        /// </summary>
        public DateTimeOffset Now => ToDateTimeOffset(Clock);

        /// <summary>
        /// Gets the next scheduled item to be executed.
        /// </summary>
        /// <returns>The next scheduled item.</returns>
        protected abstract IScheduledItem<TAbsolute>? GetNext();

        object? IServiceProvider.GetService(Type serviceType) => GetService(serviceType);

        /// <summary>
        /// Discovers scheduler services by interface type. The base class implementation supports
        /// only the IStopwatchProvider service. To influence service discovery - such as adding
        /// support for other scheduler services - derived types can override this method.
        /// </summary>
        /// <param name="serviceType">Scheduler service interface type to discover.</param>
        /// <returns>Object implementing the requested service, if available; null otherwise.</returns>
        protected virtual object? GetService(Type serviceType)
        {
            if (serviceType == typeof(IStopwatchProvider))
            {
                return this;
            }

            return null;
        }

        /// <summary>
        /// Starts a new stopwatch object.
        /// </summary>
        /// <returns>New stopwatch object; started at the time of the request.</returns>
        public IStopwatch StartStopwatch()
        {
            var start = ClockToDateTimeOffset();
            return new VirtualTimeStopwatch(this, start);
        }

        private DateTimeOffset ClockToDateTimeOffset() => ToDateTimeOffset(Clock);

        private sealed class VirtualTimeStopwatch : IStopwatch
        {
            private readonly VirtualTimeSchedulerBase<TAbsolute, TRelative> _parent;
            private readonly DateTimeOffset _start;

            public VirtualTimeStopwatch(VirtualTimeSchedulerBase<TAbsolute, TRelative> parent, DateTimeOffset start)
            {
                _parent = parent;
                _start = start;
            }

            public TimeSpan Elapsed => _parent.ClockToDateTimeOffset() - _start;
        }
    }

    /// <summary>
    /// Base class for virtual time schedulers using a priority queue for scheduled items.
    /// </summary>
    /// <typeparam name="TAbsolute">Absolute time representation type.</typeparam>
    /// <typeparam name="TRelative">Relative time representation type.</typeparam>
    public abstract class VirtualTimeScheduler<TAbsolute, TRelative> : VirtualTimeSchedulerBase<TAbsolute, TRelative>
        where TAbsolute : IComparable<TAbsolute>
    {
        private readonly SchedulerQueue<TAbsolute> _queue = new();

        /// <summary>
        /// Creates a new virtual time scheduler with the default value of TAbsolute as the initial clock value.
        /// </summary>
        protected VirtualTimeScheduler()
        {
        }

        /// <summary>
        /// Creates a new virtual time scheduler.
        /// </summary>
        /// <param name="initialClock">Initial value for the clock.</param>
        /// <param name="comparer">Comparer to determine causality of events based on absolute time.</param>
        /// <exception cref="ArgumentNullException"><paramref name="comparer"/> is <c>null</c>.</exception>
        protected VirtualTimeScheduler(TAbsolute initialClock, IComparer<TAbsolute> comparer)
            : base(initialClock, comparer)
        {
        }

        /// <summary>
        /// Gets the next scheduled item to be executed.
        /// </summary>
        /// <returns>The next scheduled item.</returns>
        protected override IScheduledItem<TAbsolute>? GetNext()
        {
            lock (_queue)
            {
                while (_queue.Count > 0)
                {
                    var next = _queue.Peek();
                    if (next.IsCanceled)
                    {
                        _queue.Dequeue();
                    }
                    else
                    {
                        return next;
                    }
                }
            }

            return null;
        }

        /// <summary>
        /// Schedules an action to be executed at dueTime.
        /// </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">Absolute time at 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 <c>null</c>.</exception>
        public override IDisposable ScheduleAbsolute<TState>(TState state, TAbsolute dueTime, Func<IScheduler, TState, IDisposable> action)
        {
            if (action == null)
            {
                throw new ArgumentNullException(nameof(action));
            }

            ScheduledItem<TAbsolute, TState>? si = null;

            var run = new Func<IScheduler, TState, IDisposable>((scheduler, state1) =>
            {
                lock (_queue)
                {
                    _queue.Remove(si!); // NB: Assigned before function is invoked.
                }

                return action(scheduler, state1);
            });

            si = new ScheduledItem<TAbsolute, TState>(this, state, run, dueTime, Comparer);

            lock (_queue)
            {
                _queue.Enqueue(si);
            }

            return si;
        }
    }
}