// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;

namespace System.Threading
{
    /// <summary>
    /// Propagates notification that operations should be canceled.
    /// </summary>
    /// <remarks>
    /// <para>
    /// A <see cref="CancellationToken"/> may be created directly in an unchangeable canceled or non-canceled state
    /// using the CancellationToken's constructors. However, to have a CancellationToken that can change
    /// from a non-canceled to a canceled state,
    /// <see cref="CancellationTokenSource">CancellationTokenSource</see> must be used.
    /// CancellationTokenSource exposes the associated CancellationToken that may be canceled by the source through its
    /// <see cref="CancellationTokenSource.Token">Token</see> property.
    /// </para>
    /// <para>
    /// Once canceled, a token may not transition to a non-canceled state, and a token whose
    /// <see cref="CanBeCanceled"/> is false will never change to one that can be canceled.
    /// </para>
    /// <para>
    /// All members of this struct are thread-safe and may be used concurrently from multiple threads.
    /// </para>
    /// </remarks>
    [DebuggerDisplay("IsCancellationRequested = {IsCancellationRequested}")]
    public readonly struct CancellationToken : IEquatable<CancellationToken>
    {
        // The backing TokenSource.
        // if null, it implicitly represents the same thing as new CancellationToken(false).
        // When required, it will be instantiated to reflect this.
        private readonly CancellationTokenSource? _source;
        // !! warning. If more fields are added, the assumptions in CreateLinkedToken may no longer be valid

        /// <summary>
        /// Returns an empty CancellationToken value.
        /// </summary>
        /// <remarks>
        /// The <see cref="CancellationToken"/> value returned by this property will be non-cancelable by default.
        /// </remarks>
        public static CancellationToken None => default;

        /// <summary>
        /// Gets whether cancellation has been requested for this token.
        /// </summary>
        /// <value>Whether cancellation has been requested for this token.</value>
        /// <remarks>
        /// <para>
        /// This property indicates whether cancellation has been requested for this token,
        /// either through the token initially being constructed in a canceled state, or through
        /// calling <see cref="CancellationTokenSource.Cancel()">Cancel</see>
        /// on the token's associated <see cref="CancellationTokenSource"/>.
        /// </para>
        /// <para>
        /// If this property is true, it only guarantees that cancellation has been requested.
        /// It does not guarantee that every registered handler
        /// has finished executing, nor that cancellation requests have finished propagating
        /// to all registered handlers.  Additional synchronization may be required,
        /// particularly in situations where related objects are being canceled concurrently.
        /// </para>
        /// </remarks>
        public bool IsCancellationRequested => _source != null && _source.IsCancellationRequested;

        /// <summary>
        /// Gets whether this token is capable of being in the canceled state.
        /// </summary>
        /// <remarks>
        /// If CanBeCanceled returns false, it is guaranteed that the token will never transition
        /// into a canceled state, meaning that <see cref="IsCancellationRequested"/> will never
        /// return true.
        /// </remarks>
        public bool CanBeCanceled => _source != null;

        /// <summary>
        /// Gets a <see cref="Threading.WaitHandle"/> that is signaled when the token is canceled.</summary>
        /// <remarks>
        /// Accessing this property causes a <see cref="Threading.WaitHandle">WaitHandle</see>
        /// to be instantiated.  It is preferable to only use this property when necessary, and to then
        /// dispose the associated <see cref="CancellationTokenSource"/> instance at the earliest opportunity (disposing
        /// the source will dispose of this allocated handle).  The handle should not be closed or disposed directly.
        /// </remarks>
        /// <exception cref="ObjectDisposedException">The associated <see
        /// cref="CancellationTokenSource">CancellationTokenSource</see> has been disposed.</exception>
        public WaitHandle WaitHandle => (_source ?? CancellationTokenSource.s_neverCanceledSource).WaitHandle;

        // public CancellationToken()
        // this constructor is implicit for structs
        //   -> this should behaves exactly as for new CancellationToken(false)

        /// <summary>
        /// Internal constructor only a CancellationTokenSource should create a CancellationToken
        /// </summary>
        internal CancellationToken(CancellationTokenSource? source) => _source = source;

        /// <summary>
        /// Initializes the <see cref="CancellationToken">CancellationToken</see>.
        /// </summary>
        /// <param name="canceled">
        /// The canceled state for the token.
        /// </param>
        /// <remarks>
        /// Tokens created with this constructor will remain in the canceled state specified
        /// by the <paramref name="canceled"/> parameter.  If <paramref name="canceled"/> is false,
        /// both <see cref="CanBeCanceled"/> and <see cref="IsCancellationRequested"/> will be false.
        /// If <paramref name="canceled"/> is true,
        /// both <see cref="CanBeCanceled"/> and <see cref="IsCancellationRequested"/> will be true.
        /// </remarks>
        public CancellationToken(bool canceled) : this(canceled ? CancellationTokenSource.s_canceledSource : null)
        {
        }

        /// <summary>
        /// Registers a delegate that will be called when this <see cref="CancellationToken">CancellationToken</see> is canceled.
        /// </summary>
        /// <remarks>
        /// <para>
        /// If this token is already in the canceled state, the
        /// delegate will be run immediately and synchronously. Any exception the delegate generates will be
        /// propagated out of this method call.
        /// </para>
        /// <para>
        /// The current <see cref="ExecutionContext">ExecutionContext</see>, if one exists, will be captured
        /// along with the delegate and will be used when executing it.
        /// </para>
        /// </remarks>
        /// <param name="callback">The delegate to be executed when the <see cref="CancellationToken">CancellationToken</see> is canceled.</param>
        /// <returns>The <see cref="CancellationTokenRegistration"/> instance that can
        /// be used to unregister the callback.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="callback"/> is null.</exception>
        public CancellationTokenRegistration Register(Action callback) => Register(callback, useSynchronizationContext: false);

        /// <summary>
        /// Registers a delegate that will be called when this
        /// <see cref="CancellationToken">CancellationToken</see> is canceled.
        /// </summary>
        /// <remarks>
        /// <para>
        /// If this token is already in the canceled state, the
        /// delegate will be run immediately and synchronously. Any exception the delegate generates will be
        /// propagated out of this method call.
        /// </para>
        /// <para>
        /// The current <see cref="ExecutionContext">ExecutionContext</see>, if one exists, will be captured
        /// along with the delegate and will be used when executing it.
        /// </para>
        /// </remarks>
        /// <param name="callback">The delegate to be executed when the <see cref="CancellationToken">CancellationToken</see> is canceled.</param>
        /// <param name="useSynchronizationContext">A Boolean value that indicates whether to capture
        /// the current <see cref="SynchronizationContext">SynchronizationContext</see> and use it
        /// when invoking the <paramref name="callback"/>.</param>
        /// <returns>The <see cref="CancellationTokenRegistration"/> instance that can
        /// be used to unregister the callback.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="callback"/> is null.</exception>
        public CancellationTokenRegistration Register(Action callback, bool useSynchronizationContext)
        {
            ArgumentNullException.ThrowIfNull(callback);
            return Register(
                (Action<object?>)(static obj => ((Action)obj!)()),
                callback,
                useSynchronizationContext,
                useExecutionContext: true);
        }

        /// <summary>
        /// Registers a delegate that will be called when this
        /// <see cref="CancellationToken">CancellationToken</see> is canceled.
        /// </summary>
        /// <remarks>
        /// <para>
        /// If this token is already in the canceled state, the
        /// delegate will be run immediately and synchronously. Any exception the delegate generates will be
        /// propagated out of this method call.
        /// </para>
        /// <para>
        /// The current <see cref="ExecutionContext">ExecutionContext</see>, if one exists, will be captured
        /// along with the delegate and will be used when executing it.
        /// </para>
        /// </remarks>
        /// <param name="callback">The delegate to be executed when the <see cref="CancellationToken">CancellationToken</see> is canceled.</param>
        /// <param name="state">The state to pass to the <paramref name="callback"/> when the delegate is invoked.  This may be null.</param>
        /// <returns>The <see cref="CancellationTokenRegistration"/> instance that can
        /// be used to unregister the callback.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="callback"/> is null.</exception>
        public CancellationTokenRegistration Register(Action<object?> callback, object? state) =>
            Register(callback, state, useSynchronizationContext: false, useExecutionContext: true);

        /// <summary>Registers a delegate that will be called when this <see cref="CancellationToken">CancellationToken</see> is canceled.</summary>
        /// <remarks>
        /// If this token is already in the canceled state, the delegate will be run immediately and synchronously. Any exception the delegate
        /// generates will be propagated out of this method call. The current <see cref="ExecutionContext">ExecutionContext</see>, if one exists,
        /// will be captured along with the delegate and will be used when executing it. The current <see cref="SynchronizationContext"/> is not captured.
        /// </remarks>
        /// <param name="callback">The delegate to be executed when the <see cref="CancellationToken">CancellationToken</see> is canceled.</param>
        /// <param name="state">The state to pass to the <paramref name="callback"/> when the delegate is invoked.  This may be null.</param>
        /// <returns>The <see cref="CancellationTokenRegistration"/> instance that can be used to unregister the callback.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="callback"/> is null.</exception>
        public CancellationTokenRegistration Register(Action<object?, CancellationToken> callback, object? state) =>
            Register(callback, state, useSynchronizationContext: false, useExecutionContext: true);

        /// <summary>
        /// Registers a delegate that will be called when this
        /// <see cref="CancellationToken">CancellationToken</see> is canceled.
        /// </summary>
        /// <remarks>
        /// <para>
        /// If this token is already in the canceled state, the
        /// delegate will be run immediately and synchronously. Any exception the delegate generates will be
        /// propagated out of this method call.
        /// </para>
        /// <para>
        /// The current <see cref="ExecutionContext">ExecutionContext</see>, if one exists,
        /// will be captured along with the delegate and will be used when executing it.
        /// </para>
        /// </remarks>
        /// <param name="callback">The delegate to be executed when the <see cref="CancellationToken">CancellationToken</see> is canceled.</param>
        /// <param name="state">The state to pass to the <paramref name="callback"/> when the delegate is invoked.  This may be null.</param>
        /// <param name="useSynchronizationContext">A Boolean value that indicates whether to capture
        /// the current <see cref="SynchronizationContext">SynchronizationContext</see> and use it
        /// when invoking the <paramref name="callback"/>.</param>
        /// <returns>The <see cref="CancellationTokenRegistration"/> instance that can
        /// be used to unregister the callback.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="callback"/> is null.</exception>
        /// <exception cref="ObjectDisposedException">The associated <see
        /// cref="CancellationTokenSource">CancellationTokenSource</see> has been disposed.</exception>
        public CancellationTokenRegistration Register(Action<object?> callback, object? state, bool useSynchronizationContext) =>
            Register(callback, state, useSynchronizationContext, useExecutionContext: true);

        /// <summary>
        /// Registers a delegate that will be called when this
        /// <see cref="CancellationToken">CancellationToken</see> is canceled.
        /// </summary>
        /// <remarks>
        /// <para>
        /// If this token is already in the canceled state, the delegate will be run immediately and synchronously.
        /// Any exception the delegate generates will be propagated out of this method call.
        /// </para>
        /// <para>
        /// <see cref="ExecutionContext">ExecutionContext</see> is not captured nor flowed
        /// to the callback's invocation.
        /// </para>
        /// </remarks>
        /// <param name="callback">The delegate to be executed when the <see cref="CancellationToken">CancellationToken</see> is canceled.</param>
        /// <param name="state">The state to pass to the <paramref name="callback"/> when the delegate is invoked.  This may be null.</param>
        /// <returns>The <see cref="CancellationTokenRegistration"/> instance that can
        /// be used to unregister the callback.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="callback"/> is null.</exception>
        public CancellationTokenRegistration UnsafeRegister(Action<object?> callback, object? state) =>
            Register(callback, state, useSynchronizationContext: false, useExecutionContext: false);

        /// <summary>Registers a delegate that will be called when this <see cref="CancellationToken">CancellationToken</see> is canceled.</summary>
        /// <remarks>
        /// If this token is already in the canceled state, the delegate will be run immediately and synchronously. Any exception the delegate
        /// generates will be propagated out of this method call. <see cref="ExecutionContext"/> is not captured nor flowed to the callback's invocation.
        /// </remarks>
        /// <param name="callback">The delegate to be executed when the <see cref="CancellationToken">CancellationToken</see> is canceled.</param>
        /// <param name="state">The state to pass to the <paramref name="callback"/> when the delegate is invoked.  This may be null.</param>
        /// <returns>The <see cref="CancellationTokenRegistration"/> instance that can be used to unregister the callback.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="callback"/> is null.</exception>
        public CancellationTokenRegistration UnsafeRegister(Action<object?, CancellationToken> callback, object? state) =>
            Register(callback, state, useSynchronizationContext: false, useExecutionContext: false);

        /// <summary>
        /// Registers a delegate that will be called when this
        /// <see cref="CancellationToken">CancellationToken</see> is canceled.
        /// </summary>
        /// <remarks>
        /// <para>
        /// If this token is already in the canceled state, the
        /// delegate will be run immediately and synchronously. Any exception the delegate generates will be
        /// propagated out of this method call.
        /// </para>
        /// </remarks>
        /// <param name="callback">The delegate to be executed when the <see cref="CancellationToken">CancellationToken</see> is canceled.</param>
        /// <param name="state">The state to pass to the <paramref name="callback"/> when the delegate is invoked.  This may be null.</param>
        /// <param name="useSynchronizationContext">A Boolean value that indicates whether to capture
        /// the current <see cref="SynchronizationContext">SynchronizationContext</see> and use it
        /// when invoking the <paramref name="callback"/>.</param>
        /// <param name="useExecutionContext">true to capture the current ExecutionContext; otherwise, false.</param>
        /// <returns>The <see cref="CancellationTokenRegistration"/> instance that can
        /// be used to unregister the callback.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="callback"/> is null.</exception>
        /// <exception cref="ObjectDisposedException">The associated <see
        /// cref="CancellationTokenSource">CancellationTokenSource</see> has been disposed.</exception>
        private CancellationTokenRegistration Register(Delegate callback, object? state, bool useSynchronizationContext, bool useExecutionContext)
        {
            ArgumentNullException.ThrowIfNull(callback);

            CancellationTokenSource? source = _source;
            return source != null ?
                source.Register(callback, state, useSynchronizationContext ? SynchronizationContext.Current : null, useExecutionContext ? ExecutionContext.Capture() : null) :
                default; // Nothing to do for tokens than can never reach the canceled state. Give back a dummy registration.
        }

        /// <summary>
        /// Determines whether the current <see cref="CancellationToken">CancellationToken</see> instance is equal to the
        /// specified token.
        /// </summary>
        /// <param name="other">The other <see cref="CancellationToken">CancellationToken</see> to which to compare this
        /// instance.</param>
        /// <returns>True if the instances are equal; otherwise, false. Two tokens are equal if they are associated
        /// with the same <see cref="CancellationTokenSource">CancellationTokenSource</see> or if they were both constructed
        /// from public CancellationToken constructors and their <see cref="IsCancellationRequested"/> values are equal.</returns>
        public bool Equals(CancellationToken other) => _source == other._source;

        /// <summary>
        /// Determines whether the current <see cref="CancellationToken">CancellationToken</see> instance is equal to the
        /// specified <see cref="object"/>.
        /// </summary>
        /// <param name="other">The other object to which to compare this instance.</param>
        /// <returns>True if <paramref name="other"/> is a <see cref="CancellationToken">CancellationToken</see>
        /// and if the two instances are equal; otherwise, false. Two tokens are equal if they are associated
        /// with the same <see cref="CancellationTokenSource">CancellationTokenSource</see> or if they were both constructed
        /// from public CancellationToken constructors and their <see cref="IsCancellationRequested"/> values are equal.</returns>
        /// <exception cref="ObjectDisposedException">An associated <see
        /// cref="CancellationTokenSource">CancellationTokenSource</see> has been disposed.</exception>
        public override bool Equals([NotNullWhen(true)] object? other) => other is CancellationToken && Equals((CancellationToken)other);

        /// <summary>
        /// Serves as a hash function for a <see cref="CancellationToken">CancellationToken</see>.
        /// </summary>
        /// <returns>A hash code for the current <see cref="CancellationToken">CancellationToken</see> instance.</returns>
        public override int GetHashCode() => (_source ?? CancellationTokenSource.s_neverCanceledSource).GetHashCode();

        /// <summary>
        /// Determines whether two <see cref="CancellationToken">CancellationToken</see> instances are equal.
        /// </summary>
        /// <param name="left">The first instance.</param>
        /// <param name="right">The second instance.</param>
        /// <returns>True if the instances are equal; otherwise, false.</returns>
        /// <exception cref="ObjectDisposedException">An associated <see
        /// cref="CancellationTokenSource">CancellationTokenSource</see> has been disposed.</exception>
        public static bool operator ==(CancellationToken left, CancellationToken right) => left.Equals(right);

        /// <summary>
        /// Determines whether two <see cref="CancellationToken">CancellationToken</see> instances are not equal.
        /// </summary>
        /// <param name="left">The first instance.</param>
        /// <param name="right">The second instance.</param>
        /// <returns>True if the instances are not equal; otherwise, false.</returns>
        /// <exception cref="ObjectDisposedException">An associated <see
        /// cref="CancellationTokenSource">CancellationTokenSource</see> has been disposed.</exception>
        public static bool operator !=(CancellationToken left, CancellationToken right) => !left.Equals(right);

        /// <summary>
        /// Throws a <see cref="OperationCanceledException">OperationCanceledException</see> if
        /// this token has had cancellation requested.
        /// </summary>
        /// <remarks>
        /// This method provides functionality equivalent to:
        /// <code>
        /// if (token.IsCancellationRequested)
        ///    throw new OperationCanceledException(token);
        /// </code>
        /// </remarks>
        /// <exception cref="OperationCanceledException">The token has had cancellation requested.</exception>
        public void ThrowIfCancellationRequested()
        {
            if (IsCancellationRequested)
                ThrowOperationCanceledException();
        }

        // Throws an OCE; separated out to enable better inlining of ThrowIfCancellationRequested
        [DoesNotReturn]
        private void ThrowOperationCanceledException() =>
            throw new OperationCanceledException(SR.OperationCanceled, this);
    }
}