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 * OPC Foundation MIT License 1.00
 *
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using System;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Logging;
using Opc.Ua;
using Opc.Ua.Client;
using Opc.Ua.Identity;
using Opc.Ua.Security.Certificates;
using Quickstarts.ConsoleReferenceClient;

namespace Quickstarts
{
    /// <summary>
    /// OPC UA Client with examples of basic functionality.
    /// </summary>
    public class UAClient : IUAClient, IDisposable
    {
        /// <summary>
        /// Initializes a new instance of the UAClient class.
        /// </summary>
        public UAClient(
            ApplicationConfiguration configuration,
            ITelemetryContext telemetry,
            Action<Array, Array>? validateResponse)
            : this(
                configuration,
                null,
                telemetry,
                validateResponse)
        {
        }

        /// <summary>
        /// Initializes a new instance of the UAClient class for reverse connections.
        /// </summary>
        public UAClient(
            ApplicationConfiguration configuration,
            ReverseConnectManager? reverseConnectManager,
            ITelemetryContext telemetry,
            Action<Array, Array>? validateResponse)
        {
            ValidateResponse = validateResponse;
            m_logger = telemetry.CreateLogger<UAClient>();
            m_telemetry = telemetry;
            m_configuration = configuration;
            // Modern global accept hook on ICertificateManager — fires for
            // every certificate validation done via this manager.
            m_configuration.CertificateManager.AcceptError = AcceptCertificate;
            m_reverseConnectManager = reverseConnectManager;
        }

        /// <inheritdoc/>
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        /// <summary>
        /// Overridable Dispose pattern implementation.
        /// </summary>
        /// <param name="disposing"></param>
        protected virtual void Dispose(bool disposing)
        {
            if (m_disposed)
            {
                return;
            }
            if (disposing)
            {
                m_reconnectHandler?.Dispose();
                Session?.Dispose();
                m_configuration.CertificateManager.AcceptError = null;
            }
            m_disposed = true;
        }

        /// <summary>
        /// Action used
        /// </summary>
        internal Action<Array, Array>? ValidateResponse { get; }

        /// <summary>
        /// Gets the client session.
        /// </summary>
        /// <remarks>
        /// Initialized to <c>null!</c> as late-init: the value is assigned by
        /// <see cref="ConnectAsync"/> before any caller observes it, and is
        /// transiently set to <c>null!</c> during reconnect/dispose. The
        /// non-nullable type matches the <see cref="IUAClient"/> contract.
        /// </remarks>
        public ISession Session { get; private set; } = null!;

        /// <summary>
        /// The session keepalive interval to be used in ms.
        /// </summary>
        public int KeepAliveInterval { get; set; } = 5000;

        /// <summary>
        /// The reconnect period to be used in ms.
        /// </summary>
        public int ReconnectPeriod { get; set; } = 1000;

        /// <summary>
        /// The reconnect period exponential backoff to be used in ms.
        /// </summary>
        public int ReconnectPeriodExponentialBackoff { get; set; } = 15000;

        /// <summary>
        /// The session lifetime.
        /// </summary>
        public uint SessionLifeTime { get; set; } = 60 * 1000;

        /// <summary>
        /// The user identity provider to use to connect to the server.
        /// </summary>
        public IClientIdentityProvider IdentityProvider { get; set; } = new AnonymousIdentityProvider();

        /// <summary>
        /// Auto accept untrusted certificates.
        /// </summary>
        public bool AutoAccept { get; set; }

        /// <summary>
        /// The file to use for log output.
        /// </summary>
        public string? LogFile { get; set; }

        /// <summary>
        /// Do a Durable Subscription Transfer
        /// </summary>
        public async Task<bool> DurableSubscriptionTransferAsync(
            string serverUrl,
            bool useSecurity = true,
            CancellationToken ct = default)
        {
            bool success = false;
            SubscriptionCollection subscriptions = [.. Session.Subscriptions];
            // Transiently null between sessions; ConnectAsync rehydrates.
            Session = null!;
            if (await ConnectAsync(serverUrl, useSecurity, ct).ConfigureAwait(false) &&
                subscriptions != null &&
                Session != null)
            {
                Console.WriteLine($"Transferring {subscriptions.Count} subscriptions from old session to new session...");
                success = await Session.TransferSubscriptionsAsync(
                    subscriptions,
                    true,
                    ct).ConfigureAwait(false);
                if (success)
                {
                    Console.WriteLine("Subscriptions transferred.");
                }
            }

            return success;
        }

        /// <summary>
        /// Creates a session with the UA server
        /// </summary>
        /// <exception cref="ObjectDisposedException"></exception>
        /// <exception cref="ArgumentNullException"><paramref name="serverUrl"/> is <c>null</c>.</exception>
        /// <exception cref="ServiceResultException"></exception>
        public async Task<bool> ConnectAsync(
            string serverUrl,
            bool useSecurity = true,
            CancellationToken ct = default)
        {
            if (m_disposed)
            {
                throw new ObjectDisposedException(nameof(UAClient));
            }

            if (serverUrl == null)
            {
                throw new ArgumentNullException(nameof(serverUrl));
            }

            try
            {
                if (Session != null && Session.Connected)
                {
                    Console.WriteLine("Session already connected!");
                }
                else
                {
                    ITransportWaitingConnection? connection = null;
                    EndpointDescription? endpointDescription = null;
                    if (m_reverseConnectManager != null)
                    {
                        Console.WriteLine($"Waiting for reverse connection to.... {serverUrl}");
                        do
                        {
                            using var cts = new CancellationTokenSource(30_000);
                            using var linkedCTS = CancellationTokenSource.CreateLinkedTokenSource(
                                ct,
                                cts.Token);
                            connection = await m_reverseConnectManager
                                .WaitForConnectionAsync(new Uri(serverUrl), null, linkedCTS.Token)
                                .ConfigureAwait(false) ??
                                throw new ServiceResultException(
                                    StatusCodes.BadTimeout,
                                    "Waiting for a reverse connection timed out.");
                            if (endpointDescription == null)
                            {
                                Console.WriteLine("Discover reverse connection endpoints....");
                                endpointDescription = await CoreClientUtils.SelectEndpointAsync(
                                    m_configuration,
                                    connection,
                                    useSecurity,
                                    m_telemetry,
                                    ct
                                ).ConfigureAwait(false) ??
                                    throw NoMatchingEndpoint(useSecurity);
                                connection = null;
                            }
                        } while (connection == null);
                    }
                    else
                    {
                        Console.WriteLine($"Connecting to... {serverUrl}");
                        endpointDescription = await CoreClientUtils.SelectEndpointAsync(
                            m_configuration,
                            serverUrl,
                            useSecurity,
                            m_telemetry,
                            ct).ConfigureAwait(false) ??
                            throw NoMatchingEndpoint(useSecurity);
                    }

                    // Get the endpoint by connecting to server's discovery endpoint.
                    // Try to find the first endopint with security.
                    var endpointConfiguration = EndpointConfiguration.Create(m_configuration);
                    var endpoint = new ConfiguredEndpoint(
                        null,
                        endpointDescription!,
                        endpointConfiguration);

                    IUserIdentity userIdentity = await CreateUserIdentityAsync(
                        endpointDescription!,
                        ct).ConfigureAwait(false);

                    // Create the session factory. - we could take it as parameter or as member
                    var sessionFactory = new DefaultSessionFactory(m_telemetry);

                    // Create the session
                    ISession session = await sessionFactory
                        .CreateAsync(
                            m_configuration,
                            connection!,
                            endpoint,
                            connection == null,
                            false,
                            m_configuration.ApplicationName!,
                            SessionLifeTime,
                            userIdentity,
                            default,
                            ct)
                        .ConfigureAwait(false);

                    // Assign the created session
                    if (session != null && session.Connected)
                    {
                        Session = session;

                        // override keep alive interval
                        Session.KeepAliveInterval = KeepAliveInterval;

                        // support transfer
                        Session.DeleteSubscriptionsOnClose = false;
                        Session.TransferSubscriptionsOnReconnect = true;

                        // set up keep alive callback.
                        Session.KeepAlive += Session_KeepAlive;

                        // prepare a reconnect handler
                        m_reconnectHandler = new SessionReconnectHandler(
                            m_telemetry,
                            true,
                            ReconnectPeriodExponentialBackoff);

                        // Session created successfully.
                        Console.WriteLine($"New Session Created with SessionName = {Session.SessionName}");
                    }
                }

                return true;
            }
            catch (Exception ex)
            {
                // Log Error
                Console.WriteLine($"Create Session Error : {ex.Message}");
                return false;
            }
        }

        /// <summary>
        /// The error reported when discovery found no endpoint that satisfies
        /// the security requirement (SelectEndpoint returned null).
        /// </summary>
        private static ServiceResultException NoMatchingEndpoint(bool useSecurity)
        {
            return useSecurity
                ? new ServiceResultException(
                    StatusCodes.BadSecurityPolicyRejected,
                    "The server offers no secure endpoint supported by this client.")
                : new ServiceResultException(
                    StatusCodes.BadNotFound,
                    "The server offers no matching endpoint.");
        }

        private ValueTask<IUserIdentity> CreateUserIdentityAsync(
            EndpointDescription endpointDescription,
            CancellationToken ct)
        {
            return IdentityProvider.AcquireIdentityAsync(
                endpointDescription,
                m_configuration.CreateMessageContext(),
                ct);
        }

        /// <summary>
        /// Disconnects the session.
        /// </summary>
        /// <param name="leaveChannelOpen">Leaves the channel open.</param>
        public async Task DisconnectAsync(bool leaveChannelOpen = false, CancellationToken ct = default)
        {
            try
            {
                if (Session != null)
                {
                    Console.WriteLine("Disconnecting...");

                    lock (m_lock)
                    {
                        Session.KeepAlive -= Session_KeepAlive;
                        m_reconnectHandler?.Dispose();
                        m_reconnectHandler = null;
                    }

                    await Session.CloseAsync(!leaveChannelOpen, ct).ConfigureAwait(false);
                    if (leaveChannelOpen)
                    {
                        // detach the channel, so it doesn't get
                        // closed when the session is disposed.
#pragma warning disable CS0618 // sample legacy path
                        Session.DetachChannel();
#pragma warning restore CS0618
                    }
                    Session.Dispose();
                    // Transiently null after dispose; matches pre-NRT behavior.
                    Session = null!;

                    // Log Session Disconnected event
                    Console.WriteLine("Session Disconnected.");
                }
                else
                {
                    Console.WriteLine("Session not created!");
                }
            }
            catch (Exception ex)
            {
                // Log Error
                m_logger.DisconnectError(ex);
            }
        }

        /// <summary>
        /// Handles a keep alive event from a session and triggers a reconnect if necessary.
        /// </summary>
        private void Session_KeepAlive(ISession session, KeepAliveEventArgs e)
        {
            try
            {
                // check for events from discarded sessions.
                if (Session == null || !Session.Equals(session))
                {
                    return;
                }

                // start reconnect sequence on communication error.
                if (ServiceResult.IsBad(e.Status))
                {
                    if (ReconnectPeriod <= 0)
                    {
                        m_logger.KeepAliveReconnectDisabled(e.Status);
                        return;
                    }

                    SessionReconnectHandler.ReconnectState state = m_reconnectHandler!
                        .BeginReconnect(
                            Session,
                            m_reverseConnectManager,
                            ReconnectPeriod,
                            Client_ReconnectComplete
                            );
                    if (state == SessionReconnectHandler.ReconnectState.Triggered)
                    {
                        m_logger.KeepAliveReconnectStatusWithPeriod(
                            e.Status,
                            state,
                            ReconnectPeriod);
                    }
                    else
                    {
                        m_logger.KeepAliveReconnectStatus(
                            e.Status,
                            state);
                    }

                    // cancel sending a new keep alive request, because reconnect is triggered.
                    e.CancelKeepAlive = true;
                }
            }
            catch (Exception exception)
            {
                m_logger.ErrorInOnKeepAlive(exception);
            }
        }

        /// <summary>
        /// Called when the reconnect attempt was successful.
        /// </summary>
        private void Client_ReconnectComplete(object? sender, EventArgs e)
        {
            // ignore callbacks from discarded objects.
            if (!ReferenceEquals(sender, m_reconnectHandler))
            {
                return;
            }

            lock (m_lock)
            {
                // if session recovered, Session property is null
                if (m_reconnectHandler!.Session != null)
                {
                    // ensure only a new instance is disposed
                    // after reactivate, the same session instance may be returned
                    if (!ReferenceEquals(Session, m_reconnectHandler.Session))
                    {
                        m_logger.ReconnectedToNewSession(m_reconnectHandler.Session.SessionId);
                        ISession session = Session;
                        Session = m_reconnectHandler.Session;
                        session?.Dispose();
                    }
                    else
                    {
                        m_logger.ReactivatedSession(m_reconnectHandler.Session.SessionId);
                    }
                }
                else
                {
                    m_logger.ReconnectKeepAliveRecovered();
                }
            }
        }

        /// <summary>
        /// Per-error accept callback invoked by the new
        /// <see cref="ICertificateValidatorEx.AcceptError"/> hook every time
        /// an untrusted certificate is received from the server. Returns
        /// <see langword="true"/> to accept the error, <see langword="false"/>
        /// to reject it.
        /// </summary>
        protected virtual bool AcceptCertificate(Certificate certificate, ServiceResult error)
        {
            // ****
            // Implement a custom logic to decide if the certificate should be
            // accepted or not. Return true to accept, false to reject.
            // ***
            m_logger.CertificateError(error);

            bool certificateAccepted =
                error.StatusCode == StatusCodes.BadCertificateUntrusted && AutoAccept;

            if (certificateAccepted)
            {
                m_logger.UntrustedCertificateAccepted(certificate.Subject);
            }
            else
            {
                m_logger.UntrustedCertificateRejected(certificate.Subject);
            }

            return certificateAccepted;
        }

        private readonly Lock m_lock = new();
        private readonly ReverseConnectManager? m_reverseConnectManager;
        private readonly ApplicationConfiguration m_configuration;
        private SessionReconnectHandler? m_reconnectHandler;
        private readonly ILogger m_logger;
        private readonly ITelemetryContext m_telemetry;
        private bool m_disposed;
    }

    internal static partial class ClientCertificateLog
    {
        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.ClientCertificate + 0, Level = LogLevel.Information,
            Message = "Untrusted Certificate accepted. Subject = {Subject}")]
        public static partial void UntrustedCertificateAccepted(this ILogger logger, string subject);

        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.ClientCertificate + 1, Level = LogLevel.Information,
            Message = "Untrusted Certificate rejected. Subject = {Subject}")]
        public static partial void UntrustedCertificateRejected(this ILogger logger, string subject);
    }

    internal static partial class ClientReconnectLog
    {
        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.ClientReconnect + 0, Level = LogLevel.Information,
            Message = "KeepAlive status {StatusCode}, reconnect status {State}, reconnect period {ReconnectPeriod}ms.")]
        public static partial void KeepAliveReconnectStatusWithPeriod(
            this ILogger logger,
            ServiceResult? statusCode,
            SessionReconnectHandler.ReconnectState state,
            int reconnectPeriod);

        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.ClientReconnect + 1, Level = LogLevel.Information,
            Message = "KeepAlive status {StatusCode}, reconnect status {State}.")]
        public static partial void KeepAliveReconnectStatus(
            this ILogger logger,
            ServiceResult? statusCode,
            SessionReconnectHandler.ReconnectState state);

        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.ClientReconnect + 2, Level = LogLevel.Error,
            Message = "Error in OnKeepAlive.")]
        public static partial void ErrorInOnKeepAlive(this ILogger logger, Exception exception);

        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.ClientReconnect + 3, Level = LogLevel.Information,
            Message = "--- RECONNECTED TO NEW SESSION --- {SessionId}")]
        public static partial void ReconnectedToNewSession(this ILogger logger, NodeId sessionId);

        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.ClientReconnect + 4, Level = LogLevel.Information,
            Message = "--- REACTIVATED SESSION --- {SessionId}")]
        public static partial void ReactivatedSession(this ILogger logger, NodeId sessionId);

        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.ClientReconnect + 5, Level = LogLevel.Information,
            Message = "--- RECONNECT KeepAlive recovered ---")]
        public static partial void ReconnectKeepAliveRecovered(this ILogger logger);
    }

    internal static partial class UAClientLog
    {
        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.UAClient + 0, Level = LogLevel.Error,
            Message = "Disconnect Error")]
        public static partial void DisconnectError(this ILogger logger, Exception ex);

        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.UAClient + 1, Level = LogLevel.Warning,
            Message = "KeepAlive status {StatusCode}, but reconnect is disabled.")]
        public static partial void KeepAliveReconnectDisabled(this ILogger logger, ServiceResult? statusCode);

        [LoggerMessage(EventId = ConsoleReferenceClientEventIds.UAClient + 2, Level = LogLevel.Information,
            Message = "{Error}")]
        public static partial void CertificateError(this ILogger logger, ServiceResult error);
    }

}