using System.Collections.Concurrent;
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Security;
using System.Net.Sockets;
using System.Runtime.InteropServices;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Text.Json;
using Microsoft.Extensions.Logging;
using NATS.NKeys;
using NATS.Server.Auth;
using NATS.Server.Auth.Jwt;
using NATS.Server.Configuration;
using NATS.Server.Events;
using NATS.Server.Gateways;
using NATS.Server.Imports;
using NATS.Server.JetStream;
using NATS.Server.JetStream.Api;
using NATS.Server.JetStream.Publish;
using NATS.Server.LeafNodes;
using NATS.Server.Monitoring;
using NATS.Server.Mqtt;
using NATS.Server.Protocol;
using NATS.Server.Routes;
using NATS.Server.Server;
using NATS.Server.Subscriptions;
using NATS.Server.Tls;
using NATS.Server.WebSocket;

namespace NATS.Server;

public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
{
    private readonly NatsOptions _options;
    private readonly ConcurrentDictionary<ulong, NatsClient> _clients = new();
    private ClosedConnectionRingBuffer _closedClients;
    private readonly ServerInfo _serverInfo;
    private readonly ILogger<NatsServer> _logger;
    private readonly ILoggerFactory _loggerFactory;
    private readonly ServerStats _stats = new();

    // Per-client deferred flush set. Collects unique clients during fan-out delivery,
    // then flushes each once. Go reference: client.go addToPCD / flushClients.
    [ThreadStatic] private static HashSet<INatsClient>? t_pcd;
    private readonly TaskCompletionSource _listeningStarted = new(TaskCreationOptions.RunContinuationsAsynchronously);
    private AuthService _authService;
    private readonly ConcurrentDictionary<string, Account> _accounts = new(StringComparer.Ordinal);

    // Config reload state
    private NatsOptions? _cliSnapshot;
    private HashSet<string> _cliFlags = [];
    private string? _configDigest;
    private readonly SemaphoreSlim _reloadMu = new(1, 1);
    private AcceptLoopErrorHandler? _acceptLoopErrorHandler;
    private readonly Account _globalAccount;
    private readonly Account _systemAccount;
    private InternalEventSystem? _eventSystem;
    private SslServerAuthenticationOptions? _sslOptions;
    private readonly TlsRateLimiter? _tlsRateLimiter;
    private readonly TlsCertificateProvider? _tlsCertProvider;
    private readonly SubjectTransform[] _subjectTransforms;
    private readonly RouteManager? _routeManager;

    /// <summary>
    /// Exposes the route manager for testing. Internal — visible to test project
    /// via InternalsVisibleTo.
    /// </summary>
    internal RouteManager? RouteManager => _routeManager;
    internal GatewayManager? GatewayManager => _gatewayManager;
    private readonly GatewayManager? _gatewayManager;
    private readonly LeafNodeManager? _leafNodeManager;
    private readonly InternalClient? _jetStreamInternalClient;
    private readonly JetStreamService? _jetStreamService;
    private readonly JetStreamApiRouter? _jetStreamApiRouter;
    private readonly StreamManager? _jetStreamStreamManager;
    private readonly ConsumerManager? _jetStreamConsumerManager;
    private readonly JetStreamPublisher? _jetStreamPublisher;
    private MqttListener? _mqttListener;
    private Socket? _listener;
    private Socket? _wsListener;
    private readonly TaskCompletionSource _wsAcceptLoopExited = new(TaskCreationOptions.RunContinuationsAsynchronously);
    private MonitorServer? _monitorServer;
    private ulong _nextClientId;
    private long _startTimeTicks;

    private readonly CancellationTokenSource _quitCts = new();
    private readonly TaskCompletionSource _shutdownComplete = new(TaskCreationOptions.RunContinuationsAsynchronously);
    private readonly TaskCompletionSource _acceptLoopExited = new(TaskCreationOptions.RunContinuationsAsynchronously);
    private int _shutdown;
    private int _activeClientCount;

    private int _lameDuck;

    private byte[] _cachedInfoLine = [];

    private readonly List<PosixSignalRegistration> _signalRegistrations = [];

    private string? _portsFilePath;
    private DateTime _configTime = DateTime.UtcNow;

    private static readonly TimeSpan AcceptMinSleep = NatsProtocol.AcceptMinSleep;
    private static readonly TimeSpan AcceptMaxSleep = NatsProtocol.AcceptMaxSleep;
    private static readonly JsonSerializerOptions s_jetStreamJsonOptions = new()
    {
        PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower,
        DefaultIgnoreCondition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingNull,
    };

    public SubList SubList => _globalAccount.SubList;
    public byte[] CachedInfoLine => _cachedInfoLine;
    public ServerStats Stats => _stats;
    public DateTime StartTime => new(Interlocked.Read(ref _startTimeTicks), DateTimeKind.Utc);
    public string ServerId => _serverInfo.ServerId;
    public string ServerName => _serverInfo.ServerName;
    public int ClientCount => _clients.Count;
    public int Port => _options.Port;

    /// <summary>
    /// Returns the actual bound port of the MQTT listener, or null if MQTT is not enabled.
    /// Used by VarzHandler for monitoring.
    /// </summary>
    public int? MqttListenerPort => _mqttListener?.Port;

    /// <summary>
    /// Returns all active MQTT client adapters for monitoring (/connz).
    /// </summary>
    public IEnumerable<Mqtt.MqttNatsClientAdapter> GetMqttAdapters()
        => _mqttListener?.GetMqttAdapters() ?? [];

    public Account SystemAccount => _systemAccount;
    public string ServerNKey { get; }
    public InternalEventSystem? EventSystem => _eventSystem;
    public bool IsShuttingDown => Volatile.Read(ref _shutdown) != 0;
    public bool IsLameDuckMode => Volatile.Read(ref _lameDuck) != 0;
    public string? ClusterListen => _routeManager?.ListenEndpoint;
    public string? GatewayListen => _gatewayManager?.ListenEndpoint;
    public string? LeafListen => _leafNodeManager?.ListenEndpoint;
    public bool IsProfilingEnabled => _options.ProfPort > 0;
    public InternalClient? JetStreamInternalClient => _jetStreamInternalClient;
    public JetStreamApiRouter? JetStreamApiRouter => _jetStreamApiRouter;
    public int JetStreamStreams => _jetStreamStreamManager?.StreamNames.Count ?? 0;
    public int JetStreamConsumers => _jetStreamConsumerManager?.ConsumerCount ?? 0;
    public Action? ReOpenLogFile { get; set; }
    public IEnumerable<NatsClient> GetClients() => _clients.Values;
    public string? ClusterName() => _options.Cluster?.Name;
    public IReadOnlyList<string> ActivePeers()
        => _routeManager?.BuildTopologySnapshot().ConnectedServerIds ?? [];

    public bool StartProfiler()
    {
        if (_options.ProfPort <= 0)
            return false;

        _logger.LogWarning("Profiling endpoint not yet supported (port: {ProfPort})", _options.ProfPort);
        return true;
    }

    public bool DisconnectClientByID(ulong clientId)
        => CloseClientById(clientId, minimalFlush: true);

    public bool LDMClientByID(ulong clientId)
        => CloseClientById(clientId, minimalFlush: false);

    public Ports PortsInfo()
    {
        var ports = new Ports();

        AddEndpoint(ports.Nats, _options.Host, _options.Port);
        AddEndpoint(ports.Monitoring, _options.MonitorHost, _options.MonitorPort);

        if (_routeManager != null)
            AddEndpoint(ports.Cluster, _routeManager.ListenEndpoint);
        else if (_options.Cluster != null)
            AddEndpoint(ports.Cluster, _options.Cluster.Host, _options.Cluster.Port);

        AddEndpoint(ports.Profile, _options.Host, _options.ProfPort);

        if (_options.WebSocket.Port >= 0)
            AddEndpoint(ports.WebSocket, _options.WebSocket.Host, _options.WebSocket.Port);

        if (_leafNodeManager != null)
            AddEndpoint(ports.LeafNodes, _leafNodeManager.ListenEndpoint);
        else if (_options.LeafNode != null)
            AddEndpoint(ports.LeafNodes, _options.LeafNode.Host, _options.LeafNode.Port);

        return ports;
    }

    public IReadOnlyList<string> GetConnectURLs()
    {
        if (!string.IsNullOrWhiteSpace(_options.ClientAdvertise))
            return [NormalizeAdvertiseUrl(_options.ClientAdvertise!, "nats")];

        var hosts = GetNonLocalIPsIfHostIsIPAny(_options.Host);
        var result = new List<string>(hosts.Count);
        foreach (var host in hosts)
            result.Add($"nats://{host}:{_options.Port}");

        return result;
    }

    public void UpdateServerINFOAndSendINFOToClients()
    {
        _serverInfo.ConnectUrls = [.. GetConnectURLs()];
        BuildCachedInfo();

        foreach (var client in _clients.Values)
        {
            if (client.ConnectReceived)
                client.QueueOutbound(_cachedInfoLine);
        }
    }

    public string ClientURL()
    {
        if (!string.IsNullOrWhiteSpace(_options.ClientAdvertise))
            return NormalizeAdvertiseUrl(_options.ClientAdvertise!, "nats");

        var host = IsWildcardHost(_options.Host) ? "127.0.0.1" : _options.Host;
        return $"nats://{host}:{_options.Port}";
    }

    public string? WebsocketURL()
    {
        if (_options.WebSocket.Port < 0)
            return null;

        if (!string.IsNullOrWhiteSpace(_options.WebSocket.Advertise))
        {
            var scheme = _options.WebSocket.NoTls ? "ws" : "wss";
            return NormalizeAdvertiseUrl(_options.WebSocket.Advertise!, scheme);
        }

        var wsHost = IsWildcardHost(_options.WebSocket.Host) ? "127.0.0.1" : _options.WebSocket.Host;
        var wsScheme = _options.WebSocket.NoTls ? "ws" : "wss";
        return $"{wsScheme}://{wsHost}:{_options.WebSocket.Port}";
    }

    public int NumRoutes() => (int)Interlocked.Read(ref _stats.Routes);

    public int NumRemotes()
        => (int)(Interlocked.Read(ref _stats.Routes) + Interlocked.Read(ref _stats.Gateways) + Interlocked.Read(ref _stats.Leafs));

    public int NumLeafNodes() => (int)Interlocked.Read(ref _stats.Leafs);
    public int NumOutboundGateways() => _gatewayManager?.NumOutboundGateways() ?? 0;
    public int NumInboundGateways() => _gatewayManager?.NumInboundGateways() ?? 0;

    public int NumSubscriptions() => _accounts.Values.Sum(acc => acc.SubscriptionCount);
    public bool JetStreamEnabled() => _jetStreamService?.IsRunning ?? false;

    public JetStreamOptions? JetStreamConfig()
    {
        if (_options.JetStream is null)
            return null;

        return new JetStreamOptions
        {
            StoreDir = _options.JetStream.StoreDir,
            MaxMemoryStore = _options.JetStream.MaxMemoryStore,
            MaxFileStore = _options.JetStream.MaxFileStore,
            MaxStreams = _options.JetStream.MaxStreams,
            MaxConsumers = _options.JetStream.MaxConsumers,
            Domain = _options.JetStream.Domain,
        };
    }

    public string StoreDir() => _options.JetStream?.StoreDir ?? string.Empty;

    public DateTime ConfigTime() => _configTime;

    public string Addr() => $"{_options.Host}:{_options.Port}";

    public string? MonitorAddr()
        => _options.MonitorPort > 0
            ? $"{_options.MonitorHost}:{_options.MonitorPort}"
            : null;

    public string? ClusterAddr() => _routeManager?.ListenEndpoint;
    public string? GatewayAddr() => _gatewayManager?.ListenEndpoint;
    public string? GetGatewayURL() => _gatewayManager?.ListenEndpoint;
    public string? GetGatewayName() => _options.Gateway?.Name;

    public string? ProfilerAddr()
        => _options.ProfPort > 0
            ? $"{_options.Host}:{_options.ProfPort}"
            : null;

    public int NumActiveAccounts() => _accounts.Values.Count(acc => acc.ClientCount > 0);

    public int NumLoadedAccounts() => _accounts.Count;

    public IReadOnlyList<ClosedClient> GetClosedClients() => _closedClients.GetAll();

    public IEnumerable<Auth.Account> GetAccounts() => _accounts.Values;
    public bool HasRemoteInterest(string subject) => _globalAccount.SubList.HasRemoteInterest(subject);
    public bool HasRemoteInterest(string account, string subject)
        => GetOrCreateAccount(account).SubList.HasRemoteInterest(account, subject);
    public bool TryCaptureJetStreamPublish(string subject, ReadOnlyMemory<byte> payload, out PubAck ack)
    {
        if (_jetStreamPublisher != null && _jetStreamPublisher.TryCapture(subject, payload, out ack))
        {
            // Only load the stored message for consumer notification when there are
            // active consumers for this stream. Avoids unnecessary LoadAsync on the hot path.
            if (ack.ErrorCode == null
                && _jetStreamConsumerManager != null
                && _jetStreamConsumerManager.HasConsumersForStream(ack.Stream)
                && _jetStreamStreamManager != null
                && _jetStreamStreamManager.TryGet(ack.Stream, out var streamHandle))
            {
                var stored = streamHandle.Store.LoadAsync(ack.Seq, default).GetAwaiter().GetResult();
                if (stored != null)
                    _jetStreamConsumerManager.OnPublished(ack.Stream, stored);
            }

            return true;
        }

        ack = new PubAck();
        return false;
    }

    /// <summary>
    /// Replicates a successful JetStream mutating API call to cluster peers via internal subjects.
    /// Maps API subjects to $JS.INTERNAL.* subjects and forwards to routes/leafnodes.
    /// Go reference: jetstream_cluster.go — RAFT proposal broadcast.
    /// </summary>
    private void TryReplicateJetStreamMutation(string apiSubject, ReadOnlyMemory<byte> payload)
    {
        string? internalSubject = null;

        if (apiSubject.StartsWith(JetStreamApiSubjects.StreamCreate, StringComparison.Ordinal))
            internalSubject = JetStreamApiSubjects.InternalStreamCreate + apiSubject[JetStreamApiSubjects.StreamCreate.Length..];
        else if (apiSubject.StartsWith(JetStreamApiSubjects.StreamDelete, StringComparison.Ordinal))
            internalSubject = JetStreamApiSubjects.InternalStreamDelete + apiSubject[JetStreamApiSubjects.StreamDelete.Length..];
        else if (apiSubject.StartsWith(JetStreamApiSubjects.StreamPurge, StringComparison.Ordinal))
            internalSubject = JetStreamApiSubjects.InternalStreamPurge + apiSubject[JetStreamApiSubjects.StreamPurge.Length..];
        else if (apiSubject.StartsWith(JetStreamApiSubjects.ConsumerCreate, StringComparison.Ordinal))
            internalSubject = JetStreamApiSubjects.InternalConsumerCreate + apiSubject[JetStreamApiSubjects.ConsumerCreate.Length..];
        else if (apiSubject.StartsWith(JetStreamApiSubjects.ConsumerDelete, StringComparison.Ordinal))
            internalSubject = JetStreamApiSubjects.InternalConsumerDelete + apiSubject[JetStreamApiSubjects.ConsumerDelete.Length..];

        if (internalSubject != null)
            ReplicateJetStreamOperation("$G", internalSubject, null, payload);
    }

    /// <summary>
    /// Forwards a JetStream replication message to all route and leaf node peers.
    /// Bypasses interest checks since replication subjects have no client subscribers.
    /// Go reference: jetstream_cluster.go — proposal broadcast to peers.
    /// </summary>
    private void ReplicateJetStreamOperation(string account, string subject, string? replyTo, ReadOnlyMemory<byte> payload)
    {
        _routeManager?.BroadcastRoutedMessageAsync(account, subject, replyTo, payload, default)
            .GetAwaiter().GetResult();
        if (_leafNodeManager != null)
        {
            var markedSubject = LeafLoopDetector.Mark(subject, ServerId);
            _leafNodeManager.ForwardMessageAsync(account, markedSubject, replyTo, payload, default)
                .GetAwaiter().GetResult();
        }
    }

    /// <summary>
    /// Handles incoming JetStream internal replication messages from cluster peers.
    /// Dispatches to the appropriate handler based on the internal subject prefix.
    /// Called from DeliverRemoteMessage for $JS.INTERNAL.* subjects.
    /// Go reference: jetstream_cluster.go — RAFT proposal apply.
    /// </summary>
    private void HandleJetStreamReplication(string subject, ReadOnlyMemory<byte> payload)
    {
        if (_jetStreamStreamManager == null || _jetStreamConsumerManager == null)
            return;

        if (subject.StartsWith(JetStreamApiSubjects.InternalStreamCreate, StringComparison.Ordinal))
        {
            var apiSubject = JetStreamApiSubjects.StreamCreate + subject[JetStreamApiSubjects.InternalStreamCreate.Length..];
            JetStream.Api.Handlers.StreamApiHandlers.HandleCreate(apiSubject, payload.Span, _jetStreamStreamManager);
        }
        else if (subject.StartsWith(JetStreamApiSubjects.InternalStreamDelete, StringComparison.Ordinal))
        {
            var streamName = subject[JetStreamApiSubjects.InternalStreamDelete.Length..];
            _jetStreamStreamManager.Delete(streamName);
        }
        else if (subject.StartsWith(JetStreamApiSubjects.InternalStreamPurge, StringComparison.Ordinal))
        {
            var streamName = subject[JetStreamApiSubjects.InternalStreamPurge.Length..];
            _jetStreamStreamManager.Purge(streamName);
        }
        else if (subject.StartsWith(JetStreamApiSubjects.InternalConsumerCreate, StringComparison.Ordinal))
        {
            var apiSubject = JetStreamApiSubjects.ConsumerCreate + subject[JetStreamApiSubjects.InternalConsumerCreate.Length..];
            JetStream.Api.Handlers.ConsumerApiHandlers.HandleCreate(apiSubject, payload.Span, _jetStreamConsumerManager);
        }
        else if (subject.StartsWith(JetStreamApiSubjects.InternalConsumerDelete, StringComparison.Ordinal))
        {
            var parts = subject[JetStreamApiSubjects.InternalConsumerDelete.Length..].Split('.');
            if (parts.Length >= 2)
                _jetStreamConsumerManager.Delete(parts[0], parts[1]);
        }
    }

    public Task WaitForReadyAsync() => _listeningStarted.Task;

    public void WaitForShutdown() => _shutdownComplete.Task.GetAwaiter().GetResult();

    internal TlsCertificateProvider? TlsCertProviderForTest => _tlsCertProvider;

    internal Task AcquireReloadLockForTestAsync() => _reloadMu.WaitAsync();

    internal void ReleaseReloadLockForTest() => _reloadMu.Release();

    internal void SetAcceptLoopErrorHandlerForTest(AcceptLoopErrorHandler handler) => _acceptLoopErrorHandler = handler;

    internal void NotifyAcceptErrorForTest(Exception ex, EndPoint? endpoint, TimeSpan delay) =>
        _acceptLoopErrorHandler?.OnAcceptError(ex, endpoint, delay);

    public async Task ShutdownAsync()
    {
        if (Interlocked.CompareExchange(ref _shutdown, 1, 0) != 0)
            return; // Already shutting down

        _logger.LogInformation("Initiating Shutdown...");

        // Publish shutdown advisory before tearing down the event system
        if (_eventSystem != null)
        {
            var shutdownSubject = string.Format(EventSubjects.ServerShutdown, _serverInfo.ServerId);
            _eventSystem.Enqueue(new PublishMessage
            {
                Subject = shutdownSubject,
                Body = new ShutdownEventMsg { Server = BuildEventServerInfo(), Reason = "Server Shutdown" },
                IsLast = true,
            });
            // Give the send loop time to process the shutdown event
            await Task.Delay(100);
            await _eventSystem.DisposeAsync();
        }

        // Signal all internal loops to stop
        await _quitCts.CancelAsync();

        // Close listeners to stop accept loops
        _listener?.Close();
        _wsListener?.Close();
        if (_routeManager != null)
            await _routeManager.DisposeAsync();
        if (_gatewayManager != null)
            await _gatewayManager.DisposeAsync();
        if (_leafNodeManager != null)
            await _leafNodeManager.DisposeAsync();
        if (_jetStreamService != null)
            await _jetStreamService.DisposeAsync();
        _stats.JetStreamEnabled = false;

        // If server was never started, accept loops never ran — signal immediately
        if (_listener == null)
            _acceptLoopExited.TrySetResult();
        if (_wsListener == null)
            _wsAcceptLoopExited.TrySetResult();

        // Wait for accept loops to exit (in parallel to avoid sequential 5s+5s waits)
        await Task.WhenAll(
            _acceptLoopExited.Task.WaitAsync(TimeSpan.FromSeconds(5)),
            _wsAcceptLoopExited.Task.WaitAsync(TimeSpan.FromSeconds(5))).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);

        // Close all client connections — flush first, then mark closed
        var flushTasks = new List<Task>();
        foreach (var client in _clients.Values)
        {
            client.MarkClosed(ClientClosedReason.ServerShutdown);
            flushTasks.Add(client.FlushAndCloseAsync(minimalFlush: true));
        }
        await Task.WhenAll(flushTasks).WaitAsync(TimeSpan.FromSeconds(2)).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);

        // Wait for active client tasks to drain (with timeout)
        if (Volatile.Read(ref _activeClientCount) > 0)
        {
            using var drainCts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
            try
            {
                while (Volatile.Read(ref _activeClientCount) > 0 && !drainCts.IsCancellationRequested)
                    await Task.Delay(50, drainCts.Token);
            }
            catch (OperationCanceledException) { }
        }

        // Stop monitor server
        if (_monitorServer != null)
            await _monitorServer.DisposeAsync();
        if (_mqttListener != null)
            await _mqttListener.DisposeAsync();

        DeletePidFile();
        DeletePortsFile();

        _logger.LogInformation("Server Exiting..");
        _shutdownComplete.TrySetResult();
    }

    public async Task LameDuckShutdownAsync()
    {
        if (IsShuttingDown || Interlocked.CompareExchange(ref _lameDuck, 1, 0) != 0)
            return;

        _logger.LogInformation("Entering lame duck mode, stop accepting new clients");

        // Close listeners to stop accepting new connections
        _listener?.Close();
        _wsListener?.Close();

        // Wait for accept loops to exit (in parallel)
        await Task.WhenAll(
            _acceptLoopExited.Task.WaitAsync(TimeSpan.FromSeconds(5)),
            _wsAcceptLoopExited.Task.WaitAsync(TimeSpan.FromSeconds(5))).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);

        var gracePeriod = _options.LameDuckGracePeriod;
        if (gracePeriod < TimeSpan.Zero) gracePeriod = -gracePeriod;

        // If no clients, go straight to shutdown
        if (_clients.IsEmpty)
        {
            await ShutdownAsync();
            return;
        }

        // Wait grace period for clients to drain naturally
        _logger.LogInformation("Waiting {GracePeriod}ms grace period", gracePeriod.TotalMilliseconds);
        try
        {
            await Task.Delay(gracePeriod, _quitCts.Token);
        }
        catch (OperationCanceledException) { return; }

        if (_clients.IsEmpty)
        {
            await ShutdownAsync();
            return;
        }

        // Stagger-close remaining clients
        var dur = _options.LameDuckDuration - gracePeriod;
        if (dur <= TimeSpan.Zero) dur = TimeSpan.FromSeconds(1);

        var clients = _clients.Values.ToList();
        var numClients = clients.Count;

        if (numClients > 0)
        {
            _logger.LogInformation("Closing {Count} existing clients over {Duration}ms",
                numClients, dur.TotalMilliseconds);

            var sleepInterval = dur.Ticks / numClients;
            if (sleepInterval < TimeSpan.TicksPerMillisecond)
                sleepInterval = TimeSpan.TicksPerMillisecond;
            if (sleepInterval > TimeSpan.TicksPerSecond)
                sleepInterval = TimeSpan.TicksPerSecond;

            for (int i = 0; i < clients.Count; i++)
            {
                clients[i].MarkClosed(ClientClosedReason.ServerShutdown);
                await clients[i].FlushAndCloseAsync(minimalFlush: true);

                if (i < clients.Count - 1)
                {
                    var jitter = Random.Shared.NextInt64(sleepInterval / 2, sleepInterval);
                    try
                    {
                        await Task.Delay(TimeSpan.FromTicks(jitter), _quitCts.Token);
                    }
                    catch (OperationCanceledException) { break; }
                }
            }
        }

        await ShutdownAsync();
    }

    /// <summary>
    /// Registers Unix signal handlers.
    /// SIGTERM → shutdown, SIGUSR2 → lame duck, SIGUSR1 → log reopen, SIGHUP → reload (stub).
    /// </summary>
    public void HandleSignals()
    {
        _signalRegistrations.Add(PosixSignalRegistration.Create(PosixSignal.SIGTERM, ctx =>
        {
            ctx.Cancel = true;
            _logger.LogInformation("Trapped SIGTERM signal");
            _ = Task.Run(async () => await ShutdownAsync());
        }));

        _signalRegistrations.Add(PosixSignalRegistration.Create(PosixSignal.SIGQUIT, ctx =>
        {
            ctx.Cancel = true;
            _logger.LogInformation("Trapped SIGQUIT signal");
            _ = Task.Run(async () => await ShutdownAsync());
        }));

        _signalRegistrations.Add(PosixSignalRegistration.Create(PosixSignal.SIGHUP, ctx =>
        {
            ctx.Cancel = true;
            _logger.LogInformation("Trapped SIGHUP signal — reloading configuration");
            _ = Task.Run(() => ReloadConfig());
        }));

        // SIGUSR1 and SIGUSR2 only on non-Windows
        if (!OperatingSystem.IsWindows())
        {
            _signalRegistrations.Add(PosixSignalRegistration.Create((PosixSignal)10, ctx =>
            {
                ctx.Cancel = true;
                _logger.LogInformation("Trapped SIGUSR1 signal — reopening log file");
                ReOpenLogFile?.Invoke();
            }));

            _signalRegistrations.Add(PosixSignalRegistration.Create((PosixSignal)12, ctx =>
            {
                ctx.Cancel = true;
                _logger.LogInformation("Trapped SIGUSR2 signal — entering lame duck mode");
                _ = Task.Run(async () => await LameDuckShutdownAsync());
            }));
        }
    }

    public NatsServer(NatsOptions options, ILoggerFactory loggerFactory)
    {
        _options = options;
        _loggerFactory = loggerFactory;
        _logger = loggerFactory.CreateLogger<NatsServer>();
        _closedClients = new ClosedConnectionRingBuffer(options.MaxClosedClients);
        _authService = AuthService.Build(options);
        _globalAccount = new Account(Account.GlobalAccountName);
        _accounts[Account.GlobalAccountName] = _globalAccount;

        // Create $SYS system account and mark it as the system account.
        // Reference: Go server/server.go — initSystemAccount, accounts.go — isSystemAccount().
        _systemAccount = new Account(Account.SystemAccountName) { IsSystemAccount = true };
        _accounts[Account.SystemAccountName] = _systemAccount;

        // If a user-defined system_account is configured, promote that account to be the
        // system account. Events published to $SYS.* will be delivered to subscribers on
        // this account. Go reference: server/server.go — configureAccounts / setSystemAccount.
        if (!string.IsNullOrEmpty(options.SystemAccount) &&
            !string.Equals(options.SystemAccount, Account.SystemAccountName, StringComparison.OrdinalIgnoreCase))
        {
            var userSysAccount = GetOrCreateAccount(options.SystemAccount);
            userSysAccount.IsSystemAccount = true;
            _systemAccount = userSysAccount;
        }

        // Create system internal client and event system
        var sysClientId = Interlocked.Increment(ref _nextClientId);
        var sysClient = new InternalClient(sysClientId, ClientKind.System, _systemAccount);
        _eventSystem = new InternalEventSystem(
            _systemAccount, sysClient,
            options.ServerName ?? $"nats-dotnet-{Environment.MachineName}",
            _loggerFactory.CreateLogger<InternalEventSystem>());

        // Generate Ed25519 server NKey identity
        using var serverKeyPair = KeyPair.CreatePair(PrefixByte.Server);
        ServerNKey = serverKeyPair.GetPublicKey();

        _serverInfo = new ServerInfo
        {
            ServerId = Guid.NewGuid().ToString("N")[..20].ToUpperInvariant(),
            ServerName = options.ServerName ?? $"nats-dotnet-{Environment.MachineName}",
            Version = NatsProtocol.Version,
            Host = options.Host,
            Port = options.Port,
            MaxPayload = options.MaxPayload,
            AuthRequired = _authService.IsAuthRequired,
        };

        if (options.Cluster != null)
        {
            _routeManager = new RouteManager(options.Cluster, _stats, _serverInfo.ServerId, ApplyRemoteSubscription,
                ProcessRoutedMessage,
                _loggerFactory.CreateLogger<RouteManager>());
            _routeManager.OnRouteRemoved += RemoveRemoteSubscriptionsForRoute;
            _routeManager.OnRouteAccountRemoved += RemoveRemoteSubscriptionsForRouteAccount;
        }

        if (options.Gateway != null)
        {
            _gatewayManager = new GatewayManager(options.Gateway, _stats, _serverInfo.ServerId, ApplyRemoteSubscription,
                ProcessGatewayMessage,
                _loggerFactory.CreateLogger<GatewayManager>());
        }

        if (options.LeafNode != null)
        {
            _leafNodeManager = new LeafNodeManager(options.LeafNode, _stats, _serverInfo.ServerId, ApplyRemoteSubscription,
                ProcessLeafMessage,
                _loggerFactory.CreateLogger<LeafNodeManager>());
        }

        if (options.JetStream != null)
        {
            _jetStreamConsumerManager = new ConsumerManager();
            _jetStreamStreamManager = new StreamManager(consumerManager: _jetStreamConsumerManager, storeDir: options.JetStream.StoreDir);
            _jetStreamConsumerManager.StreamManager = _jetStreamStreamManager;
            var jsClientId = Interlocked.Increment(ref _nextClientId);
            _jetStreamInternalClient = new InternalClient(jsClientId, ClientKind.JetStream, _systemAccount);
            _jetStreamService = new JetStreamService(options.JetStream, _jetStreamInternalClient);
            _jetStreamApiRouter = new JetStreamApiRouter(_jetStreamStreamManager, _jetStreamConsumerManager);
            _jetStreamPublisher = new JetStreamPublisher(_jetStreamStreamManager);
        }

        if (options.HasTls)
        {
            _tlsCertProvider = new TlsCertificateProvider(options.TlsCert!, options.TlsKey);
            _sslOptions = TlsHelper.BuildServerAuthOptions(options);
            _tlsCertProvider.SwapSslOptions(_sslOptions);

            // OCSP stapling: build a certificate context so the runtime can
            // fetch and cache a fresh OCSP response and staple it during the
            // TLS handshake.  offline:false tells the runtime to contact the
            // OCSP responder; if the responder is unreachable we fall back to
            // no stapling rather than refusing all connections.
            var certContext = TlsHelper.BuildCertificateContext(options, offline: false);
            if (certContext != null)
            {
                _sslOptions.ServerCertificateContext = certContext;
                _logger.LogInformation("OCSP stapling enabled (mode: {OcspMode})", options.OcspConfig!.Mode);
            }

            _serverInfo.TlsRequired = !options.AllowNonTls;
            _serverInfo.TlsAvailable = options.AllowNonTls;
            _serverInfo.TlsVerify = options.TlsVerify;

            if (options.TlsRateLimit > 0)
                _tlsRateLimiter = new TlsRateLimiter(options.TlsRateLimit);
        }

        // Compile subject transforms
        if (options.SubjectMappings is { Count: > 0 })
        {
            var transforms = new List<SubjectTransform>();
            foreach (var (source, dest) in options.SubjectMappings)
            {
                var t = SubjectTransform.Create(source, dest);
                if (t != null)
                    transforms.Add(t);
                else
                    _logger.LogWarning("Invalid subject mapping: {Source} -> {Dest}", source, dest);
            }
            _subjectTransforms = transforms.ToArray();
            if (_subjectTransforms.Length > 0)
                _logger.LogInformation("Compiled {Count} subject transform(s)", _subjectTransforms.Length);
        }
        else
        {
            _subjectTransforms = [];
        }

        BuildCachedInfo();

        // Store initial config digest for reload change detection
        if (options.ConfigFile != null)
        {
            try
            {
                var (_, digest) = NatsConfParser.ParseFileWithDigest(options.ConfigFile);
                _configDigest = digest;
                _configTime = DateTime.UtcNow;
            }
            catch (Exception ex)
            {
                _logger.LogWarning(ex, "Could not compute initial config digest for {ConfigFile}", options.ConfigFile);
            }
        }
    }

    private void BuildCachedInfo()
    {
        var infoJson = System.Text.Json.JsonSerializer.Serialize(_serverInfo);
        _cachedInfoLine = Encoding.ASCII.GetBytes($"INFO {infoJson}\r\n");
    }

    private static string NormalizeAdvertiseUrl(string advertise, string defaultScheme)
    {
        if (advertise.Contains("://", StringComparison.Ordinal))
            return advertise;

        return $"{defaultScheme}://{advertise}";
    }

    private static bool IsWildcardHost(string host)
        => host == "0.0.0.0" || host == "::";

    internal static IReadOnlyList<string> GetNonLocalIPsIfHostIsIPAny(string host)
    {
        if (!IsWildcardHost(host))
            return [host];

        var addresses = new HashSet<string>(StringComparer.Ordinal);
        foreach (var netIf in NetworkInterface.GetAllNetworkInterfaces())
        {
            if (netIf.OperationalStatus != OperationalStatus.Up)
                continue;

            IPInterfaceProperties? props;
            try
            {
                props = netIf.GetIPProperties();
            }
            catch
            {
                continue;
            }

            foreach (var uni in props.UnicastAddresses)
            {
                var addr = uni.Address;
                if (IPAddress.IsLoopback(addr) || addr.IsIPv6LinkLocal || addr.IsIPv6Multicast)
                    continue;
                if (addr.AddressFamily is not (AddressFamily.InterNetwork or AddressFamily.InterNetworkV6))
                    continue;
                addresses.Add(addr.ToString());
            }
        }

        if (addresses.Count == 0)
            addresses.Add("127.0.0.1");

        return [.. addresses.OrderBy(static a => a, StringComparer.Ordinal)];
    }

    private bool CloseClientById(ulong clientId, bool minimalFlush)
    {
        if (!_clients.TryGetValue(clientId, out var client))
            return false;

        client.MarkClosed(ClientClosedReason.ServerShutdown);
        _ = client.FlushAndCloseAsync(minimalFlush);
        return true;
    }

    private static void AddEndpoint(List<string> targets, string? host, int port)
    {
        if (string.IsNullOrWhiteSpace(host) || port <= 0)
            return;

        targets.Add($"{host}:{port}");
    }

    private static void AddEndpoint(List<string> targets, string? endpoint)
    {
        if (!string.IsNullOrWhiteSpace(endpoint))
            targets.Add(endpoint);
    }

    public async Task StartAsync(CancellationToken ct)
    {
        using var linked = CancellationTokenSource.CreateLinkedTokenSource(ct, _quitCts.Token);

        _listener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        _listener.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
        _listener.Bind(new IPEndPoint(
            _options.Host == "0.0.0.0" ? IPAddress.Any : IPAddress.Parse(_options.Host),
            _options.Port));
        Interlocked.Exchange(ref _startTimeTicks, DateTime.UtcNow.Ticks);
        _listener.Listen(128);

        // Resolve ephemeral port if port=0
        if (_options.Port == 0)
        {
            var actualPort = ((IPEndPoint)_listener.LocalEndPoint!).Port;
            _options.Port = actualPort;
            _serverInfo.Port = actualPort;
            BuildCachedInfo();
        }

        _logger.LogInformation("Listening for client connections on {Host}:{Port}", _options.Host, _options.Port);

        // Warn about stub features
        StartProfiler();

        if (_options.MonitorPort > 0)
        {
            _monitorServer = new MonitorServer(this, _options, _stats, _loggerFactory);
            await _monitorServer.StartAsync(linked.Token);
        }

        WritePidFile();
        WritePortsFile();

        WsAuthConfig.Apply(_options.WebSocket);
        var wsValidation = WebSocketOptionsValidator.Validate(_options);
        if (!wsValidation.IsValid)
            throw new InvalidOperationException($"Invalid websocket options: {string.Join("; ", wsValidation.Errors)}");

        if (_options.WebSocket.Port >= 0)
        {
            _wsListener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            _wsListener.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
            _wsListener.Bind(new IPEndPoint(
                _options.WebSocket.Host == "0.0.0.0" ? IPAddress.Any : IPAddress.Parse(_options.WebSocket.Host),
                _options.WebSocket.Port));
            _wsListener.Listen(128);

            if (_options.WebSocket.Port == 0)
            {
                _options.WebSocket.Port = ((IPEndPoint)_wsListener.LocalEndPoint!).Port;
            }

            _logger.LogInformation("Listening for WebSocket clients on {Host}:{Port}",
                _options.WebSocket.Host, _options.WebSocket.Port);

            if (_options.WebSocket.NoTls)
                _logger.LogWarning("WebSocket not configured with TLS. DO NOT USE IN PRODUCTION!");

            _ = RunWebSocketAcceptLoopAsync(linked.Token);
        }
        else
        {
            // No WebSocket listener — signal immediately so shutdown doesn't wait
            _wsAcceptLoopExited.TrySetResult();
        }

        if (_routeManager != null)
            await _routeManager.StartAsync(linked.Token);
        if (_gatewayManager != null)
            await _gatewayManager.StartAsync(linked.Token);
        if (_leafNodeManager != null)
            await _leafNodeManager.StartAsync(linked.Token);
        if (_options.Mqtt is { Port: > 0 } mqttOptions)
        {
            var mqttHost = string.IsNullOrWhiteSpace(mqttOptions.Host) ? _options.Host : mqttOptions.Host;

            // Create MQTT JetStream components if JetStream is enabled
            MqttStreamInitializer? mqttStreamInit = null;
            MqttConsumerManager? mqttConsumerMgr = null;
            if (_jetStreamStreamManager != null && _jetStreamConsumerManager != null)
            {
                mqttStreamInit = new Mqtt.MqttStreamInitializer(_jetStreamStreamManager);
                mqttConsumerMgr = new Mqtt.MqttConsumerManager(_jetStreamStreamManager, _jetStreamConsumerManager);
            }

            _mqttListener = new MqttListener(
                mqttHost,
                mqttOptions.Port,
                _authService,
                mqttOptions,
                mqttStreamInit,
                mqttConsumerMgr,
                router: this);
            _mqttListener.AllocateClientId = () => Interlocked.Increment(ref _nextClientId);
            _mqttListener.ResolveAccount = name => GetOrCreateAccount(name ?? Auth.Account.GlobalAccountName);
            await _mqttListener.StartAsync(linked.Token);
        }
        if (_jetStreamService != null)
        {
            await _jetStreamService.StartAsync(linked.Token);
            _stats.JetStreamEnabled = true;
        }

        _listeningStarted.TrySetResult();

        _eventSystem?.Start(this);
        _eventSystem?.InitEventTracking(this);

        var tmpDelay = AcceptMinSleep;

        try
        {
            while (!linked.Token.IsCancellationRequested)
            {
                Socket socket;
                try
                {
                    socket = await _listener!.AcceptAsync(linked.Token);
                    tmpDelay = AcceptMinSleep; // Reset on success
                }
                catch (OperationCanceledException)
                {
                    break;
                }
                catch (ObjectDisposedException)
                {
                    break;
                }
                catch (SocketException ex)
                {
                    if (IsShuttingDown || IsLameDuckMode)
                        break;

                    _acceptLoopErrorHandler?.OnAcceptError(ex, _listener?.LocalEndPoint, tmpDelay);
                    _logger.LogError(ex, "Temporary accept error, sleeping {Delay}ms", tmpDelay.TotalMilliseconds);
                    try { await Task.Delay(tmpDelay, linked.Token); }
                    catch (OperationCanceledException) { break; }

                    tmpDelay = TimeSpan.FromTicks(Math.Min(tmpDelay.Ticks * 2, AcceptMaxSleep.Ticks));
                    continue;
                }

                // Check MaxConnections
                if (_options.MaxConnections > 0 && _clients.Count >= _options.MaxConnections)
                {
                    _logger.LogWarning("Client connection rejected: maximum connections ({MaxConnections}) exceeded",
                        _options.MaxConnections);
                    try
                    {
                        var stream = new NetworkStream(socket, ownsSocket: false);
                        var errBytes = Encoding.ASCII.GetBytes(
                            $"-ERR '{NatsProtocol.ErrMaxConnectionsExceeded}'\r\n");
                        await stream.WriteAsync(errBytes, linked.Token);
                        await stream.FlushAsync(linked.Token);
                        stream.Dispose();
                    }
                    catch (Exception ex2)
                    {
                        _logger.LogDebug(ex2, "Failed to send -ERR to rejected client");
                    }
                    finally
                    {
                        socket.Dispose();
                    }
                    continue;
                }

                var clientId = Interlocked.Increment(ref _nextClientId);
                Interlocked.Increment(ref _stats.TotalConnections);
                Interlocked.Increment(ref _activeClientCount);

                _logger.LogDebug("Client {ClientId} connected from {RemoteEndpoint}", clientId, socket.RemoteEndPoint);

                _ = AcceptClientAsync(socket, clientId, linked.Token);
            }
        }
        catch (OperationCanceledException)
        {
            _logger.LogDebug("Accept loop cancelled, server shutting down");
        }
        finally
        {
            _acceptLoopExited.TrySetResult();
        }
    }

    private async Task AcceptClientAsync(Socket socket, ulong clientId, CancellationToken ct)
    {
        var reloadLockHeld = false;
        NatsClient? client = null;
        try
        {
            await _reloadMu.WaitAsync(ct);
            reloadLockHeld = true;

            // Rate limit TLS handshakes
            if (_tlsRateLimiter != null)
                await _tlsRateLimiter.WaitAsync(ct);

            var networkStream = new NetworkStream(socket, ownsSocket: false);

            // TLS negotiation (no-op if not configured)
            var (stream, infoAlreadySent) = await TlsConnectionWrapper.NegotiateAsync(
                socket, networkStream, _options, _sslOptions, _serverInfo,
                _loggerFactory.CreateLogger("NATS.Server.Tls"), ct);

            // Extract TLS state
            TlsConnectionState? tlsState = null;
            if (stream is SslStream ssl)
            {
                tlsState = new TlsConnectionState(
                    ssl.SslProtocol.ToString(),
                    ssl.NegotiatedCipherSuite.ToString(),
                    ssl.RemoteCertificate as X509Certificate2);
            }

            // Build per-client ServerInfo with nonce if NKey auth is configured
            byte[]? nonce = null;
            var clientInfo = _serverInfo;
            if (_authService.NonceRequired)
            {
                var rawNonce = _authService.GenerateNonce();
                var nonceStr = _authService.EncodeNonce(rawNonce);
                // The client signs the nonce string (ASCII), not the raw bytes
                nonce = Encoding.ASCII.GetBytes(nonceStr);
                clientInfo = new ServerInfo
                {
                    ServerId = _serverInfo.ServerId,
                    ServerName = _serverInfo.ServerName,
                    Version = _serverInfo.Version,
                    Host = _serverInfo.Host,
                    Port = _serverInfo.Port,
                    MaxPayload = _serverInfo.MaxPayload,
                    AuthRequired = _serverInfo.AuthRequired,
                    TlsRequired = _serverInfo.TlsRequired,
                    TlsAvailable = _serverInfo.TlsAvailable,
                    TlsVerify = _serverInfo.TlsVerify,
                    Nonce = nonceStr,
                };
            }

            var clientLogger = _loggerFactory.CreateLogger($"NATS.Server.NatsClient[{clientId}]");
            client = new NatsClient(clientId, stream, socket, _options, clientInfo,
                _authService, nonce, clientLogger, _stats);
            client.Router = this;
            client.TlsState = tlsState;
            client.InfoAlreadySent = infoAlreadySent;
            _clients[clientId] = client;
        }
        catch (Exception ex)
        {
            if (client is null)
            {
                var earlyReason = _options.HasTls
                    ? ClientClosedReason.TlsHandshakeError
                    : ClientClosedReason.ReadError;
                TrackEarlyClosedClient(socket, clientId, earlyReason);
            }

            _logger.LogDebug(ex, "Failed to accept client {ClientId}", clientId);
            try { socket.Shutdown(SocketShutdown.Both); } catch { }
            socket.Dispose();
            return;
        }
        finally
        {
            if (reloadLockHeld)
                _reloadMu.Release();
        }

        try
        {
            if (client != null)
                await RunClientAsync(client, ct);
        }
        catch (Exception ex)
        {
            _logger.LogDebug(ex, "Failed to accept client {ClientId}", clientId);
            try { socket.Shutdown(SocketShutdown.Both); } catch { }
            socket.Dispose();
        }
    }

    private async Task RunWebSocketAcceptLoopAsync(CancellationToken ct)
    {
        var tmpDelay = AcceptMinSleep;
        try
        {
            while (!ct.IsCancellationRequested)
            {
                Socket socket;
                try
                {
                    socket = await _wsListener!.AcceptAsync(ct);
                    tmpDelay = AcceptMinSleep;
                }
                catch (OperationCanceledException) { break; }
                catch (ObjectDisposedException) { break; }
                catch (SocketException ex)
                {
                    if (IsShuttingDown || IsLameDuckMode) break;
                    _acceptLoopErrorHandler?.OnAcceptError(ex, _wsListener?.LocalEndPoint, tmpDelay);
                    _logger.LogError(ex, "Temporary WebSocket accept error, sleeping {Delay}ms", tmpDelay.TotalMilliseconds);
                    try { await Task.Delay(tmpDelay, ct); } catch (OperationCanceledException) { break; }
                    tmpDelay = TimeSpan.FromTicks(Math.Min(tmpDelay.Ticks * 2, AcceptMaxSleep.Ticks));
                    continue;
                }

                if (_options.MaxConnections > 0 && _clients.Count >= _options.MaxConnections)
                {
                    socket.Dispose();
                    continue;
                }

                var clientId = Interlocked.Increment(ref _nextClientId);
                Interlocked.Increment(ref _stats.TotalConnections);
                Interlocked.Increment(ref _activeClientCount);

                _ = AcceptWebSocketClientAsync(socket, clientId, ct);
            }
        }
        finally
        {
            _wsAcceptLoopExited.TrySetResult();
        }
    }

    private async Task AcceptWebSocketClientAsync(Socket socket, ulong clientId, CancellationToken ct)
    {
        try
        {
            var networkStream = new NetworkStream(socket, ownsSocket: false);
            Stream stream = networkStream;

            // TLS negotiation if configured
            if (_sslOptions != null && !_options.WebSocket.NoTls)
            {
                var (tlsStream, _) = await TlsConnectionWrapper.NegotiateAsync(
                    socket, networkStream, _options, _sslOptions, _serverInfo,
                    _loggerFactory.CreateLogger("NATS.Server.Tls"), ct);
                stream = tlsStream;
            }

            // HTTP upgrade handshake
            var upgradeResult = await WsUpgrade.TryUpgradeAsync(stream, stream, _options.WebSocket, ct);
            if (!upgradeResult.Success)
            {
                _logger.LogDebug("WebSocket upgrade failed for client {ClientId}", clientId);
                socket.Dispose();
                Interlocked.Decrement(ref _activeClientCount);
                return;
            }

            // Create WsConnection wrapper
            var wsConn = new WsConnection(stream,
                compress: upgradeResult.Compress,
                maskRead: upgradeResult.MaskRead,
                maskWrite: upgradeResult.MaskWrite,
                browser: upgradeResult.Browser,
                noCompFrag: upgradeResult.NoCompFrag);

            var clientLogger = _loggerFactory.CreateLogger($"NATS.Server.NatsClient[{clientId}]");
            var client = new NatsClient(clientId, wsConn, socket, _options, _serverInfo,
                _authService, null, clientLogger, _stats);
            client.Router = this;
            client.IsWebSocket = true;
            client.WsInfo = upgradeResult;
            _clients[clientId] = client;

            await RunClientAsync(client, ct);
        }
        catch (Exception ex)
        {
            _logger.LogDebug(ex, "Failed to accept WebSocket client {ClientId}", clientId);
            try { socket.Shutdown(SocketShutdown.Both); } catch { }
            socket.Dispose();
            Interlocked.Decrement(ref _activeClientCount);
        }
    }

    private async Task RunClientAsync(NatsClient client, CancellationToken ct)
    {
        try
        {
            await client.RunAsync(ct);
        }
        catch (Exception ex)
        {
            _logger.LogDebug(ex, "Client {ClientId} disconnected with error", client.Id);
        }
        finally
        {
            _logger.LogDebug("Client {ClientId} disconnected (reason: {CloseReason})", client.Id, client.CloseReason);
            RemoveClient(client);
            Interlocked.Decrement(ref _activeClientCount);
        }
    }

    public void OnLocalSubscription(string account, string subject, string? queue)
    {
        _routeManager?.PropagateLocalSubscription(account, subject, queue);
        _gatewayManager?.PropagateLocalSubscription(account, subject, queue);
        _leafNodeManager?.PropagateLocalSubscription(account, subject, queue);
    }

    public void OnLocalUnsubscription(string account, string subject, string? queue)
    {
        _routeManager?.PropagateLocalUnsubscription(account, subject, queue);
        _gatewayManager?.PropagateLocalUnsubscription(account, subject, queue);
        _leafNodeManager?.PropagateLocalUnsubscription(account, subject, queue);
    }

    private void ApplyRemoteSubscription(RemoteSubscription sub)
    {
        var account = GetOrCreateAccount(sub.Account);
        account.SubList.ApplyRemoteSub(sub);
    }

    private void RemoveRemoteSubscriptionsForRoute(string routeId)
    {
        foreach (var account in _accounts.Values)
            account.SubList.RemoveRemoteSubs(routeId);
    }

    private void RemoveRemoteSubscriptionsForRouteAccount(string routeId, string accountName)
    {
        if (_accounts.TryGetValue(accountName, out var account))
            account.SubList.RemoveRemoteSubsForAccount(routeId, accountName);
    }

    private void ProcessRoutedMessage(RouteMessage message)
    {
        DeliverRemoteMessage(message.Account, message.Subject, message.ReplyTo, message.Payload);
    }

    private void ProcessGatewayMessage(GatewayMessage message)
    {
        var replyTo = message.ReplyTo;
        if (ReplyMapper.TryRestoreGatewayReply(replyTo, out var restoredReply))
            replyTo = restoredReply;
        else if (ReplyMapper.HasGatewayReplyPrefix(replyTo))
            replyTo = null;

        DeliverRemoteMessage(message.Account, message.Subject, replyTo, message.Payload);
    }

    private void ProcessLeafMessage(LeafMessage message)
    {
        if (LeafLoopDetector.IsLooped(message.Subject, ServerId))
            return;

        var subject = message.Subject;
        if (LeafLoopDetector.TryUnmark(subject, out var unmarked))
            subject = unmarked;
        else if (LeafLoopDetector.HasLoopMarker(subject))
            return;

        DeliverRemoteMessage(message.Account, subject, message.ReplyTo, message.Payload);
    }

    private void DeliverRemoteMessage(string account, string subject, string? replyTo, ReadOnlyMemory<byte> payload)
    {
        // Handle internal JetStream cluster replication messages.
        // Go reference: jetstream_cluster.go — RAFT proposal apply dispatches to handlers.
        if (subject.StartsWith(JetStreamApiSubjects.InternalPrefix, StringComparison.Ordinal))
        {
            HandleJetStreamReplication(subject, payload);
            return;
        }

        // Capture routed data messages into local JetStream streams.
        // Skip $JS. subjects — those are API calls, not stream data.
        if (!subject.StartsWith("$JS.", StringComparison.Ordinal))
            TryCaptureJetStreamPublish(subject, payload, out _);

        var targetAccount = GetOrCreateAccount(account);
        var result = targetAccount.SubList.Match(subject);

        foreach (var sub in result.PlainSubs)
            DeliverMessage(sub, subject, replyTo, default, payload);

        foreach (var queueGroup in result.QueueSubs)
        {
            if (queueGroup.Length == 0)
                continue;

            var sub = queueGroup[0];
            DeliverMessage(sub, subject, replyTo, default, payload);
        }
    }

    public void ProcessMessage(string subject, string? replyTo, ReadOnlyMemory<byte> headers,
        ReadOnlyMemory<byte> payload, INatsClient sender)
    {
        // Cast to NatsClient for operations that require it (JetStream pub-ack, stats).
        // Non-NatsClient senders (e.g. MqttNatsClientAdapter) skip those code paths.
        var natsClient = sender as NatsClient;

        if (replyTo != null
            && subject.StartsWith("$JS.API", StringComparison.Ordinal)
            && _jetStreamApiRouter != null)
        {
            // Pull consumer MSG.NEXT requires special handling: deliver individual
            // HMSG messages to the client's reply inbox instead of a single JSON blob.
            // Go reference: consumer.go:4276 processNextMsgRequest
            if (subject.StartsWith(JetStream.Api.JetStreamApiSubjects.ConsumerNext, StringComparison.Ordinal)
                && _jetStreamConsumerManager != null
                && _jetStreamStreamManager != null
                && natsClient != null)
            {
                Interlocked.Increment(ref _stats.JetStreamApiTotal);
                DeliverPullFetchMessages(subject, replyTo, payload, natsClient);
                return;
            }

            var response = _jetStreamApiRouter.Route(subject, payload.Span);
            Interlocked.Increment(ref _stats.JetStreamApiTotal);
            if (response.Error != null)
            {
                Interlocked.Increment(ref _stats.JetStreamApiErrors);
            }

            // Replicate successful mutating operations to cluster peers.
            // Go reference: jetstream_cluster.go — RAFT proposal replication.
            if (response.Error == null)
                TryReplicateJetStreamMutation(subject, payload);

            var data = JsonSerializer.SerializeToUtf8Bytes(response.ToWireFormat(), s_jetStreamJsonOptions);
            ProcessMessage(replyTo, null, default, data, sender);
            return;
        }

        if (TryCaptureJetStreamPublish(subject, payload, out var pubAck))
        {
            natsClient?.RecordJetStreamPubAck(pubAck);

            // Replicate data messages to cluster peers so their JetStream stores also capture them.
            // Route forwarding below is gated on subscriber interest, which JetStream streams don't
            // create, so we must explicitly push data to replicas.
            // Go reference: jetstream_cluster.go — RAFT propose entry replication.
            if (pubAck.ErrorCode == null)
                ReplicateJetStreamOperation("$G", subject, null, payload);

            // Send pub ack response to the reply subject (request-reply pattern).
            // Go reference: server/jetstream.go — jsPubAckResponse sent to reply.
            if (replyTo != null)
            {
                if (JetStream.Publish.JetStreamPubAckFormatter.IsSimpleSuccess(pubAck))
                {
                    // Fast path: hand-rolled UTF-8 formatter avoids JsonSerializer overhead.
                    Span<byte> ackBuf = stackalloc byte[256];
                    var ackLen = JetStream.Publish.JetStreamPubAckFormatter.FormatSuccess(ackBuf, pubAck.Stream, pubAck.Seq);
                    ProcessMessage(replyTo, null, default, ackBuf[..ackLen].ToArray(), sender);
                }
                else
                {
                    var ackData = JsonSerializer.SerializeToUtf8Bytes(pubAck, s_jetStreamJsonOptions);
                    ProcessMessage(replyTo, null, default, ackData, sender);
                }

                return;
            }
        }

        // Apply subject transforms
        if (_subjectTransforms.Length > 0)
        {
            foreach (var transform in _subjectTransforms)
            {
                var mapped = transform.Apply(subject);
                if (mapped != null)
                {
                    subject = mapped;
                    break; // First matching transform wins
                }
            }
        }

        var senderAccount = sender.Account ?? _globalAccount;
        if (_routeManager != null && senderAccount.SubList.HasRemoteInterest(senderAccount.Name, subject))
            _routeManager.ForwardRoutedMessageAsync(senderAccount.Name, subject, replyTo, payload, default).GetAwaiter().GetResult();
        if (_gatewayManager != null && senderAccount.SubList.HasRemoteInterest(senderAccount.Name, subject))
        {
            var mappedReplyTo = ReplyMapper.ToGatewayReply(replyTo, ServerId);
            _gatewayManager.ForwardMessageAsync(senderAccount.Name, subject, mappedReplyTo, payload, default).GetAwaiter().GetResult();
        }

        if (_leafNodeManager != null && senderAccount.SubList.HasRemoteInterest(senderAccount.Name, subject))
        {
            var markedSubject = LeafLoopDetector.Mark(subject, ServerId);
            _leafNodeManager.ForwardMessageAsync(senderAccount.Name, markedSubject, replyTo, payload, default).GetAwaiter().GetResult();
        }

        var subList = sender.Account?.SubList ?? _globalAccount.SubList;
        var result = subList.Match(subject);
        var delivered = false;
        int deliveredCount = 0;

        // Pre-encode subject bytes once for all fan-out deliveries (one alloc per publish, not per delivery).
        var subjectBytes = Encoding.ASCII.GetBytes(subject);

        // Per-client deferred flush: collect unique clients during fan-out, signal each once.
        // Go reference: client.go:3905 addToPCD / client.go:1324 flushClients.
        var pcd = t_pcd ??= new HashSet<INatsClient>();
        pcd.Clear();

        // Deliver to plain subscribers
        var messageSize = payload.Length + headers.Length;
        foreach (var sub in result.PlainSubs)
        {
            if (sub.Client == null || sub.Client == sender && !(sender.ClientOpts?.Echo ?? true))
                continue;

            DeliverMessage(sub, subjectBytes, sub.SidBytes, subject, replyTo, headers, payload, pcd);
            delivered = true;
            deliveredCount++;
        }

        // Deliver to one member of each queue group (round-robin)
        foreach (var queueGroup in result.QueueSubs)
        {
            if (queueGroup.Length == 0) continue;

            // Simple round-robin -- pick based on total delivered across group
            if (natsClient != null)
            {
                var idx = Math.Abs((int)Interlocked.Increment(ref natsClient.OutMsgs)) % queueGroup.Length;
                // Undo the OutMsgs increment -- it will be incremented properly in SendMessageNoFlush
                Interlocked.Decrement(ref natsClient.OutMsgs);

                for (int attempt = 0; attempt < queueGroup.Length; attempt++)
                {
                    var sub = queueGroup[(idx + attempt) % queueGroup.Length];
                    if (sub.Client != null && (sub.Client != sender || (sender.ClientOpts?.Echo ?? true)))
                    {
                        DeliverMessage(sub, subjectBytes, sub.SidBytes, subject, replyTo, headers, payload, pcd);
                        delivered = true;
                        deliveredCount++;
                        break;
                    }
                }
            }
            else
            {
                // Non-NatsClient sender: simple first-match
                foreach (var sub in queueGroup)
                {
                    if (sub.Client != null && sub.Client != sender)
                    {
                        DeliverMessage(sub, subjectBytes, sub.SidBytes, subject, replyTo, headers, payload, pcd);
                        delivered = true;
                        deliveredCount++;
                        break;
                    }
                }
            }
        }

        // Batch server-wide stats once per publish (instead of per-delivery Interlocked ops).
        if (deliveredCount > 0)
        {
            Interlocked.Add(ref _stats.OutMsgs, (long)deliveredCount);
            Interlocked.Add(ref _stats.OutBytes, (long)messageSize * deliveredCount);
        }

        // Flush all unique clients once after fan-out.
        // Go reference: client.go:1324 flushClients — iterates pcd map, one signal per client.
        foreach (var client in pcd)
            client.SignalFlush();
        pcd.Clear();

        // Check for service imports that match this subject.
        // When a client in the importer account publishes to a subject
        // that matches a service import "From" pattern, we forward the
        // message to the destination (exporter) account's subscribers
        // using the mapped "To" subject.
        if (sender.Account != null)
        {
            foreach (var kvp in sender.Account.Imports.Services)
            {
                foreach (var si in kvp.Value)
                {
                    if (si.Invalid) continue;
                    if (SubjectMatch.MatchLiteral(subject, si.From))
                    {
                        ProcessServiceImport(si, subject, replyTo, headers, payload, sender.Account);
                        delivered = true;
                    }
                }
            }
        }

        // No-responders: if nobody received the message and the publisher
        // opted in, send back a 503 status HMSG on the reply subject.
        if (!delivered && replyTo != null && sender.ClientOpts?.NoResponders == true && natsClient != null)
        {
            SendNoResponders(natsClient, replyTo);
        }
    }

    /// <summary>
    /// Handles $JS.API.CONSUMER.MSG.NEXT by delivering individual HMSG messages
    /// to the client's reply inbox. Go reference: consumer.go:4276 processNextMsgRequest.
    /// </summary>
    private void DeliverPullFetchMessages(string subject, string replyTo, ReadOnlyMemory<byte> payload, NatsClient sender)
    {
        var prefix = JetStream.Api.JetStreamApiSubjects.ConsumerNext;
        var remainder = subject[prefix.Length..];
        var split = remainder.Split('.', 2, StringSplitOptions.RemoveEmptyEntries);
        if (split.Length != 2)
        {
            var notFoundHeader = System.Text.Encoding.UTF8.GetBytes("NATS/1.0 404 No Messages\r\n\r\n");
            ProcessMessage(replyTo, null, (ReadOnlyMemory<byte>)notFoundHeader, default, sender);
            return;
        }

        var (streamName, consumerName) = (split[0], split[1]);

        // Parse batch request
        int batch = 1;
        int expiresMs = 0;
        bool noWait = false;
        int idleHeartbeatMs = 0;
        if (payload.Length > 0)
        {
            try
            {
                using var doc = System.Text.Json.JsonDocument.Parse(payload);
                if (doc.RootElement.TryGetProperty("batch", out var batchEl) && batchEl.TryGetInt32(out var b))
                    batch = Math.Max(b, 1);
                if (doc.RootElement.TryGetProperty("no_wait", out var nwEl) && nwEl.ValueKind == System.Text.Json.JsonValueKind.True)
                    noWait = true;
                if (doc.RootElement.TryGetProperty("expires", out var expEl) && expEl.TryGetInt64(out var expNs))
                    expiresMs = (int)(expNs / 1_000_000);
                if (doc.RootElement.TryGetProperty("idle_heartbeat", out var hbEl) && hbEl.TryGetInt64(out var hbNs))
                    idleHeartbeatMs = (int)(hbNs / 1_000_000);
            }
            catch (System.Text.Json.JsonException ex)
            {
                _logger.LogDebug(ex, "Malformed JSON in pull request payload, using defaults");
            }
        }

        // Find the sender's inbox subscription so we can deliver directly.
        // Go reference: consumer.go deliverMsg — delivers directly to the client, bypassing pub/sub echo checks.
        var subList = sender.Account?.SubList ?? _globalAccount.SubList;
        var matchResult = subList.Match(replyTo);
        Subscription? inboxSub = null;
        foreach (var sub in matchResult.PlainSubs)
        {
            if (sub.Client == sender)
            {
                inboxSub = sub;
                break;
            }
        }

        if (inboxSub == null)
            return;

        if (noWait || expiresMs <= 0)
        {
            // Synchronous path for no_wait (used by FetchAsync client path).
            // Fetch all immediately available messages and return.
            var fetchResult = _jetStreamConsumerManager!.FetchAsync(
                streamName, consumerName,
                new JetStream.Consumers.PullFetchRequest { Batch = batch, NoWait = true },
                _jetStreamStreamManager!, default).GetAwaiter().GetResult();

            DeliverFetchedMessages(inboxSub, streamName, consumerName, replyTo, fetchResult.Messages);

            // Send terminal status
            ReadOnlyMemory<byte> statusHeader = fetchResult.Messages.Count == 0
                ? System.Text.Encoding.UTF8.GetBytes("NATS/1.0 404 No Messages\r\n\r\n")
                : System.Text.Encoding.UTF8.GetBytes("NATS/1.0 408 Request Timeout\r\n\r\n");
            DeliverMessage(inboxSub, replyTo, null, statusHeader, default);
        }
        else
        {
            // Async path for ConsumeAsync: deliver messages incrementally without blocking
            // the client's read loop. Go reference: consumer.go processNextMsgRequest —
            // registers a waiting request and returns to the read loop; messages are delivered
            // asynchronously as they become available.
            var capturedSub = inboxSub;
            _ = Task.Run(() => DeliverPullFetchMessagesAsync(
                streamName, consumerName, batch, expiresMs, idleHeartbeatMs,
                replyTo, capturedSub, sender));
        }
    }

    /// <summary>
    /// Background task that delivers pull fetch messages incrementally.
    /// Polls the stream store for messages and delivers each one as it becomes available.
    /// Sends idle heartbeats to keep the client connection alive.
    /// Go reference: consumer.go — waiting request fulfillment loop.
    /// </summary>
    private async Task DeliverPullFetchMessagesAsync(
        string streamName, string consumerName, int batch, int expiresMs, int idleHeartbeatMs,
        string replyTo, Subscription inboxSub, NatsClient sender)
    {
        using var expiresCts = new CancellationTokenSource(TimeSpan.FromMilliseconds(expiresMs));
        var ct = expiresCts.Token;

        if (!_jetStreamConsumerManager!.TryGet(streamName, consumerName, out var consumer))
        {
            DeliverMessage(inboxSub, replyTo, null,
                System.Text.Encoding.UTF8.GetBytes("NATS/1.0 404 No Messages\r\n\r\n"), default);
            return;
        }

        if (!_jetStreamStreamManager!.TryGet(streamName, out var streamHandle))
        {
            DeliverMessage(inboxSub, replyTo, null,
                System.Text.Encoding.UTF8.GetBytes("NATS/1.0 404 No Messages\r\n\r\n"), default);
            return;
        }

        // Resolve initial sequence if needed
        if (consumer.NextSequence == 1)
        {
            var state = await streamHandle.Store.GetStateAsync(ct);
            consumer.NextSequence = consumer.Config.DeliverPolicy switch
            {
                JetStream.Models.DeliverPolicy.Last when state.LastSeq > 0 => state.LastSeq,
                JetStream.Models.DeliverPolicy.New when consumer.Config.OptStartSeq > 0 => consumer.Config.OptStartSeq,
                JetStream.Models.DeliverPolicy.New when state.LastSeq > 0 => state.LastSeq + 1,
                JetStream.Models.DeliverPolicy.ByStartSequence when consumer.Config.OptStartSeq > 0 => consumer.Config.OptStartSeq,
                _ => state.FirstSeq > 0 ? state.FirstSeq : 1,
            };
        }

        // Use cached CompiledFilter from ConsumerHandle (avoids per-fetch allocation)
        var compiledFilter = consumer.CompiledFilter;
        var sequence = consumer.NextSequence;
        ReadOnlyMemory<byte> minHeaders = "NATS/1.0\r\n\r\n"u8.ToArray();
        var ackPrefix = $"$JS.ACK.{streamName}.{consumerName}.1.";
        int deliverySeq = 0;
        int delivered = 0;
        var lastDeliveryTime = DateTime.UtcNow;
        var hbInterval = idleHeartbeatMs > 0 ? TimeSpan.FromMilliseconds(idleHeartbeatMs) : TimeSpan.FromSeconds(15);

        try
        {
            while (delivered < batch && !ct.IsCancellationRequested)
            {
                var message = await streamHandle.Store.LoadAsync(sequence, ct);
                if (message != null)
                {
                    // Check filter
                    if (!compiledFilter.Matches(message.Subject))
                    {
                        sequence++;
                        continue;
                    }

                    // Skip already-acked messages
                    if (message.Sequence <= consumer.AckProcessor.AckFloor)
                    {
                        sequence++;
                        continue;
                    }

                    deliverySeq++;
                    delivered++;
                    var tsNanos = new DateTimeOffset(message.TimestampUtc).ToUnixTimeMilliseconds() * 1_000_000L;
                    var numPending = batch - delivered;
                    var ackReply = BuildAckReply(ackPrefix, message.Sequence, deliverySeq, tsNanos, numPending);

                    // Bypass DeliverMessage — we already know the target client is sender.
                    // Skip permission check and auto-unsub overhead for JS delivery inbox.
                    // Go reference: consumer.go — batch delivery with deferred flush.
                    sender.SendMessageNoFlush(message.Subject, inboxSub.Sid, ackReply, minHeaders, message.Payload);

                    // Batch flush every 64 messages to amortize write-loop wakeup cost.
                    if ((delivered & 63) == 0)
                        sender.SignalFlush();

                    if (consumer.Config.AckPolicy is JetStream.Models.AckPolicy.Explicit or JetStream.Models.AckPolicy.All)
                    {
                        if (consumer.Config.MaxAckPending > 0 && consumer.AckProcessor.PendingCount >= consumer.Config.MaxAckPending)
                            break;
                        consumer.AckProcessor.Register(message.Sequence, consumer.Config.AckWaitMs);
                    }

                    sequence++;
                    lastDeliveryTime = DateTime.UtcNow;
                }
                else
                {
                    // Flush any buffered messages before blocking on the signal.
                    // Without this, messages queued via SendMessageNoFlush would sit
                    // in the buffer until the next batch boundary or loop exit.
                    sender.SignalFlush();

                    // No message available — send idle heartbeat if needed
                    if (DateTime.UtcNow - lastDeliveryTime >= hbInterval)
                    {
                        // Go reference: consumer.go sendIdleHeartbeat — status 100 with headers
                        var hbHeader = System.Text.Encoding.UTF8.GetBytes(
                            "NATS/1.0 100 Idle Heartbeat\r\nNats-Last-Consumer: " + consumerName +
                            "\r\nNats-Last-Stream: " + streamName + "\r\n\r\n");
                        DeliverMessage(inboxSub, replyTo, null, hbHeader, default);
                        lastDeliveryTime = DateTime.UtcNow;
                    }

                    // Wait for publisher to signal a new message, with a heartbeat-interval
                    // timeout so the heartbeat check is re-evaluated periodically.
                    // Go reference: consumer.go — channel signaling from publisher.
                    try
                    {
                        await streamHandle.WaitForPublishAsync(ct).WaitAsync(hbInterval, ct).ConfigureAwait(false);
                    }
                    catch (TimeoutException)
                    {
                        // Heartbeat interval elapsed — loop back to re-check
                    }
                }
            }
        }
        catch (OperationCanceledException) when (expiresCts.IsCancellationRequested)
        {
            // ExpiresMs timeout — expected
        }

        // Final flush for any remaining batched messages
        sender.SignalFlush();

        consumer.NextSequence = sequence;

        // Send terminal status directly to sender (last message, includes flush)
        ReadOnlyMemory<byte> statusHeader = delivered == 0
            ? System.Text.Encoding.UTF8.GetBytes("NATS/1.0 404 No Messages\r\n\r\n")
            : System.Text.Encoding.UTF8.GetBytes("NATS/1.0 408 Request Timeout\r\n\r\n");
        sender.SendMessage(replyTo, inboxSub.Sid, null, statusHeader, default);
    }

    private void DeliverFetchedMessages(Subscription inboxSub, string streamName, string consumerName,
        string replyTo, IReadOnlyList<JetStream.Storage.StoredMessage> messages)
    {
        ReadOnlyMemory<byte> minHeaders = "NATS/1.0\r\n\r\n"u8.ToArray();
        int deliverySeq = 0;
        int numPending = messages.Count;

        // Pre-compute constant ack prefix to avoid per-message string interpolation.
        // Go reference: consumer.go — ack reply format is $JS.ACK.<stream>.<consumer>.1.<seq>.<deliverySeq>.<ts>.<pending>
        var ackPrefix = $"$JS.ACK.{streamName}.{consumerName}.1.";

        // Use pcd pattern: all messages go to the same client, one flush after the loop.
        var pcd = t_pcd ??= new HashSet<INatsClient>();
        pcd.Clear();

        foreach (var msg in messages)
        {
            deliverySeq++;
            numPending--;

            var tsNanos = new DateTimeOffset(msg.TimestampUtc).ToUnixTimeMilliseconds() * 1_000_000L;
            var ackReply = BuildAckReply(ackPrefix, msg.Sequence, deliverySeq, tsNanos, numPending);

            DeliverMessage(inboxSub, msg.Subject, ackReply, minHeaders, msg.Payload, pcd);
        }

        // Flush once after all messages delivered
        foreach (var client in pcd)
            client.SignalFlush();
        pcd.Clear();
    }

    /// <summary>
    /// Fast-path overload using pre-encoded subject and SID bytes to avoid per-delivery encoding.
    /// Used by ProcessMessage fan-out loop.
    /// </summary>
    private void DeliverMessage(Subscription sub, ReadOnlySpan<byte> subjectBytes, ReadOnlySpan<byte> sidBytes,
        string subject, string? replyTo,
        ReadOnlyMemory<byte> headers, ReadOnlyMemory<byte> payload,
        HashSet<INatsClient>? pcd = null)
    {
        var client = sub.Client;
        if (client == null) return;

        // Auto-unsub: only track when a limit is set (common case is MaxMessages == 0).
        if (sub.MaxMessages > 0)
        {
            var count = Interlocked.Increment(ref sub.MessageCount);
            if (count > sub.MaxMessages)
            {
                var subList = client.Account?.SubList ?? _globalAccount.SubList;
                subList.Remove(sub);
                client.RemoveSubscription(sub.Sid);
                return;
            }
        }

        if (client.Permissions?.IsDeliveryAllowed(subject) == false)
            return;

        if (pcd != null)
        {
            if (client is NatsClient nc)
                nc.SendMessageNoFlush(subjectBytes, sidBytes, replyTo, headers, payload);
            else
                client.SendMessageNoFlush(subject, sub.Sid, replyTo, headers, payload);
            pcd.Add(client);
        }
        else
        {
            client.SendMessage(subject, sub.Sid, replyTo, headers, payload);
        }

        if (replyTo != null && client.Permissions?.ResponseTracker != null)
        {
            if (client.Permissions.IsPublishAllowed(replyTo) == false)
                client.Permissions.ResponseTracker.RegisterReply(replyTo);
        }
    }

    private void DeliverMessage(Subscription sub, string subject, string? replyTo,
        ReadOnlyMemory<byte> headers, ReadOnlyMemory<byte> payload,
        HashSet<INatsClient>? pcd = null)
    {
        var client = sub.Client;
        if (client == null) return;

        // Auto-unsub: only track when a limit is set (common case is MaxMessages == 0).
        if (sub.MaxMessages > 0)
        {
            var count = Interlocked.Increment(ref sub.MessageCount);
            if (count > sub.MaxMessages)
            {
                var subList = client.Account?.SubList ?? _globalAccount.SubList;
                subList.Remove(sub);
                client.RemoveSubscription(sub.Sid);
                return;
            }
        }

        if (client.Permissions?.IsDeliveryAllowed(subject) == false)
            return;

        if (pcd != null)
        {
            client.SendMessageNoFlush(subject, sub.Sid, replyTo, headers, payload);
            pcd.Add(client);
        }
        else
        {
            client.SendMessage(subject, sub.Sid, replyTo, headers, payload);
        }

        if (replyTo != null && client.Permissions?.ResponseTracker != null)
        {
            if (client.Permissions.IsPublishAllowed(replyTo) == false)
                client.Permissions.ResponseTracker.RegisterReply(replyTo);
        }
    }

    /// <summary>
    /// Builds an ack reply subject from pre-computed prefix and per-message values.
    /// Uses stack-based formatting to avoid string interpolation boxing/allocations.
    /// </summary>
    private static string BuildAckReply(string ackPrefix, ulong sequence, int deliverySeq, long tsNanos, int numPending)
    {
        // Max digits: ulong=20, int=11, long=20, int=11 + 3 dots = 65 chars max for suffix
        Span<char> buf = stackalloc char[ackPrefix.Length + 65];
        ackPrefix.AsSpan().CopyTo(buf);
        var pos = ackPrefix.Length;
        sequence.TryFormat(buf[pos..], out var w); pos += w; buf[pos++] = '.';
        deliverySeq.TryFormat(buf[pos..], out w); pos += w; buf[pos++] = '.';
        tsNanos.TryFormat(buf[pos..], out w); pos += w; buf[pos++] = '.';
        numPending.TryFormat(buf[pos..], out w); pos += w;
        return new string(buf[..pos]);
    }

    /// <summary>
    /// Processes a service import by transforming the subject from the importer's
    /// subject space to the exporter's subject space, then delivering to matching
    /// subscribers in the destination account.
    /// Reference: Go server/accounts.go addServiceImport / processServiceImport.
    /// </summary>
    public void ProcessServiceImport(ServiceImport si, string subject, string? replyTo,
        ReadOnlyMemory<byte> headers, ReadOnlyMemory<byte> payload, Account? sourceAccount = null)
    {
        if (si.Invalid) return;

        // Transform subject: map from importer subject space to exporter subject space
        string targetSubject;
        if (si.Transform != null)
        {
            var transformed = si.Transform.Apply(subject);
            targetSubject = transformed ?? si.To;
        }
        else if (si.UsePub)
        {
            targetSubject = subject;
        }
        else
        {
            // Default: use the "To" subject from the import definition.
            // For wildcard imports (e.g. "requests.>" -> "api.>"), we need
            // to map the specific subject tokens from the source pattern to
            // the destination pattern.
            targetSubject = MapImportSubject(subject, si.From, si.To);
        }

        // Set up a temporary reverse service import so that responses from the
        // destination (exporter) account can route back to the source (importer)
        // account. This handles request-reply across account boundaries.
        // Go reference: client.go setupResponseServiceImport
        if (replyTo != null && sourceAccount != null && !si.IsResponse)
        {
            SetupResponseServiceImport(si.DestinationAccount, sourceAccount, replyTo, si.Export);
        }

        // Service latency tracking: when the response arrives back, compute elapsed
        // time and publish a latency metric to the configured subject.
        // Go reference: client.go processServiceImport — latency tracking path.
        if (si.IsResponse && si.Tracking && si.TimestampTicks > 0)
        {
            var elapsed = TimeSpan.FromTicks(Environment.TickCount64 * TimeSpan.TicksPerMillisecond - si.TimestampTicks);
            PublishServiceLatency(si, elapsed);
        }

        // Match against destination account's SubList
        var destSubList = si.DestinationAccount.SubList;
        var result = destSubList.Match(targetSubject);

        // Deliver to plain subscribers in the destination account
        foreach (var sub in result.PlainSubs)
        {
            if (sub.Client == null) continue;
            DeliverMessage(sub, targetSubject, replyTo, headers, payload);
        }

        // Deliver to one member of each queue group
        foreach (var queueGroup in result.QueueSubs)
        {
            if (queueGroup.Length == 0) continue;
            var sub = queueGroup[0]; // Simple selection: first available
            if (sub.Client != null)
                DeliverMessage(sub, targetSubject, replyTo, headers, payload);
        }
    }

    /// <summary>
    /// Creates a temporary reverse service import in the exporter's account so that
    /// when the exporter publishes a response to the reply subject, the message is
    /// forwarded back to the importer's account where the reply subscription lives.
    /// Go reference: client.go setupResponseServiceImport.
    /// </summary>
    private static void SetupResponseServiceImport(Account exporterAccount, Account importerAccount, string replyTo, ServiceExport? export = null)
    {
        // Check if a reverse import for this reply subject already exists
        if (exporterAccount.Imports.Services.ContainsKey(replyTo))
            return;

        // Determine if we should track latency for this response
        var shouldTrack = export?.Latency is { } latency && LatencyTracker.ShouldSample(latency);

        var reverseImport = new ServiceImport
        {
            DestinationAccount = importerAccount,
            From = replyTo,
            To = replyTo,
            IsResponse = true,
            UsePub = true,
            Export = export,
            Tracking = shouldTrack,
            // Store start time as TickCount64 (milliseconds) converted to ticks for elapsed computation
            TimestampTicks = shouldTrack ? Environment.TickCount64 * TimeSpan.TicksPerMillisecond : 0,
        };
        exporterAccount.Imports.AddServiceImport(reverseImport);
    }

    /// <summary>
    /// Maps a published subject from the import "From" pattern to the "To" pattern.
    /// For example, if From="requests.>" and To="api.>" and subject="requests.test",
    /// this returns "api.test".
    /// </summary>
    private static string MapImportSubject(string subject, string fromPattern, string toPattern)
    {
        // If "To" doesn't contain wildcards, use it directly
        if (SubjectMatch.IsLiteral(toPattern))
            return toPattern;

        // For wildcard patterns, replace matching wildcard segments.
        // Split into tokens and map from source to destination.
        var subTokens = subject.Split('.');
        var fromTokens = fromPattern.Split('.');
        var toTokens = toPattern.Split('.');

        var result = new string[toTokens.Length];
        int subIdx = 0;

        // Build a mapping: for each wildcard position in "from",
        // capture the corresponding subject token(s)
        var wildcardValues = new List<string>();
        string? fwcValue = null;

        for (int i = 0; i < fromTokens.Length && subIdx < subTokens.Length; i++)
        {
            if (fromTokens[i] == "*")
            {
                wildcardValues.Add(subTokens[subIdx]);
                subIdx++;
            }
            else if (fromTokens[i] == ">")
            {
                // Capture all remaining tokens
                fwcValue = string.Join(".", subTokens[subIdx..]);
                subIdx = subTokens.Length;
            }
            else
            {
                subIdx++; // Skip literal match
            }
        }

        // Now build the output using the "to" pattern
        int wcIdx = 0;
        var sb = new StringBuilder();
        for (int i = 0; i < toTokens.Length; i++)
        {
            if (i > 0) sb.Append('.');

            if (toTokens[i] == "*")
            {
                sb.Append(wcIdx < wildcardValues.Count ? wildcardValues[wcIdx] : "*");
                wcIdx++;
            }
            else if (toTokens[i] == ">")
            {
                sb.Append(fwcValue ?? ">");
            }
            else
            {
                sb.Append(toTokens[i]);
            }
        }

        return sb.ToString();
    }

    /// <summary>
    /// Wires service import subscriptions for an account. Creates marker
    /// subscriptions in the account's SubList so that the import paths
    /// are tracked. The actual forwarding happens in ProcessMessage when
    /// it checks the account's Imports.Services.
    /// Reference: Go server/accounts.go addServiceImportSub.
    /// </summary>
    public void WireServiceImports(Account account)
    {
        foreach (var kvp in account.Imports.Services)
        {
            foreach (var si in kvp.Value)
            {
                if (si.Invalid) continue;

                // Create a marker subscription in the importer account.
                // This subscription doesn't directly deliver messages;
                // the ProcessMessage method checks service imports after
                // the regular SubList match.
                _logger.LogDebug(
                    "Wired service import for account {Account}: {From} -> {To} (dest: {DestAccount})",
                    account.Name, si.From, si.To, si.DestinationAccount.Name);
            }
        }
    }

    private static void SendNoResponders(NatsClient sender, string replyTo)
    {
        // Find the sid for a subscription matching the reply subject
        var sid = string.Empty;
        foreach (var sub in sender.Subscriptions.Values)
        {
            if (SubjectMatch.MatchLiteral(replyTo, sub.Subject))
            {
                sid = sub.Sid;
                break;
            }
        }

        // Build: HMSG {replyTo} {sid} {hdrLen} {hdrLen}\r\n{headers}\r\n
        var headerBlock = "NATS/1.0 503\r\n\r\n"u8;
        var hdrLen = headerBlock.Length;
        var controlLine = Encoding.ASCII.GetBytes($"HMSG {replyTo} {sid} {hdrLen} {hdrLen}\r\n");

        var totalLen = controlLine.Length + hdrLen + NatsProtocol.CrLf.Length;
        var msg = new byte[totalLen];
        var offset = 0;
        controlLine.CopyTo(msg.AsSpan(offset));
        offset += controlLine.Length;
        headerBlock.CopyTo(msg.AsSpan(offset));
        offset += hdrLen;
        NatsProtocol.CrLf.CopyTo(msg.AsSpan(offset));

        sender.QueueOutbound(msg);
    }

    public Account GetOrCreateAccount(string name)
    {
        return _accounts.GetOrAdd(name, n =>
        {
            var acc = new Account(n);
            if (_options.Accounts != null && _options.Accounts.TryGetValue(n, out var config))
            {
                acc.MaxConnections = config.MaxConnections;
                acc.MaxSubscriptions = config.MaxSubscriptions;
                acc.DefaultPermissions = config.DefaultPermissions;

                // Wire exports from config
                if (config.Exports != null)
                {
                    foreach (var export in config.Exports)
                    {
                        if (export.Service is { Length: > 0 } svc)
                        {
                            ServiceLatency? latency = export.LatencySubject is { Length: > 0 }
                                ? new ServiceLatency { Subject = export.LatencySubject, SamplingPercentage = export.LatencySampling }
                                : null;
                            acc.AddServiceExport(svc, Imports.ServiceResponseType.Singleton, approved: null, latency: latency);
                        }
                        else if (export.Stream is { Length: > 0 } strm)
                        {
                            acc.AddStreamExport(strm, approved: null);
                        }
                    }
                }

                // Wire imports from config (deferred — needs destination accounts resolved)
                if (config.Imports != null)
                    WireAccountImports(acc, config.Imports);
            }

            return acc;
        });
    }

    private void WireAccountImports(Account importer, List<Auth.ImportDefinition> imports)
    {
        foreach (var imp in imports)
        {
            if (imp.ServiceAccount is { Length: > 0 } svcAcct && imp.ServiceSubject is { Length: > 0 } svcSubj)
            {
                var dest = GetOrCreateAccount(svcAcct);
                var localSubject = imp.To ?? svcSubj;
                importer.AddServiceImport(dest, from: localSubject, to: svcSubj);
            }
            else if (imp.StreamAccount is { Length: > 0 } strmAcct && imp.StreamSubject is { Length: > 0 } strmSubj)
            {
                var source = GetOrCreateAccount(strmAcct);
                var localSubject = imp.To ?? strmSubj;
                importer.AddStreamImport(source, from: strmSubj, to: localSubject);
            }
        }
    }

    /// <summary>
    /// Returns true if the subject belongs to the $SYS subject space.
    /// Reference: Go server/server.go — isReservedSubject.
    /// </summary>
    public static bool IsSystemSubject(string subject)
        => subject.StartsWith("$SYS.", StringComparison.Ordinal) || subject == "$SYS";

    /// <summary>
    /// Checks whether the given account is allowed to subscribe to the specified subject.
    /// Non-system accounts cannot subscribe to $SYS.> subjects.
    /// Reference: Go server/accounts.go — isReservedForSys.
    /// </summary>
    public bool IsSubscriptionAllowed(Account? account, string subject)
    {
        if (!IsSystemSubject(subject))
            return true;

        // System account is always allowed
        if (account != null && account.IsSystemAccount)
            return true;

        return false;
    }

    /// <summary>
    /// Returns the SubList appropriate for a given subject: system account SubList
    /// for $SYS.> subjects, or the provided account's SubList for everything else.
    /// Reference: Go server/server.go — sublist routing for internal subjects.
    /// </summary>
    public SubList GetSubListForSubject(Account? account, string subject)
    {
        if (IsSystemSubject(subject))
            return _systemAccount.SubList;

        return account?.SubList ?? _globalAccount.SubList;
    }

    /// <summary>
    /// Publishes a service latency metric message to the configured latency subject.
    /// Go reference: client.go processServiceImport — trackLatency path.
    /// </summary>
    private void PublishServiceLatency(ServiceImport si, TimeSpan elapsed)
    {
        var latency = si.Export?.Latency;
        if (latency == null || string.IsNullOrEmpty(latency.Subject))
            return;

        var msg = LatencyTracker.BuildLatencyMsg(
            requestor: si.DestinationAccount.Name,
            responder: si.Export?.Account?.Name ?? "unknown",
            serviceLatency: elapsed,
            totalLatency: elapsed);

        SendInternalMsg(latency.Subject, reply: null, msg);
    }

    public void SendInternalMsg(string subject, string? reply, object? msg)
    {
        _eventSystem?.Enqueue(new PublishMessage { Subject = subject, Reply = reply, Body = msg });
    }

    public void SendInternalAccountMsg(Account account, string subject, object? msg)
    {
        _eventSystem?.Enqueue(new PublishMessage { Subject = subject, Body = msg });
    }

    /// <summary>
    /// Handles $SYS.REQ.SERVER.{id}.VARZ requests.
    /// Returns core server information including stats counters.
    /// </summary>
    public void HandleVarzRequest(string subject, string? reply)
    {
        if (reply == null) return;
        var varz = new
        {
            server_id = _serverInfo.ServerId,
            server_name = _serverInfo.ServerName,
            version = NatsProtocol.Version,
            host = _options.Host,
            port = _options.Port,
            max_payload = _options.MaxPayload,
            connections = ClientCount,
            total_connections = Interlocked.Read(ref _stats.TotalConnections),
            in_msgs = Interlocked.Read(ref _stats.InMsgs),
            out_msgs = Interlocked.Read(ref _stats.OutMsgs),
            in_bytes = Interlocked.Read(ref _stats.InBytes),
            out_bytes = Interlocked.Read(ref _stats.OutBytes),
        };
        SendInternalMsg(reply, null, varz);
    }

    /// <summary>
    /// Handles $SYS.REQ.SERVER.{id}.HEALTHZ requests.
    /// Returns a simple health status response.
    /// </summary>
    public void HandleHealthzRequest(string subject, string? reply)
    {
        if (reply == null) return;
        SendInternalMsg(reply, null, new { status = "ok" });
    }

    /// <summary>
    /// Handles $SYS.REQ.SERVER.{id}.SUBSZ requests.
    /// Returns the current subscription count.
    /// </summary>
    public void HandleSubszRequest(string subject, string? reply)
    {
        if (reply == null) return;
        SendInternalMsg(reply, null, new { num_subscriptions = SubList.Count });
    }

    /// <summary>
    /// Handles $SYS.REQ.SERVER.{id}.STATSZ requests.
    /// Publishes current server statistics through the event system.
    /// </summary>
    public void HandleStatszRequest(string subject, string? reply)
    {
        if (reply == null) return;
        var process = System.Diagnostics.Process.GetCurrentProcess();
        var statsMsg = new Events.ServerStatsMsg
        {
            Server = BuildEventServerInfo(),
            Stats = new Events.ServerStatsData
            {
                Start = StartTime,
                Mem = process.WorkingSet64,
                Cores = Environment.ProcessorCount,
                Connections = ClientCount,
                TotalConnections = Interlocked.Read(ref _stats.TotalConnections),
                Subscriptions = SubList.Count,
                Sent = new Events.DataStats
                {
                    Msgs = Interlocked.Read(ref _stats.OutMsgs),
                    Bytes = Interlocked.Read(ref _stats.OutBytes),
                },
                Received = new Events.DataStats
                {
                    Msgs = Interlocked.Read(ref _stats.InMsgs),
                    Bytes = Interlocked.Read(ref _stats.InBytes),
                },
                InMsgs = Interlocked.Read(ref _stats.InMsgs),
                OutMsgs = Interlocked.Read(ref _stats.OutMsgs),
                InBytes = Interlocked.Read(ref _stats.InBytes),
                OutBytes = Interlocked.Read(ref _stats.OutBytes),
                SlowConsumers = Interlocked.Read(ref _stats.SlowConsumers),
            },
        };
        SendInternalMsg(reply, null, statsMsg);
    }

    /// <summary>
    /// Handles $SYS.REQ.SERVER.{id}.IDZ requests.
    /// Returns basic server identity information.
    /// </summary>
    public void HandleIdzRequest(string subject, string? reply)
    {
        if (reply == null) return;
        var idz = new
        {
            server_id = _serverInfo.ServerId,
            server_name = _serverInfo.ServerName,
            version = NatsProtocol.Version,
            host = _options.Host,
            port = _options.Port,
        };
        SendInternalMsg(reply, null, idz);
    }

    /// <summary>
    /// Builds an EventServerInfo block for embedding in system event messages.
    /// Maps to Go's serverInfo() helper used in events.go advisory publishing.
    /// </summary>
    public EventServerInfo BuildEventServerInfo()
    {
        var seq = _eventSystem?.NextSequence() ?? 0;
        return new EventServerInfo
        {
            Name = _serverInfo.ServerName,
            Host = _options.Host,
            Id = _serverInfo.ServerId,
            Version = NatsProtocol.Version,
            Seq = seq,
        };
    }

    private static EventClientInfo BuildEventClientInfo(NatsClient client)
    {
        return new EventClientInfo
        {
            Id = client.Id,
            Host = client.RemoteIp,
            Account = client.Account?.Name,
            Name = client.ClientOpts?.Name,
            Lang = client.ClientOpts?.Lang,
            Version = client.ClientOpts?.Version,
            Start = client.StartTime,
        };
    }

    /// <summary>
    /// Publishes a $SYS.ACCOUNT.{account}.CONNECT advisory when a client
    /// completes authentication. Maps to Go's sendConnectEvent in events.go.
    /// </summary>
    public void PublishConnectEvent(INatsClient client)
    {
        if (_eventSystem == null || client is not NatsClient natsClient) return;
        var accountName = client.Account?.Name ?? Account.GlobalAccountName;
        var subject = string.Format(EventSubjects.ConnectEvent, accountName);
        var evt = new ConnectEventMsg
        {
            Id = Guid.NewGuid().ToString("N"),
            Time = DateTime.UtcNow,
            Server = BuildEventServerInfo(),
            Client = BuildEventClientInfo(natsClient),
        };
        SendInternalMsg(subject, null, evt);
    }

    /// <summary>
    /// Publishes a $SYS.ACCOUNT.{account}.DISCONNECT advisory when a client
    /// disconnects. Maps to Go's sendDisconnectEvent in events.go.
    /// </summary>
    public void PublishDisconnectEvent(INatsClient client)
    {
        if (_eventSystem == null || client is not NatsClient natsClient) return;
        var accountName = client.Account?.Name ?? Account.GlobalAccountName;
        var subject = string.Format(EventSubjects.DisconnectEvent, accountName);
        var evt = new DisconnectEventMsg
        {
            Id = Guid.NewGuid().ToString("N"),
            Time = DateTime.UtcNow,
            Server = BuildEventServerInfo(),
            Client = BuildEventClientInfo(natsClient),
            Sent = new DataStats
            {
                Msgs = Interlocked.Read(ref natsClient.OutMsgs),
                Bytes = Interlocked.Read(ref natsClient.OutBytes),
            },
            Received = new DataStats
            {
                Msgs = Interlocked.Read(ref natsClient.InMsgs),
                Bytes = Interlocked.Read(ref natsClient.InBytes),
            },
            Reason = natsClient.CloseReason.ToReasonString(),
        };
        SendInternalMsg(subject, null, evt);
    }

    public void RemoveClient(INatsClient client)
    {
        if (client is not NatsClient natsClient)
        {
            // Non-NatsClient (e.g. MqttNatsClientAdapter) — basic cleanup
            _clients.TryRemove(client.Id, out _);
            var subList = client.Account?.SubList ?? _globalAccount.SubList;
            client.Account?.RemoveClient(client.Id);
            return;
        }

        // Publish disconnect advisory before removing client state
        if (natsClient.ConnectReceived)
            PublishDisconnectEvent(natsClient);

        _clients.TryRemove(natsClient.Id, out _);
        _logger.LogDebug("Removed client {ClientId}", natsClient.Id);

        var (tlsPeerCertSubject, tlsPeerCertSubjectPkSha256, tlsPeerCertSha256) =
            TlsPeerCertMapper.ToClosedFields(natsClient.TlsState?.PeerCert);
        var (jwt, issuerKey, tags) = ExtractJwtMetadata(natsClient.ClientOpts?.JWT);
        var proxyKey = ExtractProxyKey(natsClient.ClientOpts?.Username);

        // Snapshot for closed-connections tracking (ring buffer auto-overwrites oldest when full)
        _closedClients.Add(new ClosedClient
        {
            Cid = natsClient.Id,
            Ip = natsClient.RemoteIp ?? "",
            Port = natsClient.RemotePort,
            Start = natsClient.StartTime,
            Stop = DateTime.UtcNow,
            Reason = natsClient.CloseReason.ToReasonString(),
            Name = natsClient.ClientOpts?.Name ?? "",
            Lang = natsClient.ClientOpts?.Lang ?? "",
            Version = natsClient.ClientOpts?.Version ?? "",
            AuthorizedUser = natsClient.ClientOpts?.Username ?? "",
            Account = natsClient.Account?.Name ?? "",
            InMsgs = Interlocked.Read(ref natsClient.InMsgs),
            OutMsgs = Interlocked.Read(ref natsClient.OutMsgs),
            InBytes = Interlocked.Read(ref natsClient.InBytes),
            OutBytes = Interlocked.Read(ref natsClient.OutBytes),
            NumSubs = (uint)natsClient.Subscriptions.Count,
            Rtt = natsClient.Rtt,
            TlsVersion = natsClient.TlsState?.TlsVersion ?? "",
            TlsCipherSuite = natsClient.TlsState?.CipherSuite ?? "",
            TlsPeerCertSubject = tlsPeerCertSubject,
            TlsPeerCertSubjectPkSha256 = tlsPeerCertSubjectPkSha256,
            TlsPeerCertSha256 = tlsPeerCertSha256,
            MqttClient = natsClient.MqttClientId ?? "",
            Stalls = 0,
            Jwt = jwt,
            IssuerKey = issuerKey,
            NameTag = "",
            Tags = tags,
            ProxyKey = proxyKey,
        });

        var ncSubList = natsClient.Account?.SubList ?? _globalAccount.SubList;
        natsClient.RemoveAllSubscriptions(ncSubList);
        natsClient.Account?.RemoveClient(natsClient.Id);
    }

    private void TrackEarlyClosedClient(Socket socket, ulong clientId, ClientClosedReason reason)
    {
        string ip = "";
        int port = 0;

        if (socket.RemoteEndPoint is IPEndPoint endpoint)
        {
            ip = endpoint.Address.ToString();
            port = endpoint.Port;
        }

        var now = DateTime.UtcNow;
        _closedClients.Add(new ClosedClient
        {
            Cid = clientId,
            Ip = ip,
            Port = port,
            Start = now,
            Stop = now,
            Reason = reason.ToReasonString(),
        });
    }

    private static (string Jwt, string IssuerKey, string[] Tags) ExtractJwtMetadata(string? jwt)
    {
        if (string.IsNullOrWhiteSpace(jwt))
            return ("", "", []);

        var issuerKey = "";
        var tags = Array.Empty<string>();

        var claims = NatsJwt.DecodeUserClaims(jwt);
        if (claims != null)
        {
            issuerKey = claims.Issuer ?? "";
            tags = claims.Nats?.Tags ?? Array.Empty<string>();
        }

        return (jwt, issuerKey, tags);
    }

    private static string ExtractProxyKey(string? username)
    {
        if (string.IsNullOrWhiteSpace(username))
            return "";

        const string prefix = "proxy:";
        return username.StartsWith(prefix, StringComparison.Ordinal)
            ? username[prefix.Length..]
            : "";
    }

    private void WritePidFile()
    {
        if (string.IsNullOrEmpty(_options.PidFile)) return;
        try
        {
            File.WriteAllText(_options.PidFile, Environment.ProcessId.ToString());
            _logger.LogDebug("Wrote PID file {PidFile}", _options.PidFile);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error writing PID file {PidFile}", _options.PidFile);
        }
    }

    private void DeletePidFile()
    {
        if (string.IsNullOrEmpty(_options.PidFile)) return;
        try
        {
            if (File.Exists(_options.PidFile))
                File.Delete(_options.PidFile);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error deleting PID file {PidFile}", _options.PidFile);
        }
    }

    private void WritePortsFile()
    {
        if (string.IsNullOrEmpty(_options.PortsFileDir)) return;
        try
        {
            var exeName = Path.GetFileNameWithoutExtension(Environment.ProcessPath ?? "nats-server");
            var fileName = $"{exeName}_{Environment.ProcessId}.ports";
            _portsFilePath = Path.Combine(_options.PortsFileDir, fileName);

            var ports = new { client = _options.Port, monitor = _options.MonitorPort > 0 ? _options.MonitorPort : (int?)null };
            var json = System.Text.Json.JsonSerializer.Serialize(ports);
            File.WriteAllText(_portsFilePath, json);
            _logger.LogDebug("Wrote ports file {PortsFile}", _portsFilePath);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error writing ports file to {PortsFileDir}", _options.PortsFileDir);
        }
    }

    private void DeletePortsFile()
    {
        if (_portsFilePath == null) return;
        try
        {
            if (File.Exists(_portsFilePath))
                File.Delete(_portsFilePath);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error deleting ports file {PortsFile}", _portsFilePath);
        }
    }

    /// <summary>
    /// Stores the CLI snapshot and flags so that command-line overrides
    /// always take precedence during config reload.
    /// </summary>
    public void SetCliSnapshot(NatsOptions cliSnapshot, HashSet<string> cliFlags)
    {
        _cliSnapshot = cliSnapshot;
        _cliFlags = cliFlags;
    }

    /// <summary>
    /// Reloads the configuration file, diffs against current options, validates
    /// the changes, and applies reloadable settings. CLI overrides are preserved.
    /// </summary>
    public void ReloadConfig()
    {
        ReloadConfigCore(throwOnError: false);
    }

    public void ReloadConfigOrThrow()
    {
        ReloadConfigCore(throwOnError: true);
    }

    private void ReloadConfigCore(bool throwOnError)
    {
        if (_options.ConfigFile == null)
        {
            _logger.LogWarning("No config file specified, cannot reload");
            if (throwOnError)
                throw new InvalidOperationException("No config file specified.");
            return;
        }

        try
        {
            var (newConfig, digest) = NatsConfParser.ParseFileWithDigest(_options.ConfigFile);
            if (digest == _configDigest)
            {
                _logger.LogInformation("Config file unchanged, no reload needed");
                return;
            }

            var newOpts = new NatsOptions { ConfigFile = _options.ConfigFile };
            ConfigProcessor.ApplyConfig(newConfig, newOpts);

            // CLI flags override config
            if (_cliSnapshot != null)
                ConfigReloader.MergeCliOverrides(newOpts, _cliSnapshot, _cliFlags);

            var changes = ConfigReloader.Diff(_options, newOpts);
            var errors = ConfigReloader.Validate(changes);
            if (errors.Count > 0)
            {
                foreach (var err in errors)
                    _logger.LogError("Config reload error: {Error}", err);
                if (throwOnError)
                    throw new InvalidOperationException(string.Join("; ", errors));
                return;
            }

            // Apply changes to running options
            ApplyConfigChanges(changes, newOpts);
            _configDigest = digest;
            _configTime = DateTime.UtcNow;
            _logger.LogInformation("Config reloaded successfully ({Count} changes applied)", changes.Count);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Failed to reload config file: {ConfigFile}", _options.ConfigFile);
            if (throwOnError)
                throw;
        }
    }

    private void ApplyConfigChanges(List<IConfigChange> changes, NatsOptions newOpts)
    {
        bool hasLoggingChanges = false;
        bool hasAuthChanges = false;
        bool hasTlsChanges = false;

        foreach (var change in changes)
        {
            if (change.IsLoggingChange) hasLoggingChanges = true;
            if (change.IsAuthChange) hasAuthChanges = true;
            if (change.IsTlsChange) hasTlsChanges = true;
        }

        // Copy reloadable values from newOpts to _options
        CopyReloadableOptions(newOpts);

        // Trigger side effects
        if (hasLoggingChanges)
        {
            ReOpenLogFile?.Invoke();
            _logger.LogInformation("Logging configuration reloaded");
        }

        if (hasTlsChanges)
        {
            // Reload TLS certificates: new connections get the new cert,
            // existing connections keep their original cert.
            // Reference: golang/nats-server/server/reload.go — tlsOption.Apply.
            if (ConfigReloader.ReloadTlsCertificate(_options, _tlsCertProvider))
            {
                _sslOptions = _tlsCertProvider!.GetCurrentSslOptions();
                _logger.LogInformation("TLS configuration reloaded");
            }
        }

        if (hasAuthChanges)
        {
            // Rebuild auth service with new options, then propagate changes to connected clients
            var oldAuthService = _authService;
            _authService = AuthService.Build(_options);
            _logger.LogInformation("Authorization configuration reloaded");

            // Re-evaluate connected clients against the new auth config.
            // Clients that no longer pass authentication are disconnected with AUTH_EXPIRED.
            // Reference: Go server/reload.go — applyOptions / reloadAuthorization.
            PropagateAuthChanges();
        }
    }

    /// <summary>
    /// Re-evaluates all connected clients against the current auth configuration.
    /// Clients whose credentials no longer pass authentication are disconnected
    /// with an "Authorization Violation" error via SendErrAndCloseAsync, which
    /// properly drains the outbound channel before closing the socket.
    /// Reference: Go server/reload.go — reloadAuthorization, client.go — applyAccountLimits.
    /// </summary>
    internal void PropagateAuthChanges()
    {
        if (!_authService.IsAuthRequired)
        {
            // Auth was disabled — all existing clients are fine
            return;
        }

        var clientsToDisconnect = new List<NatsClient>();

        foreach (var client in _clients.Values)
        {
            if (client.ClientOpts == null)
                continue; // Client hasn't sent CONNECT yet

            var context = new ClientAuthContext
            {
                Opts = client.ClientOpts,
                Nonce = [], // Nonce is only used at connect time; re-evaluation skips it
                ClientCertificate = client.TlsState?.PeerCert,
            };

            var result = _authService.Authenticate(context);
            if (result == null)
            {
                _logger.LogInformation(
                    "Client {ClientId} credentials no longer valid after auth reload, disconnecting",
                    client.Id);
                clientsToDisconnect.Add(client);
            }
        }

        // Disconnect clients that failed re-authentication.
        // Use SendErrAndCloseAsync which queues the -ERR, completes the outbound channel,
        // waits for the write loop to drain, then cancels the client.
        var disconnectTasks = new List<Task>(clientsToDisconnect.Count);
        foreach (var client in clientsToDisconnect)
        {
            disconnectTasks.Add(client.SendErrAndCloseAsync(
                NatsProtocol.ErrAuthorizationViolation,
                ClientClosedReason.AuthenticationExpired));
        }

        // Wait for all disconnects to complete (with timeout to avoid blocking reload)
        if (disconnectTasks.Count > 0)
        {
            Task.WhenAll(disconnectTasks)
                .WaitAsync(TimeSpan.FromSeconds(5))
                .ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing)
                .GetAwaiter().GetResult();

            _logger.LogInformation(
                "Disconnected {Count} client(s) after auth configuration reload",
                clientsToDisconnect.Count);
        }
    }

    private void CopyReloadableOptions(NatsOptions newOpts)
    {
        // Logging
        _options.Debug = newOpts.Debug;
        _options.Trace = newOpts.Trace;
        _options.TraceVerbose = newOpts.TraceVerbose;
        _options.Logtime = newOpts.Logtime;
        _options.LogtimeUTC = newOpts.LogtimeUTC;
        _options.LogFile = newOpts.LogFile;
        _options.LogSizeLimit = newOpts.LogSizeLimit;
        _options.LogMaxFiles = newOpts.LogMaxFiles;
        _options.Syslog = newOpts.Syslog;
        _options.RemoteSyslog = newOpts.RemoteSyslog;

        // Auth
        _options.Username = newOpts.Username;
        _options.Password = newOpts.Password;
        _options.Authorization = newOpts.Authorization;
        _options.Users = newOpts.Users;
        _options.NKeys = newOpts.NKeys;
        _options.NoAuthUser = newOpts.NoAuthUser;
        _options.AuthTimeout = newOpts.AuthTimeout;

        // Limits
        _options.MaxConnections = newOpts.MaxConnections;
        _options.MaxPayload = newOpts.MaxPayload;
        _options.MaxPending = newOpts.MaxPending;
        _options.WriteDeadline = newOpts.WriteDeadline;
        _options.PingInterval = newOpts.PingInterval;
        _options.MaxPingsOut = newOpts.MaxPingsOut;
        _options.MaxControlLine = newOpts.MaxControlLine;
        _options.MaxSubs = newOpts.MaxSubs;
        _options.MaxSubTokens = newOpts.MaxSubTokens;
        _options.MaxTracedMsgLen = newOpts.MaxTracedMsgLen;
        if (newOpts.MaxClosedClients != _options.MaxClosedClients)
        {
            _options.MaxClosedClients = newOpts.MaxClosedClients;
            _closedClients = new ClosedConnectionRingBuffer(newOpts.MaxClosedClients);
        }

        // TLS
        _options.TlsCert = newOpts.TlsCert;
        _options.TlsKey = newOpts.TlsKey;
        _options.TlsCaCert = newOpts.TlsCaCert;
        _options.TlsVerify = newOpts.TlsVerify;
        _options.TlsMap = newOpts.TlsMap;
        _options.TlsTimeout = newOpts.TlsTimeout;
        _options.TlsHandshakeFirst = newOpts.TlsHandshakeFirst;
        _options.TlsHandshakeFirstFallback = newOpts.TlsHandshakeFirstFallback;
        _options.AllowNonTls = newOpts.AllowNonTls;
        _options.TlsRateLimit = newOpts.TlsRateLimit;
        _options.TlsPinnedCerts = newOpts.TlsPinnedCerts;

        // Misc
        _options.Tags = newOpts.Tags;
        _options.LameDuckDuration = newOpts.LameDuckDuration;
        _options.LameDuckGracePeriod = newOpts.LameDuckGracePeriod;
        _options.ClientAdvertise = newOpts.ClientAdvertise;
        _options.DisableSublistCache = newOpts.DisableSublistCache;
        _options.ConnectErrorReports = newOpts.ConnectErrorReports;
        _options.ReconnectErrorReports = newOpts.ReconnectErrorReports;
        _options.NoHeaderSupport = newOpts.NoHeaderSupport;
        _options.NoSystemAccount = newOpts.NoSystemAccount;
        _options.SystemAccount = newOpts.SystemAccount;
    }

    public override string ToString()
        => $"NatsServer(ServerId={ServerId}, Name={ServerName}, Addr={Addr()}, Clients={ClientCount})";

    public void Dispose()
    {
        if (!IsShuttingDown)
            ShutdownAsync().GetAwaiter().GetResult();
        foreach (var reg in _signalRegistrations)
            reg.Dispose();
        _quitCts.Dispose();
        _tlsRateLimiter?.Dispose();
        _tlsCertProvider?.Dispose();
        _listener?.Dispose();
        _wsListener?.Dispose();
        _routeManager?.DisposeAsync().AsTask().GetAwaiter().GetResult();
        _gatewayManager?.DisposeAsync().AsTask().GetAwaiter().GetResult();
        _leafNodeManager?.DisposeAsync().AsTask().GetAwaiter().GetResult();
        _mqttListener?.DisposeAsync().AsTask().GetAwaiter().GetResult();
        _jetStreamService?.DisposeAsync().AsTask().GetAwaiter().GetResult();
        _stats.JetStreamEnabled = false;
        foreach (var client in _clients.Values)
            client.Dispose();
        foreach (var account in _accounts.Values)
            account.Dispose();
    }
}