// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved. See License.txt in the project root for license information.

using System.Collections.Generic;
using System.Reactive.Disposables;

namespace System.Reactive.Concurrency
{
    /// <summary>
    /// Base class for historical schedulers, which are virtual time schedulers that use DateTimeOffset for absolute time and TimeSpan for relative time.
    /// </summary>
    public abstract class HistoricalSchedulerBase : VirtualTimeSchedulerBase<DateTimeOffset, TimeSpan>
    {
        /// <summary>
        /// Creates a new historical scheduler with the minimum value of DateTimeOffset as the initial clock value.
        /// </summary>
        protected HistoricalSchedulerBase()
            : base(DateTimeOffset.MinValue, Comparer<DateTimeOffset>.Default)
        {
        }

        /// <summary>
        /// Creates a new historical scheduler with the specified initial clock value.
        /// </summary>
        /// <param name="initialClock">Initial clock value.</param>
        protected HistoricalSchedulerBase(DateTimeOffset initialClock)
            : base(initialClock, Comparer<DateTimeOffset>.Default)
        {
        }

        /// <summary>
        /// Creates a new historical 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>
        protected HistoricalSchedulerBase(DateTimeOffset initialClock, IComparer<DateTimeOffset> comparer)
            : base(initialClock, 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 override DateTimeOffset Add(DateTimeOffset absolute, TimeSpan relative)
        {
            return absolute.Add(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 override DateTimeOffset ToDateTimeOffset(DateTimeOffset absolute)
        {
            return 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 override TimeSpan ToRelative(TimeSpan timeSpan)
        {
            return timeSpan;
        }
    }

    /// <summary>
    /// Provides a virtual time scheduler that uses DateTimeOffset for absolute time and TimeSpan for relative time.
    /// </summary>
    public class HistoricalScheduler : HistoricalSchedulerBase
    {
        private readonly SchedulerQueue<DateTimeOffset> queue = new SchedulerQueue<DateTimeOffset>();

        /// <summary>
        /// Creates a new historical scheduler with the minimum value of DateTimeOffset as the initial clock value.
        /// </summary>
        public HistoricalScheduler()
            : base()
        {
        }

        /// <summary>
        /// Creates a new historical scheduler with the specified initial clock value.
        /// </summary>
        /// <param name="initialClock">Initial value for the clock.</param>
        public HistoricalScheduler(DateTimeOffset initialClock)
            : base(initialClock)
        {
        }

        /// <summary>
        /// Creates a new historical scheduler with the specified initial clock value.
        /// </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 null.</exception>
        public HistoricalScheduler(DateTimeOffset initialClock, IComparer<DateTimeOffset> comparer)
            : base(initialClock, comparer)
        {
        }

        /// <summary>
        /// Gets the next scheduled item to be executed.
        /// </summary>
        /// <returns>The next scheduled item.</returns>
        protected override IScheduledItem<DateTimeOffset> GetNext()
        {
            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 null.</exception>
        public override IDisposable ScheduleAbsolute<TState>(TState state, DateTimeOffset dueTime, Func<IScheduler, TState, IDisposable> action)
        {
            if (action == null)
                throw new ArgumentNullException("action");

            var si = default(ScheduledItem<DateTimeOffset, TState>);

            var run = new Func<IScheduler, TState, IDisposable>((scheduler, state1) =>
            {
                queue.Remove(si);
                return action(scheduler, state1);
            });

            si = new ScheduledItem<DateTimeOffset, TState>(this, state, run, dueTime, Comparer);
            queue.Enqueue(si);

            return Disposable.Create(si.Cancel);
        }
    }
}