/* ========================================================================
 * Copyright (c) 2005-2025 The OPC Foundation, Inc. All rights reserved.
 *
 * OPC Foundation MIT License 1.00
 *
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use,
 * copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following
 * conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
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 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 *
 * The complete license agreement can be found here:
 * http://opcfoundation.org/License/MIT/1.00/
 * ======================================================================*/

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
using Opc.Ua.Security.Certificates;

namespace Opc.Ua.Server
{
    /// <summary>
    /// Allows application components to receive notifications when changes to sessions occur.
    /// </summary>
    /// <remarks>
    /// Sinks that receive these events must not block the thread.
    /// </remarks>
    public interface ISessionManager : IDisposable
    {
        /// <summary>
        /// Raised after a new session is created.
        /// </summary>
        event SessionEventHandler SessionCreated;

        /// <summary>
        /// Raised whenever a session is activated and the user identity or preferred locales changed.
        /// </summary>
        event SessionEventHandler SessionActivated;

        /// <summary>
        /// Raised before a session is closed.
        /// </summary>
        event SessionEventHandler SessionClosing;

        /// <summary>
        /// Raised after diagnostics of an existing session were changed.
        /// </summary>
        event SessionEventHandler SessionDiagnosticsChanged;

        /// <summary>
        /// Raised to signal a channel that the session is still alive.
        /// </summary>
        event SessionEventHandler SessionChannelKeepAlive;

        /// <summary>
        /// Raised before the user identity for a session is changed.
        /// </summary>
        [Obsolete(
            "Replaced by IUserTokenAuthenticator + IServerIdentityRegistry. " +
            "Register authenticators via services.AddIdentityAuthenticator<T>() or " +
            "server.CurrentInstance.IdentityRegistry.Register(...). See Docs/IdentityProviders.md.")]
        event ImpersonateEventHandler ImpersonateUser;

        /// <summary>
        /// Raised to validate a session-less request.
        /// </summary>
        event EventHandler<ValidateSessionLessRequestEventArgs> ValidateSessionLessRequest;

        /// <summary>
        /// Starts the session manager.
        /// </summary>
        ValueTask StartupAsync(CancellationToken cancellationToken = default);

        /// <summary>
        /// Stops the session manager and closes all sessions.
        /// </summary>
        void Shutdown();

        /// <summary>
        /// Clears all tracked failed authentication attempts and lockouts.
        /// Intended for diagnostic and test scenarios.
        /// </summary>
        void ClearAuthenticationLockouts();

        /// <summary>
        /// Returns all of the sessions known to the session manager.
        /// </summary>
        /// <returns>A list of the sessions.</returns>
        IList<ISession> GetSessions();

        /// <summary>
        /// Find and return a session specified by authentication token
        /// </summary>
        /// <returns>The requested session.</returns>
        ISession? GetSession(NodeId authenticationToken);

        /// <summary>
        /// Creates a new session.
        /// </summary>
        ValueTask<CreateSessionResult> CreateSessionAsync(
            OperationContext context,
            Certificate serverCertificate,
            string? sessionName,
            ByteString clientNonce,
            ApplicationDescription? clientDescription,
            string? endpointUrl,
            Certificate? clientCertificate,
            CertificateCollection? clientCertificateChain,
            double requestedSessionTimeout,
            uint maxResponseMessageSize,
            CancellationToken cancellationToken = default);

        /// <summary>
        /// Activates an existing session
        /// </summary>
        ValueTask<(bool IdentityContextChanged, ByteString ServerNonce, ServiceResult ActivationStatus)> ActivateSessionAsync(
            OperationContext context,
            NodeId authenticationToken,
            SignatureData? clientSignature,
            ExtensionObject userIdentityToken,
            SignatureData? userTokenSignature,
            ArrayOf<string> localeIds,
            CancellationToken cancellationToken = default);

        /// <summary>
        /// Closes the specified session.
        /// </summary>
        /// <remarks>
        /// This method should not throw an exception if the session no longer exists.
        /// </remarks>
        ValueTask CloseSessionAsync(NodeId sessionId, CancellationToken cancellationToken = default);

        /// <summary>
        /// Validates request header and returns a request context.
        /// </summary>
        /// <remarks>
        /// This method verifies that the session id is valid and that it uses secure channel id
        /// associated with current thread. It also verifies that the timestamp is not too
        /// and that the sequence number is not out of order (update requests only).
        /// </remarks>
        ValueTask<OperationContext> ValidateRequestAsync(
            RequestHeader? requestHeader,
            SecureChannelContext secureChannelContext,
            RequestType requestType,
            RequestLifetime requestLifetime);

        /// <summary>
        /// Triggers the <see cref="SessionDiagnosticsChanged"/> event so subscribers can react.
        /// </summary>
        void RaiseSessionDiagnosticsChangedEvent(ISession session);
    }

    /// <summary>
    /// The result of a call to <see cref="ISessionManager.CreateSessionAsync"/>.
    /// </summary>
    public class CreateSessionResult
    {
        /// <summary>
        /// The created Session.
        /// </summary>
        public required ISession Session { get; init; }

        /// <summary>
        /// The SessionID assigned to the session.
        /// </summary>
        public required NodeId SessionId { get; init; }

        /// <summary>
        /// The authentication token used to identify and authorize the client.
        /// </summary>
        public required NodeId AuthenticationToken { get; init; }

        /// <summary>
        /// The server nonce of the session.
        /// </summary>
        public required ByteString ServerNonce { get; init; }

        /// <summary>
        /// The revised session timeout.
        /// </summary>
        public required double RevisedSessionTimeout { get; init; }
    }

    /// <summary>
    /// The possible reasons for a session related event.
    /// </summary>
    public enum SessionEventReason
    {
        /// <summary>
        /// A new session was created.
        /// </summary>
        Created,

        /// <summary>
        /// A session is being activated with a new user identity.
        /// </summary>
        Impersonating,

        /// <summary>
        /// A session was activated and the user identity or preferred locales changed.
        /// </summary>
        Activated,

        /// <summary>
        /// The diagnostics of an existing session were changed.
        /// </summary>
        DiagnosticsChanged,

        /// <summary>
        /// A session is about to be closed.
        /// </summary>
        Closing,

        /// <summary>
        /// A keep alive to signal a channel that the session is still active.
        /// Triggered by the session manager based on <see cref="ServerConfiguration.MinSessionTimeout"/>.
        /// </summary>
        ChannelKeepAlive
    }

    /// <summary>
    /// The delegate for functions used to receive session related events.
    /// </summary>
    public delegate void SessionEventHandler(ISession session, SessionEventReason reason);

    /// <summary>
    /// A class which provides the event arguments for session related event.
    /// </summary>
    public class ImpersonateEventArgs : EventArgs
    {
        /// <summary>
        /// Creates a new instance.
        /// </summary>
        public ImpersonateEventArgs(
            IUserIdentityTokenHandler? newIdentityHandler,
            UserTokenPolicy? userTokenPolicy,
            EndpointDescription? endpointDescription = null)
        {
            UserIdentityTokenHandler = newIdentityHandler!;
            UserTokenPolicy = userTokenPolicy!;
            EndpointDescription = endpointDescription!;
        }

        /// <summary>
        /// The new user identity for the session.
        /// </summary>
        public IUserIdentityTokenHandler UserIdentityTokenHandler { get; }

        /// <summary>
        /// The new user identity for the session.
        /// </summary>
        [Obsolete("Use UserIdentityTokenHandler to process the token.")]
        public UserIdentityToken NewIdentity => UserIdentityTokenHandler.Token;

        /// <summary>
        /// The user token policy selected by the client.
        /// </summary>
        public UserTokenPolicy UserTokenPolicy { get; }

        /// <summary>
        /// An application defined handle that can be used for access control operations.
        /// </summary>
        public IUserIdentity Identity { get; set; } = null!;

        /// <summary>
        /// An application defined handle that can be used for access control operations.
        /// </summary>
        public IUserIdentity EffectiveIdentity { get; set; } = null!;

        /// <summary>
        /// Set to indicate that an error occurred validating the identity and that it should be rejected.
        /// </summary>
        public ServiceResult IdentityValidationError { get; set; } = null!;

        /// <summary>
        /// Get the EndpointDescription
        /// </summary>
        public EndpointDescription EndpointDescription { get; }
    }

    /// <summary>
    /// The delegate for functions used to receive impersonation events.
    /// </summary>
    public delegate void ImpersonateEventHandler(ISession session, ImpersonateEventArgs args);

    /// <summary>
    /// A class which provides the event arguments for session related event.
    /// </summary>
    public class ValidateSessionLessRequestEventArgs : EventArgs
    {
        /// <summary>
        /// Creates a new instance.
        /// </summary>
        public ValidateSessionLessRequestEventArgs(
            NodeId authenticationToken,
            RequestType requestType)
        {
            AuthenticationToken = authenticationToken;
            RequestType = requestType;
        }

        /// <summary>
        /// The request type for the request.
        /// </summary>
        public RequestType RequestType { get; }

        /// <summary>
        /// The new user identity for the session.
        /// </summary>
        public NodeId AuthenticationToken { get; }

        /// <summary>
        /// The identity to associate with the session-less request.
        /// </summary>
        public IUserIdentity Identity { get; set; } = null!;

        /// <summary>
        /// Set to indicate that an error occurred validating the session-less request and that it should be rejected.
        /// </summary>
        public ServiceResult Error { get; set; } = null!;
    }
}