// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the Apache 2.0 License.
// See the LICENSE file in the project root for more information. 

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

namespace System.Reactive.Concurrency
{
    /// <summary>
    /// Abstract base class for scheduled work items.
    /// </summary>
    /// <typeparam name="TAbsolute">Absolute time representation type.</typeparam>
    public abstract class ScheduledItem<TAbsolute> : IScheduledItem<TAbsolute>, IComparable<ScheduledItem<TAbsolute>>
        where TAbsolute : IComparable<TAbsolute>
    {
        private readonly SingleAssignmentDisposable _disposable = new SingleAssignmentDisposable();
        private readonly IComparer<TAbsolute> _comparer;

        /// <summary>
        /// Creates a new scheduled work item to run at the specified time.
        /// </summary>
        /// <param name="dueTime">Absolute time at which the work item has to be executed.</param>
        /// <param name="comparer">Comparer used to compare work items based on their scheduled time.</param>
        /// <exception cref="ArgumentNullException"><paramref name="comparer"/> is <c>null</c>.</exception>
        protected ScheduledItem(TAbsolute dueTime, IComparer<TAbsolute> comparer)
        {
            if (comparer == null)
                throw new ArgumentNullException(nameof(comparer));

            DueTime = dueTime;
            _comparer = comparer;
        }

        /// <summary>
        /// Gets the absolute time at which the item is due for invocation.
        /// </summary>
        public TAbsolute DueTime { get; }

        /// <summary>
        /// Invokes the work item.
        /// </summary>
        public void Invoke()
        {
            if (!_disposable.IsDisposed)
            {
                _disposable.Disposable = InvokeCore();
            }
        }

        /// <summary>
        /// Implement this method to perform the work item invocation, returning a disposable object for deep cancellation.
        /// </summary>
        /// <returns>Disposable object used to cancel the work item and/or derived work items.</returns>
        protected abstract IDisposable InvokeCore();

        #region Inequality

        /// <summary>
        /// Compares the work item with another work item based on absolute time values.
        /// </summary>
        /// <param name="other">Work item to compare the current work item to.</param>
        /// <returns>Relative ordering between this and the specified work item.</returns>
        /// <remarks>The inequality operators are overloaded to provide results consistent with the <see cref="IComparable"/> implementation. Equality operators implement traditional reference equality semantics.</remarks>
        public int CompareTo(ScheduledItem<TAbsolute> other)
        {
            // MSDN: By definition, any object compares greater than null, and two null references compare equal to each other.
            if (ReferenceEquals(other, null))
            {
                return 1;
            }

            return _comparer.Compare(DueTime, other.DueTime);
        }

        /// <summary>
        /// Determines whether one specified <see cref="ScheduledItem{TAbsolute}" /> object is due before a second specified <see cref="ScheduledItem{TAbsolute}" /> object.
        /// </summary>
        /// <param name="left">The first object to compare.</param>
        /// <param name="right">The second object to compare.</param>
        /// <returns><c>true</c> if the <see cref="DueTime"/> value of left is earlier than the <see cref="DueTime"/> value of right; otherwise, <c>false</c>.</returns>
        /// <remarks>This operator provides results consistent with the <see cref="IComparable"/> implementation.</remarks>
        public static bool operator <(ScheduledItem<TAbsolute> left, ScheduledItem<TAbsolute> right) => Comparer<ScheduledItem<TAbsolute>>.Default.Compare(left, right) < 0;

        /// <summary>
        /// Determines whether one specified <see cref="ScheduledItem{TAbsolute}" /> object is due before or at the same of a second specified <see cref="ScheduledItem{TAbsolute}" /> object.
        /// </summary>
        /// <param name="left">The first object to compare.</param>
        /// <param name="right">The second object to compare.</param>
        /// <returns><c>true</c> if the <see cref="DueTime"/> value of left is earlier than or simultaneous with the <see cref="DueTime"/> value of right; otherwise, <c>false</c>.</returns>
        /// <remarks>This operator provides results consistent with the <see cref="IComparable"/> implementation.</remarks>
        public static bool operator <=(ScheduledItem<TAbsolute> left, ScheduledItem<TAbsolute> right) => Comparer<ScheduledItem<TAbsolute>>.Default.Compare(left, right) <= 0;

        /// <summary>
        /// Determines whether one specified <see cref="ScheduledItem{TAbsolute}" /> object is due after a second specified <see cref="ScheduledItem{TAbsolute}" /> object.
        /// </summary>
        /// <param name="left">The first object to compare.</param>
        /// <param name="right">The second object to compare.</param>
        /// <returns><c>true</c> if the <see cref="DueTime"/> value of left is later than the <see cref="DueTime"/> value of right; otherwise, <c>false</c>.</returns>
        /// <remarks>This operator provides results consistent with the <see cref="IComparable"/> implementation.</remarks>
        public static bool operator >(ScheduledItem<TAbsolute> left, ScheduledItem<TAbsolute> right) => Comparer<ScheduledItem<TAbsolute>>.Default.Compare(left, right) > 0;

        /// <summary>
        /// Determines whether one specified <see cref="ScheduledItem{TAbsolute}" /> object is due after or at the same time of a second specified <see cref="ScheduledItem{TAbsolute}" /> object.
        /// </summary>
        /// <param name="left">The first object to compare.</param>
        /// <param name="right">The second object to compare.</param>
        /// <returns><c>true</c> if the <see cref="DueTime"/> value of left is later than or simultaneous with the <see cref="DueTime"/> value of right; otherwise, <c>false</c>.</returns>
        /// <remarks>This operator provides results consistent with the <see cref="IComparable"/> implementation.</remarks>
        public static bool operator >=(ScheduledItem<TAbsolute> left, ScheduledItem<TAbsolute> right) => Comparer<ScheduledItem<TAbsolute>>.Default.Compare(left, right) >= 0;

        #endregion

        #region Equality

        /// <summary>
        /// Determines whether two specified <see cref="ScheduledItem{TAbsolute, TValue}" /> objects are equal.
        /// </summary>
        /// <param name="left">The first object to compare.</param>
        /// <param name="right">The second object to compare.</param>
        /// <returns><c>true</c> if both <see cref="ScheduledItem{TAbsolute, TValue}" /> are equal; otherwise, <c>false</c>.</returns>
        /// <remarks>This operator does not provide results consistent with the IComparable implementation. Instead, it implements reference equality.</remarks>
        public static bool operator ==(ScheduledItem<TAbsolute> left, ScheduledItem<TAbsolute> right) => ReferenceEquals(left, right);

        /// <summary>
        /// Determines whether two specified <see cref="ScheduledItem{TAbsolute, TValue}" /> objects are inequal.
        /// </summary>
        /// <param name="left">The first object to compare.</param>
        /// <param name="right">The second object to compare.</param>
        /// <returns><c>true</c> if both <see cref="ScheduledItem{TAbsolute, TValue}" /> are inequal; otherwise, <c>false</c>.</returns>
        /// <remarks>This operator does not provide results consistent with the IComparable implementation. Instead, it implements reference equality.</remarks>
        public static bool operator !=(ScheduledItem<TAbsolute> left, ScheduledItem<TAbsolute> right) => !(left == right);

        /// <summary>
        /// Determines whether a <see cref="ScheduledItem{TAbsolute}" /> object is equal to the specified object.
        /// </summary>
        /// <param name="obj">The object to compare to the current <see cref="ScheduledItem{TAbsolute}" /> object.</param>
        /// <returns><c>true</c> if the obj parameter is a <see cref="ScheduledItem{TAbsolute}" /> object and is equal to the current <see cref="ScheduledItem{TAbsolute}" /> object; otherwise, <c>false</c>.</returns>
        public override bool Equals(object obj) => ReferenceEquals(this, obj);

        /// <summary>
        /// Returns the hash code for the current <see cref="ScheduledItem{TAbsolute}" /> object.
        /// </summary>
        /// <returns>A 32-bit signed integer hash code.</returns>
        public override int GetHashCode() => base.GetHashCode();

        #endregion

        /// <summary>
        /// Cancels the work item by disposing the resource returned by <see cref="InvokeCore"/> as soon as possible.
        /// </summary>
        public void Cancel() => _disposable.Dispose();

        /// <summary>
        /// Gets whether the work item has received a cancellation request.
        /// </summary>
        public bool IsCanceled => _disposable.IsDisposed;
    }

    /// <summary>
    /// Represents a scheduled work item based on the materialization of an IScheduler.Schedule method call.
    /// </summary>
    /// <typeparam name="TAbsolute">Absolute time representation type.</typeparam>
    /// <typeparam name="TValue">Type of the state passed to the scheduled action.</typeparam>
    public sealed class ScheduledItem<TAbsolute, TValue> : ScheduledItem<TAbsolute>
        where TAbsolute : IComparable<TAbsolute>
    {
        private readonly IScheduler _scheduler;
        private readonly TValue _state;
        private readonly Func<IScheduler, TValue, IDisposable> _action;

        /// <summary>
        /// Creates a materialized work item.
        /// </summary>
        /// <param name="scheduler">Recursive scheduler to invoke the scheduled action with.</param>
        /// <param name="state">State to pass to the scheduled action.</param>
        /// <param name="action">Scheduled action.</param>
        /// <param name="dueTime">Time at which to run the scheduled action.</param>
        /// <param name="comparer">Comparer used to compare work items based on their scheduled time.</param>
        /// <exception cref="ArgumentNullException"><paramref name="scheduler"/> or <paramref name="action"/> or <paramref name="comparer"/> is <c>null</c>.</exception>
        public ScheduledItem(IScheduler scheduler, TValue state, Func<IScheduler, TValue, IDisposable> action, TAbsolute dueTime, IComparer<TAbsolute> comparer)
            : base(dueTime, comparer)
        {
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));
            if (action == null)
                throw new ArgumentNullException(nameof(action));

            _scheduler = scheduler;
            _state = state;
            _action = action;
        }

        /// <summary>
        /// Creates a materialized work item.
        /// </summary>
        /// <param name="scheduler">Recursive scheduler to invoke the scheduled action with.</param>
        /// <param name="state">State to pass to the scheduled action.</param>
        /// <param name="action">Scheduled action.</param>
        /// <param name="dueTime">Time at which to run the scheduled action.</param>
        /// <exception cref="ArgumentNullException"><paramref name="scheduler"/> or <paramref name="action"/> is <c>null</c>.</exception>
        public ScheduledItem(IScheduler scheduler, TValue state, Func<IScheduler, TValue, IDisposable> action, TAbsolute dueTime)
            : this(scheduler, state, action, dueTime, Comparer<TAbsolute>.Default)
        {
        }

        /// <summary>
        /// Invokes the scheduled action with the supplied recursive scheduler and state.
        /// </summary>
        /// <returns>Cancellation resource returned by the scheduled action.</returns>
        protected override IDisposable InvokeCore() => _action(_scheduler, _state);
    }
}