using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.Abstractions;
using Opc.Ua;
using Opc.Ua.Client;
using Opc.Ua.Configuration;
using ZB.MOM.WW.OtOpcUa.Core.Abstractions;

namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;

/// <summary>
///     OPC UA Client (gateway) driver. Opens a <see cref="Session"/> against a remote OPC UA
///     server and re-exposes its address space through the local OtOpcUa server. PR 66 ships
///     the scaffold: <see cref="IDriver"/> only (connect / close / health). Browse, read,
///     write, subscribe, and probe land in PRs 67-69.
/// </summary>
/// <remarks>
///     <para>
///         Builds its own <see cref="ApplicationConfiguration"/> rather than reusing
///         <c>Client.Shared</c> — Client.Shared is oriented at the interactive CLI; this
///         driver is an always-on service component with different session-lifetime needs
///         (keep-alive monitor, session transfer on reconnect, multi-year uptime).
///     </para>
///     <para>
///         <b>Session lifetime</b>: a single <see cref="Session"/> per driver instance.
///         Subscriptions multiplex onto that session; SDK reconnect handler takes the session
///         down and brings it back up on remote-server restart — the driver must re-send
///         subscriptions + TransferSubscriptions on reconnect to avoid dangling
///         monitored-item handles. That mechanic lands in PR 69.
///     </para>
/// </remarks>
public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IAlarmSource, IHistoryProvider, IDisposable, IAsyncDisposable
{
    private readonly ILogger<OpcUaClientDriver> _logger;

    /// <param name="options">Driver configuration.</param>
    /// <param name="driverInstanceId">Stable logical ID from the config DB.</param>
    /// <param name="logger">Optional logger; defaults to NullLogger when not supplied.</param>
    public OpcUaClientDriver(OpcUaClientDriverOptions options, string driverInstanceId,
        ILogger<OpcUaClientDriver>? logger = null)
    {
        _options = options;
        _driverInstanceId = driverInstanceId;
        _logger = logger ?? NullLogger<OpcUaClientDriver>.Instance;
    }

    private readonly OpcUaClientDriverOptions _options;
    private readonly string _driverInstanceId;
    // ---- IAlarmSource state ----

    private readonly System.Collections.Concurrent.ConcurrentDictionary<long, RemoteAlarmSubscription> _alarmSubscriptions = new();
    private long _nextAlarmSubscriptionId;

    /// <summary>Occurs when an alarm event is received from the remote OPC UA server.</summary>
    public event EventHandler<AlarmEventArgs>? OnAlarmEvent;

    // ---- ISubscribable + IHostConnectivityProbe state ----

    private readonly System.Collections.Concurrent.ConcurrentDictionary<long, RemoteSubscription> _subscriptions = new();
    private long _nextSubscriptionId;
    private readonly object _probeLock = new();
    private HostState _hostState = HostState.Unknown;
    private DateTime _hostStateChangedUtc = DateTime.UtcNow;
    private KeepAliveEventHandler? _keepAliveHandler;

    /// <summary>Occurs when a monitored item's data value changes on the remote OPC UA server.</summary>
    public event EventHandler<DataChangeEventArgs>? OnDataChange;
    /// <summary>Occurs when the host connectivity status of the remote OPC UA server changes.</summary>
    public event EventHandler<HostStatusChangedEventArgs>? OnHostStatusChanged;

    // OPC UA StatusCode constants the driver surfaces for local-side faults. Upstream-server
    // StatusCodes are passed through verbatim per driver-specs.md §8 "cascading quality" —
    // downstream clients need to distinguish 'remote source down' from 'local driver failure'.
    private const uint StatusBadNodeIdInvalid = 0x80330000u;
    private const uint StatusBadInternalError = 0x80020000u;
    private const uint StatusBadCommunicationError = 0x80050000u;

    private readonly SemaphoreSlim _gate = new(1, 1);

    /// <summary>Active OPC UA session. Null until <see cref="InitializeAsync"/> returns cleanly.</summary>
    internal ISession? Session { get; private set; }

    /// <summary>Per-connection gate. PRs 67+ serialize read/write/browse on this.</summary>
    internal SemaphoreSlim Gate => _gate;

    private DriverHealth _health = new(DriverState.Unknown, null, null);
    private bool _disposed;
    /// <summary>URL of the endpoint the driver actually connected to. Exposed via <see cref="HostName"/>.</summary>
    private string? _connectedEndpointUrl;
    /// <summary>
    ///     Cert-validation delegate wired when <see cref="OpcUaClientDriverOptions.AutoAcceptCertificates"/>
    ///     is <c>true</c>. Stored so <see cref="Dispose"/> / <see cref="DisposeAsync"/> can
    ///     detach it from the (potentially process-shared) <see cref="CertificateValidator"/>
    ///     and avoid leaking the closure (Driver.OpcUaClient-012).
    /// </summary>
    private CertificateValidationEventHandler? _certValidationHandler;
    /// <summary>The <see cref="CertificateValidator"/> that owns <see cref="_certValidationHandler"/>.</summary>
    private CertificateValidator? _certValidatorRef;
    /// <summary>
    ///     Approximate count of discovered nodes (folders + variables). Updated by
    ///     <see cref="DiscoverAsync"/> and used to report a non-zero
    ///     <see cref="GetMemoryFootprint"/> to the Core allocation-slope detector
    ///     (Driver.OpcUaClient-013).
    /// </summary>
    private volatile int _discoveredNodeCount;
    /// <summary>
    ///     SDK-provided reconnect handler that owns the retry loop + session-transfer machinery
    ///     when the session's keep-alive channel reports a bad status. Null outside the
    ///     reconnecting window; constructed lazily inside the keep-alive handler. Guarded by
    ///     <see cref="_probeLock"/> — keep-alive callbacks fire from the SDK timer thread and
    ///     can race a check-then-set if left unsynchronized (Driver.OpcUaClient-005).
    /// </summary>
    private SessionReconnectHandler? _reconnectHandler;

    /// <summary>
    ///     Bidirectional namespace map built at connect time from <c>session.NamespaceUris</c>.
    ///     Stored NodeIds embed the server-stable namespace <b>URI</b> rather than the
    ///     session-relative <c>ns=N</c> index, so a remote-server namespace-table reorder
    ///     across a restart does not silently re-point stored references at the wrong
    ///     namespace (driver-specs.md §8 "Namespace Remapping", finding Driver.OpcUaClient-004).
    ///     Null until <see cref="InitializeAsync"/> returns cleanly.
    /// </summary>
    private NamespaceMap? _namespaceMap;

    /// <summary>Gets the stable logical identifier for this driver instance from the config database.</summary>
    public string DriverInstanceId => _driverInstanceId;
    /// <summary>Gets the driver type identifier.</summary>
    public string DriverType => "OpcUaClient";

    /// <summary>Initializes the OPC UA client driver with the given configuration.</summary>
    /// <param name="driverConfigJson">JSON-serialized driver configuration.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
    {
        _health = new DriverHealth(DriverState.Initializing, null, null);
        try
        {
            // Enforce the Equipment-vs-SystemPlatform choice at startup per driver-specs.md
            // §8 "Namespace Assignment" — a misconfigured remote fails draft validation here,
            // not as a runtime surprise.
            ValidateNamespaceKind(_options);

            var appConfig = await BuildApplicationConfigurationAsync(cancellationToken).ConfigureAwait(false);
            var candidates = ResolveEndpointCandidates(_options);

            var identity = BuildUserIdentity(_options);

            // Failover sweep: try each endpoint in order, return the session from the first
            // one that successfully connects. Per-endpoint failures are captured so the final
            // aggregate exception names every URL that was tried and why — critical diag for
            // operators debugging 'why did the failover pick #3?'.
            var attemptErrors = new List<string>(candidates.Count);
            ISession? session = null;
            string? connectedUrl = null;
            foreach (var url in candidates)
            {
                try
                {
                    session = await OpenSessionOnEndpointAsync(
                        appConfig, url, _options.SecurityPolicy, _options.SecurityMode,
                        identity, cancellationToken).ConfigureAwait(false);
                    connectedUrl = url;
                    break;
                }
                catch (Exception ex)
                {
                    attemptErrors.Add($"{url} -> {ex.GetType().Name}: {ex.Message}");
                }
            }

            if (session is null)
                throw new AggregateException(
                    "OPC UA Client failed to connect to any of the configured endpoints. " +
                    "Tried:\n  " + string.Join("\n  ", attemptErrors),
                    attemptErrors.Select(e => new InvalidOperationException(e)));

            // Wire the session's keep-alive channel into HostState + the reconnect trigger.
            // OPC UA keep-alives are authoritative for session liveness: the SDK pings on
            // KeepAliveInterval and sets KeepAliveStopped when N intervals elapse without a
            // response. On a bad keep-alive the driver spins up a SessionReconnectHandler
            // which transparently retries + swaps the underlying session. Subscriptions move
            // via TransferSubscriptions so local MonitoredItem handles stay valid.
            _keepAliveHandler = OnKeepAlive;
            session.KeepAlive += _keepAliveHandler;

            // Build the bidirectional namespace map from the freshly negotiated session's
            // NamespaceUris (driver-specs.md §8 "Namespace Remapping"). Stored NodeIds carry
            // the namespace URI, not the session-relative ns=N index, so a remote namespace
            // reorder across a restart can't silently misaddress nodes.
            _namespaceMap = NamespaceMap.FromSession(session);

            Session = session;
            _connectedEndpointUrl = connectedUrl;
            _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
            TransitionTo(HostState.Running);
        }
        catch (Exception ex)
        {
            try { if (Session is Session s) await s.CloseAsync().ConfigureAwait(false); } catch { }
            Session = null;
            _health = new DriverHealth(DriverState.Faulted, null, ex.Message);
            throw;
        }
    }

    /// <summary>
    ///     Build a minimal in-memory <see cref="ApplicationConfiguration"/>. Certificates live
    ///     under the OS user profile — on Windows that's <c>%LocalAppData%\OtOpcUa\pki</c>
    ///     — so multiple driver instances in the same OtOpcUa server process share one
    ///     certificate store without extra config.
    /// </summary>
    private async Task<ApplicationConfiguration> BuildApplicationConfigurationAsync(CancellationToken ct)
    {
        // The default ctor is obsolete in favour of the ITelemetryContext overload; suppress
        // locally rather than plumbing a telemetry context all the way through the driver
        // surface — the driver emits no per-request telemetry of its own and the SDK's
        // internal fallback is fine for a gateway use case.
#pragma warning disable CS0618
        var app = new ApplicationInstance
        {
            ApplicationName = _options.SessionName,
            ApplicationType = ApplicationType.Client,
        };
#pragma warning restore CS0618

        var pkiRoot = Path.Combine(
            Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
            "OtOpcUa", "pki");

        var config = new ApplicationConfiguration
        {
            ApplicationName = _options.SessionName,
            ApplicationType = ApplicationType.Client,
            ApplicationUri = _options.ApplicationUri,
            SecurityConfiguration = new SecurityConfiguration
            {
                ApplicationCertificate = new CertificateIdentifier
                {
                    StoreType = CertificateStoreType.Directory,
                    StorePath = Path.Combine(pkiRoot, "own"),
                    SubjectName = $"CN={_options.SessionName}",
                },
                TrustedPeerCertificates = new CertificateTrustList
                {
                    StoreType = CertificateStoreType.Directory,
                    StorePath = Path.Combine(pkiRoot, "trusted"),
                },
                TrustedIssuerCertificates = new CertificateTrustList
                {
                    StoreType = CertificateStoreType.Directory,
                    StorePath = Path.Combine(pkiRoot, "issuers"),
                },
                RejectedCertificateStore = new CertificateTrustList
                {
                    StoreType = CertificateStoreType.Directory,
                    StorePath = Path.Combine(pkiRoot, "rejected"),
                },
                AutoAcceptUntrustedCertificates = _options.AutoAcceptCertificates,
            },
            TransportQuotas = new TransportQuotas { OperationTimeout = (int)_options.Timeout.TotalMilliseconds },
            ClientConfiguration = new ClientConfiguration
            {
                DefaultSessionTimeout = (int)_options.SessionTimeout.TotalMilliseconds,
            },
            DisableHiResClock = true,
        };

        await config.ValidateAsync(ApplicationType.Client, ct).ConfigureAwait(false);

        // AutoAccept=true is a dev-only escape hatch. Emit a prominent warning so a
        // production misconfiguration is immediately visible in logs (Driver.OpcUaClient-012).
        if (_options.AutoAcceptCertificates)
        {
            _logger.LogWarning(
                "OpcUaClientDriver '{DriverInstanceId}': AutoAcceptCertificates=true — all " +
                "remote server certificate errors are accepted, including expired / wrong-host " +
                "/ chain-incomplete. This MUST be false in production to prevent MITM attacks " +
                "against the opc.tcp channel.",
                _driverInstanceId);

            // Accept the full set of certificate-validation error codes: a real dev cert can
            // fail with BadCertificateChainIncomplete, BadCertificateTimeInvalid, or
            // BadCertificateHostNameInvalid, not only BadCertificateUntrusted. Only accepting
            // the latter would silently fail for those certs (Driver.OpcUaClient-012).
            CertificateValidationEventHandler handler = (_, e) => e.Accept = true;
            config.CertificateValidator.CertificateValidation += handler;
            // Store refs so ShutdownAsync + Dispose can detach the delegate and avoid
            // leaking a closure on a potentially process-shared validator.
            _certValidationHandler = handler;
            _certValidatorRef = config.CertificateValidator;
        }

        // Ensure an application certificate exists. The SDK auto-generates one if missing.
        app.ApplicationConfiguration = config;
        await app.CheckApplicationInstanceCertificatesAsync(silent: true, lifeTimeInMonths: null, ct)
            .ConfigureAwait(false);

        return config;
    }

    /// <summary>
    ///     Resolve the ordered failover candidate list. <c>EndpointUrls</c> wins when
    ///     non-empty; otherwise fall back to <c>EndpointUrl</c> as a single-URL shortcut so
    ///     existing single-endpoint configs keep working without migration.
    /// </summary>
    /// <param name="opts">Driver options containing endpoint configuration.</param>
    internal static IReadOnlyList<string> ResolveEndpointCandidates(OpcUaClientDriverOptions opts)
    {
        if (opts.EndpointUrls is { Count: > 0 }) return opts.EndpointUrls;
        return [opts.EndpointUrl];
    }

    /// <summary>
    ///     Enforce the §8 "Namespace Assignment" rule at startup. An <c>Equipment</c>-kind
    ///     instance gateways raw equipment data and therefore needs a config-driven UNS
    ///     mapping table (remote nodes don't conform to UNS); a <c>SystemPlatform</c>-kind
    ///     instance gateways processed data whose hierarchy is preserved verbatim, so a
    ///     UNS mapping table is meaningless and rejected. Throwing here surfaces the
    ///     misconfiguration as a draft-validation failure rather than a runtime surprise.
    /// </summary>
    /// <param name="opts">Driver options containing namespace configuration.</param>
    internal static void ValidateNamespaceKind(OpcUaClientDriverOptions opts)
    {
        switch (opts.TargetNamespaceKind)
        {
            case OpcUaTargetNamespaceKind.Equipment:
                if (opts.UnsMappingTable is null || opts.UnsMappingTable.Count == 0)
                    throw new InvalidOperationException(
                        "OpcUaClient driver configured with TargetNamespaceKind=Equipment but no " +
                        "UnsMappingTable: §8 requires a config-driven remote-to-UNS mapping table " +
                        "because remote nodes do not conform to UNS by default. Provide a mapping " +
                        "table or set TargetNamespaceKind=SystemPlatform if the remote exposes " +
                        "processed data.");
                break;
            case OpcUaTargetNamespaceKind.SystemPlatform:
                if (opts.UnsMappingTable is { Count: > 0 })
                    throw new InvalidOperationException(
                        "OpcUaClient driver configured with TargetNamespaceKind=SystemPlatform but " +
                        "a UnsMappingTable was supplied: processed data preserves its own hierarchy " +
                        "and a UNS mapping table is ambiguous here. Clear the mapping table or set " +
                        "TargetNamespaceKind=Equipment if the remote exposes raw equipment data.");
                break;
            default:
                throw new ArgumentOutOfRangeException(
                    nameof(opts), opts.TargetNamespaceKind, "Unknown TargetNamespaceKind.");
        }
    }

    /// <summary>
    ///     Build the user-identity token from the driver options. Split out of
    ///     <see cref="InitializeAsync"/> so the failover sweep reuses one identity across
    ///     every endpoint attempt — generating it N times would re-unlock the user cert's
    ///     private key N times, wasteful + keeps the password in memory longer.
    /// </summary>
    /// <param name="options">Driver options containing authentication configuration.</param>
    internal static UserIdentity BuildUserIdentity(OpcUaClientDriverOptions options) =>
        options.AuthType switch
        {
            OpcUaAuthType.Anonymous => new UserIdentity(new AnonymousIdentityToken()),
            OpcUaAuthType.Username => new UserIdentity(
                options.Username ?? string.Empty,
                System.Text.Encoding.UTF8.GetBytes(options.Password ?? string.Empty)),
            OpcUaAuthType.Certificate => BuildCertificateIdentity(options),
            _ => new UserIdentity(new AnonymousIdentityToken()),
        };

    /// <summary>
    ///     Open a session against a single endpoint URL. Bounded by
    ///     <see cref="OpcUaClientDriverOptions.PerEndpointConnectTimeout"/> so the failover
    ///     sweep doesn't spend its full budget on one dead server. Moved out of
    ///     <see cref="InitializeAsync"/> so the failover loop body stays readable.
    /// </summary>
    private async Task<ISession> OpenSessionOnEndpointAsync(
        ApplicationConfiguration appConfig,
        string endpointUrl,
        OpcUaSecurityPolicy policy,
        OpcUaSecurityMode mode,
        UserIdentity identity,
        CancellationToken ct)
    {
        using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
        cts.CancelAfter(_options.PerEndpointConnectTimeout);

        var selected = await SelectMatchingEndpointAsync(
            appConfig, endpointUrl, policy, mode, cts.Token).ConfigureAwait(false);
        var endpointConfig = EndpointConfiguration.Create(appConfig);
        endpointConfig.OperationTimeout = (int)_options.Timeout.TotalMilliseconds;
        var endpoint = new ConfiguredEndpoint(null, selected, endpointConfig);

        var session = await new DefaultSessionFactory(telemetry: null!).CreateAsync(
            appConfig,
            endpoint,
            false,                                              // updateBeforeConnect
            _options.SessionName,
            (uint)_options.SessionTimeout.TotalMilliseconds,
            identity,
            null,                                               // preferredLocales
            cts.Token).ConfigureAwait(false);

        session.KeepAliveInterval = (int)_options.KeepAliveInterval.TotalMilliseconds;
        return session;
    }

    /// <summary>
    ///     Select the remote endpoint matching both the requested <paramref name="policy"/>
    ///     and <paramref name="mode"/>. The SDK's <c>CoreClientUtils.SelectEndpointAsync</c>
    ///     only honours a boolean "use security" flag; we need policy-aware matching so an
    ///     operator asking for <c>Basic256Sha256</c> against a server that also offers
    ///     <c>Basic128Rsa15</c> doesn't silently end up on the weaker cipher.
    /// </summary>
    private static async Task<EndpointDescription> SelectMatchingEndpointAsync(
        ApplicationConfiguration appConfig,
        string endpointUrl,
        OpcUaSecurityPolicy policy,
        OpcUaSecurityMode mode,
        CancellationToken ct)
    {
        // GetEndpoints returns everything the server advertises; policy + mode filter is
        // applied client-side so the selection is explicit and fails loudly if the operator
        // asks for a combination the server doesn't publish. DiscoveryClient.CreateAsync
        // is the non-obsolete path in SDK 1.5.378; the synchronous Create(..) variants are
        // all deprecated.
        using var client = await DiscoveryClient.CreateAsync(
            appConfig, new Uri(endpointUrl), Opc.Ua.DiagnosticsMasks.None, ct).ConfigureAwait(false);
        var all = await client.GetEndpointsAsync(null, ct).ConfigureAwait(false);

        var wantedPolicyUri = MapSecurityPolicy(policy);
        var wantedMode = mode switch
        {
            OpcUaSecurityMode.None => MessageSecurityMode.None,
            OpcUaSecurityMode.Sign => MessageSecurityMode.Sign,
            OpcUaSecurityMode.SignAndEncrypt => MessageSecurityMode.SignAndEncrypt,
            _ => throw new ArgumentOutOfRangeException(nameof(mode)),
        };

        var match = all.FirstOrDefault(e =>
            e.SecurityPolicyUri == wantedPolicyUri && e.SecurityMode == wantedMode);

        if (match is null)
        {
            var advertised = string.Join(", ", all
                .Select(e => $"{ShortPolicyName(e.SecurityPolicyUri)}/{e.SecurityMode}"));
            throw new InvalidOperationException(
                $"No endpoint at '{endpointUrl}' matches SecurityPolicy={policy} + SecurityMode={mode}. " +
                $"Server advertises: {advertised}");
        }
        return match;
    }

    /// <summary>
    ///     Build a <see cref="UserIdentity"/> carrying a client user-authentication
    ///     certificate loaded from <see cref="OpcUaClientDriverOptions.UserCertificatePath"/>.
    ///     Used when the remote server's endpoint advertises Certificate-type user tokens.
    ///     Fails fast if the path is missing, the file doesn't exist, or the certificate
    ///     lacks a private key (the private key is required to sign the user-token
    ///     challenge during session activation).
    /// </summary>
    /// <param name="options">Driver options containing certificate configuration.</param>
    internal static UserIdentity BuildCertificateIdentity(OpcUaClientDriverOptions options)
    {
        if (string.IsNullOrWhiteSpace(options.UserCertificatePath))
            throw new InvalidOperationException(
                "OpcUaAuthType.Certificate requires OpcUaClientDriverOptions.UserCertificatePath to be set.");
        if (!System.IO.File.Exists(options.UserCertificatePath))
            throw new System.IO.FileNotFoundException(
                $"User certificate not found at '{options.UserCertificatePath}'.",
                options.UserCertificatePath);

        // X509CertificateLoader (new in .NET 9) is the only non-obsolete way to load a PFX
        // since the legacy X509Certificate2 ctors are marked obsolete on net10. Passes the
        // password through verbatim; PEM files with external keys fall back to
        // LoadCertificateFromFile which picks up the adjacent .key if present.
        var cert = System.Security.Cryptography.X509Certificates.X509CertificateLoader
            .LoadPkcs12FromFile(options.UserCertificatePath, options.UserCertificatePassword);

        if (!cert.HasPrivateKey)
            throw new InvalidOperationException(
                $"User certificate at '{options.UserCertificatePath}' has no private key — " +
                "the private key is required to sign the OPC UA user-token challenge at session activation.");

        return new UserIdentity(cert);
    }

    /// <summary>Convert a driver <see cref="OpcUaSecurityPolicy"/> to the OPC UA policy URI.</summary>
    /// <param name="policy">The driver security policy to map.</param>
    internal static string MapSecurityPolicy(OpcUaSecurityPolicy policy) => policy switch
    {
        OpcUaSecurityPolicy.None => SecurityPolicies.None,
        OpcUaSecurityPolicy.Basic128Rsa15 => SecurityPolicies.Basic128Rsa15,
        OpcUaSecurityPolicy.Basic256 => SecurityPolicies.Basic256,
        OpcUaSecurityPolicy.Basic256Sha256 => SecurityPolicies.Basic256Sha256,
        OpcUaSecurityPolicy.Aes128_Sha256_RsaOaep => SecurityPolicies.Aes128_Sha256_RsaOaep,
        OpcUaSecurityPolicy.Aes256_Sha256_RsaPss => SecurityPolicies.Aes256_Sha256_RsaPss,
        _ => throw new ArgumentOutOfRangeException(nameof(policy), policy, null),
    };

    private static string ShortPolicyName(string policyUri) =>
        policyUri?.Substring(policyUri.LastIndexOf('#') + 1) ?? "(null)";

    /// <summary>Reinitializes the driver with new configuration, shutting down and restarting the session.</summary>
    /// <param name="driverConfigJson">JSON-serialized driver configuration.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
    {
        await ShutdownAsync(cancellationToken).ConfigureAwait(false);
        await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
    }

    /// <summary>Gracefully shuts down the OPC UA session, unsubscribing all active monitoring items and closing the connection.</summary>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task ShutdownAsync(CancellationToken cancellationToken)
    {
        // Tear down remote subscriptions first — otherwise Session.Close will try and may fail
        // with BadSubscriptionIdInvalid noise in the upstream log. _subscriptions is cleared
        // whether or not the wire-side delete succeeds since the local handles are useless
        // after close anyway. Before deleting each subscription we detach the Notification
        // handlers we attached at subscribe time so the SDK's invocation list no longer
        // holds the driver instance through the closure (Driver.OpcUaClient-014).
        foreach (var rs in _subscriptions.Values)
        {
            DetachNotificationHandlers(rs.ItemHandlers);
            try { await rs.Subscription.DeleteAsync(silent: true, cancellationToken).ConfigureAwait(false); }
            catch { /* best-effort */ }
        }
        _subscriptions.Clear();

        foreach (var ras in _alarmSubscriptions.Values)
        {
            try { ras.EventItem.Notification -= ras.Handler; }
            catch { /* best-effort */ }
            try { await ras.Subscription.DeleteAsync(silent: true, cancellationToken).ConfigureAwait(false); }
            catch { /* best-effort */ }
        }
        _alarmSubscriptions.Clear();

        // Abort any in-flight reconnect attempts before touching the session — BeginReconnect's
        // retry loop holds a reference to the current session and would fight Session.CloseAsync
        // if left spinning. Take the handler under _probeLock so a keep-alive callback racing
        // through OnKeepAlive can't arm a fresh handler after we've torn this one down
        // (Driver.OpcUaClient-005).
        SessionReconnectHandler? handlerToCancel;
        lock (_probeLock)
        {
            handlerToCancel = _reconnectHandler;
            _reconnectHandler = null;
        }
        try { handlerToCancel?.CancelReconnect(); } catch { }
        handlerToCancel?.Dispose();

        // Take the session reference under _probeLock before touching it, so we can't race
        // an OnReconnectComplete that is simultaneously swapping to a new session
        // (Driver.OpcUaClient-006). We clear Session to null here so any concurrent caller
        // that checks inside _gate sees null immediately after shutdown begins.
        ISession? sessionToClose;
        lock (_probeLock)
        {
            sessionToClose = Session;
            if (_keepAliveHandler is not null && sessionToClose is not null)
            {
                try { sessionToClose.KeepAlive -= _keepAliveHandler; } catch { }
            }
            _keepAliveHandler = null;
            Session = null;
        }

        try { if (sessionToClose is Session s) await s.CloseAsync(cancellationToken).ConfigureAwait(false); }
        catch { /* best-effort */ }
        try { sessionToClose?.Dispose(); } catch { }
        _namespaceMap = null;
        _connectedEndpointUrl = null;

        // Detach the cert-validation handler so the (potentially process-shared)
        // CertificateValidator doesn't hold a delegate to a shutting-down driver
        // (Driver.OpcUaClient-012).
        if (_certValidationHandler is not null && _certValidatorRef is not null)
        {
            try { _certValidatorRef.CertificateValidation -= _certValidationHandler; } catch { }
            _certValidationHandler = null;
            _certValidatorRef = null;
        }

        TransitionTo(HostState.Unknown);
        _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
    }

    /// <summary>Gets the current health status of the OPC UA client driver.</summary>
    public DriverHealth GetHealth() => _health;

    /// <summary>
    ///     Returns an approximate in-driver memory footprint for the Core allocation-slope
    ///     detector. Each discovered node (folder or variable) contributes ~512 bytes to cover
    ///     the <see cref="DriverAttributeInfo"/> record, the browse-name string, and the stable
    ///     <c>nsu=</c> reference string stored in the address-space builder. The real number
    ///     depends on string length + box sizes; the constant is conservative enough that a
    ///     10k-node remote server reports ~5 MB — well within the budget and detectable by the
    ///     Core slope alarm (Driver.OpcUaClient-013).
    /// </summary>
    public long GetMemoryFootprint() => _discoveredNodeCount * 512L;

    /// <summary>
    ///     Drops the discovered-node count so the Core's cache-budget enforcement can request
    ///     a flush when footprint budget is breached. The OPC UA Client driver holds no
    ///     independently-flushable cache beyond what the address-space builder retains — a
    ///     flush here resets the footprint counter and signals the Core that re-discovery
    ///     will rebuild it cleanly from the remote server.
    /// </summary>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken)
    {
        _discoveredNodeCount = 0;
        return Task.CompletedTask;
    }

    // ---- IReadable ----

    /// <summary>Reads the current values of the specified nodes from the remote OPC UA server.</summary>
    /// <param name="fullReferences">Fully-qualified node identifiers to read.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task<IReadOnlyList<DataValueSnapshot>> ReadAsync(
        IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
    {
        // Make sure a session exists before queuing on the gate, but do NOT bind the wire
        // call to this reference — a reconnect can swap Session while we wait on _gate. The
        // session actually used is re-read inside the gate (Driver.OpcUaClient-001/-006).
        _ = RequireSession();
        var results = new DataValueSnapshot[fullReferences.Count];
        var now = DateTime.UtcNow;

        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            // Re-read Session inside the critical section: if a reconnect completed while we
            // were blocked on _gate, OnReconnectComplete has already swapped in the new
            // session. NodeId parsing is namespace-relative, so it must also use the current
            // session's namespace table.
            var session = Session;
            if (session is null)
            {
                for (var i = 0; i < fullReferences.Count; i++)
                    results[i] = new DataValueSnapshot(null, StatusBadCommunicationError, null, now);
                return results;
            }

            // Parse NodeIds against the live session. Tags whose reference doesn't parse get
            // BadNodeIdInvalid and are omitted from the wire request — saves a round-trip for
            // a fault the driver can detect locally.
            var toSend = new ReadValueIdCollection();
            var indexMap = new List<int>(fullReferences.Count); // maps wire-index -> results-index
            for (var i = 0; i < fullReferences.Count; i++)
            {
                if (!TryParseNodeId(session, fullReferences[i], out var nodeId))
                {
                    results[i] = new DataValueSnapshot(null, StatusBadNodeIdInvalid, null, now);
                    continue;
                }
                toSend.Add(new ReadValueId { NodeId = nodeId, AttributeId = Attributes.Value });
                indexMap.Add(i);
            }

            if (toSend.Count == 0) return results;

            try
            {
                var resp = await session.ReadAsync(
                    requestHeader: null,
                    maxAge: 0,
                    timestampsToReturn: TimestampsToReturn.Both,
                    nodesToRead: toSend,
                    ct: cancellationToken).ConfigureAwait(false);

                var values = resp.Results;
                for (var w = 0; w < values.Count; w++)
                {
                    var r = indexMap[w];
                    var dv = values[w];
                    // Preserve the upstream StatusCode verbatim — including Bad codes per
                    // §8's cascading-quality rule. Also preserve SourceTimestamp so downstream
                    // clients can detect stale upstream data.
                    results[r] = new DataValueSnapshot(
                        Value: dv.Value,
                        StatusCode: dv.StatusCode.Code,
                        SourceTimestampUtc: dv.SourceTimestamp == DateTime.MinValue ? null : dv.SourceTimestamp,
                        ServerTimestampUtc: dv.ServerTimestamp == DateTime.MinValue ? now : dv.ServerTimestamp);
                }
                _health = new DriverHealth(DriverState.Healthy, now, null);
            }
            catch (Exception ex)
            {
                // Transport / timeout / session-dropped — fan out the same fault across every
                // tag in this batch. Per-tag StatusCode stays BadCommunicationError (not
                // BadInternalError) so operators distinguish "upstream unreachable" from
                // "driver bug".
                for (var w = 0; w < indexMap.Count; w++)
                {
                    var r = indexMap[w];
                    results[r] = new DataValueSnapshot(null, StatusBadCommunicationError, null, now);
                }
                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
            }
        }
        finally { _gate.Release(); }
        return results;
    }

    // ---- IWritable ----

    /// <summary>Writes values to the specified nodes on the remote OPC UA server.</summary>
    /// <param name="writes">Write requests specifying nodes and values to write.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task<IReadOnlyList<WriteResult>> WriteAsync(
        IReadOnlyList<Core.Abstractions.WriteRequest> writes, CancellationToken cancellationToken)
    {
        // See ReadAsync — the wire call must use the session current inside the gate, not a
        // reference captured before WaitAsync (Driver.OpcUaClient-001/-006).
        _ = RequireSession();
        var results = new WriteResult[writes.Count];

        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            var session = Session;
            if (session is null)
            {
                // Writes are non-idempotent (decision #44/#45) — but here the request never
                // reached the wire, so BadCommunicationError ("definitely did not happen") is
                // the honest code.
                for (var i = 0; i < writes.Count; i++)
                    results[i] = new WriteResult(StatusBadCommunicationError);
                return results;
            }

            var toSend = new WriteValueCollection();
            var indexMap = new List<int>(writes.Count);
            for (var i = 0; i < writes.Count; i++)
            {
                if (!TryParseNodeId(session, writes[i].FullReference, out var nodeId))
                {
                    results[i] = new WriteResult(StatusBadNodeIdInvalid);
                    continue;
                }
                toSend.Add(new WriteValue
                {
                    NodeId = nodeId,
                    AttributeId = Attributes.Value,
                    Value = new DataValue(new Variant(writes[i].Value)),
                });
                indexMap.Add(i);
            }

            if (toSend.Count == 0) return results;

            try
            {
                var resp = await session.WriteAsync(
                    requestHeader: null,
                    nodesToWrite: toSend,
                    ct: cancellationToken).ConfigureAwait(false);

                var codes = resp.Results;
                for (var w = 0; w < codes.Count; w++)
                {
                    var r = indexMap[w];
                    // Pass upstream WriteResult StatusCode through verbatim. Success codes
                    // include Good (0) and any warning-level Good* variants; anything with
                    // the severity bits set is a Bad.
                    results[r] = new WriteResult(codes[w].Code);
                }
            }
            catch (OperationCanceledException)
            {
                // Timeout / cancellation after the wire request may have been dispatched.
                // Writes are non-idempotent (decision #44/#45) — BadTimeout ("outcome unknown,
                // do not blindly retry") is more honest than BadCommunicationError ("definitely
                // did not happen"). Downstream callers that need retry semantics check for
                // BadTimeout and can decide whether to re-issue (Driver.OpcUaClient-009).
                const uint StatusBadTimeout = 0x800A0000u;
                for (var w = 0; w < indexMap.Count; w++)
                    results[indexMap[w]] = new WriteResult(StatusBadTimeout);
            }
            catch (Exception)
            {
                // Pre-wire transport failure — the write definitely did not reach the server.
                for (var w = 0; w < indexMap.Count; w++)
                    results[indexMap[w]] = new WriteResult(StatusBadCommunicationError);
            }
        }
        finally { _gate.Release(); }
        return results;
    }

    /// <summary>
    ///     Parse a tag's full-reference string as a NodeId, resolved against the
    ///     <paramref name="session"/>'s <i>current</i> namespace table. Accepts both the
    ///     server-stable <c>nsu=&lt;uri&gt;;…</c> form the driver persists (see
    ///     <see cref="NamespaceMap.ToStableReference"/>) and plain OPC UA serialized forms
    ///     (<c>ns=2;s=…</c>, <c>i=2253</c>, <c>ns=4;g=…</c>, <c>ns=3;b=…</c>). Resolving the
    ///     <c>nsu=…</c> form against the current session re-binds it through that session's
    ///     URI table, so a remote namespace-table reorder across a restart is transparently
    ///     corrected (driver-specs.md §8). Empty + malformed strings return false; the driver
    ///     surfaces that as <see cref="StatusBadNodeIdInvalid"/> without a wire round-trip.
    /// </summary>
    /// <param name="session">The OPC UA session to resolve the node ID against.</param>
    /// <param name="fullReference">The full reference string to parse.</param>
    /// <param name="nodeId">The parsed node ID when successful.</param>
    internal static bool TryParseNodeId(ISession session, string fullReference, out NodeId nodeId) =>
        NamespaceMap.TryResolve(session, fullReference, out nodeId);

    /// <summary>
    ///     Render a discovered NodeId in the server-stable form persisted into the local
    ///     address space. Falls back to the raw serialized NodeId if the namespace map is not
    ///     yet built (it always is by the time <see cref="DiscoverAsync"/> runs).
    /// </summary>
    private string StableReference(NodeId nodeId) =>
        _namespaceMap?.ToStableReference(nodeId) ?? nodeId.ToString() ?? string.Empty;

    private ISession RequireSession() =>
        Session ?? throw new InvalidOperationException("OpcUaClientDriver not initialized");

    // ---- ITagDiscovery ----

    /// <summary>Discovers the remote OPC UA server's address space and materializes it through the supplied builder.</summary>
    /// <param name="builder">Address space builder for materializing discovered nodes.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task DiscoverAsync(IAddressSpaceBuilder builder, CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(builder);
        // Confirm a session exists before queuing; the session actually browsed is re-read
        // inside the gate so a reconnect mid-wait can't leave us browsing a closed session
        // (Driver.OpcUaClient-001/-006).
        _ = RequireSession();

        var rootFolder = builder.Folder("Remote", "Remote");
        var visited = new HashSet<NodeId>();
        var discovered = 0;
        var pendingVariables = new List<PendingVariable>();

        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            var session = Session
                ?? throw new InvalidOperationException(
                    "OpcUaClient session was lost before discovery could browse the remote server.");

            var root = !string.IsNullOrEmpty(_options.BrowseRoot)
                ? NodeId.Parse(session.MessageContext, _options.BrowseRoot)
                : ObjectIds.ObjectsFolder;

            // Pass 1: browse hierarchy + create folders inline, collect variables into a
            // pending list. Defers variable registration until attributes are resolved — the
            // address-space builder's Variable call is the one-way commit, so doing it only
            // once per variable (with correct DataType/SecurityClass/IsArray) avoids the
            // alternative (register with placeholders + mutate later) which the
            // IAddressSpaceBuilder contract doesn't expose.
            await BrowseRecursiveAsync(session, root, rootFolder, visited,
                depth: 0,
                discovered: () => discovered, increment: () => discovered++,
                pendingVariables: pendingVariables,
                ct: cancellationToken).ConfigureAwait(false);

            // Pass 2: batch-read DataType + AccessLevel + ValueRank + Historizing per
            // variable. One wire request for up to ~N variables; for 10k-node servers this is
            // still a couple of hundred ms total since the SDK chunks ReadAsync automatically.
            await EnrichAndRegisterVariablesAsync(session, pendingVariables, cancellationToken)
                .ConfigureAwait(false);

            // Update the footprint counter so GetMemoryFootprint() returns a real estimate
            // after each discovery pass (Driver.OpcUaClient-013).
            _discoveredNodeCount = discovered;
        }
        finally { _gate.Release(); }
    }

    /// <summary>
    ///     A variable collected during the browse pass, waiting for attribute enrichment
    ///     before being registered on the address-space builder.
    /// </summary>
    private readonly record struct PendingVariable(
        IAddressSpaceBuilder ParentFolder,
        string BrowseName,
        string DisplayName,
        NodeId NodeId);

    private async Task BrowseRecursiveAsync(
        ISession session, NodeId node, IAddressSpaceBuilder folder, HashSet<NodeId> visited,
        int depth, Func<int> discovered, Action increment,
        List<PendingVariable> pendingVariables, CancellationToken ct)
    {
        if (depth >= _options.MaxBrowseDepth) return;
        if (discovered() >= _options.MaxDiscoveredNodes) return;
        if (!visited.Add(node)) return;

        var browseDescriptions = new BrowseDescriptionCollection
        {
            new()
            {
                NodeId = node,
                BrowseDirection = BrowseDirection.Forward,
                ReferenceTypeId = ReferenceTypeIds.HierarchicalReferences,
                IncludeSubtypes = true,
                NodeClassMask = (uint)(NodeClass.Object | NodeClass.Variable),
                ResultMask = (uint)(BrowseResultMask.BrowseName | BrowseResultMask.DisplayName
                                    | BrowseResultMask.NodeClass | BrowseResultMask.TypeDefinition),
            }
        };

        ReferenceDescriptionCollection refs;
        try
        {
            var resp = await session.BrowseAsync(
                requestHeader: null,
                view: null,
                requestedMaxReferencesPerNode: 0,
                nodesToBrowse: browseDescriptions,
                ct: ct).ConfigureAwait(false);

            if (resp.Results.Count == 0) return;
            var result = resp.Results[0];
            refs = result.References;

            // Follow browse continuation points. OPC UA servers cap the references returned
            // per node in a single response; when a folder has more children than the cap,
            // BrowseResult.ContinuationPoint is non-empty and the remainder must be pulled
            // with BrowseNext. Without this loop a large remote folder is silently truncated
            // and discovered tags go missing from the local address space
            // (Driver.OpcUaClient-003).
            var continuationPoint = result.ContinuationPoint;
            while (continuationPoint is { Length: > 0 })
            {
                var next = await session.BrowseNextAsync(
                    requestHeader: null,
                    releaseContinuationPoints: false,
                    continuationPoints: [continuationPoint],
                    ct: ct).ConfigureAwait(false);

                if (next.Results.Count == 0) break;
                var nextResult = next.Results[0];
                if (nextResult.References is { Count: > 0 })
                    refs.AddRange(nextResult.References);
                continuationPoint = nextResult.ContinuationPoint;
            }
        }
        catch
        {
            // Transient browse failure on a sub-tree — don't kill the whole discovery, just
            // skip this branch. The driver's health surface will reflect the cascade via the
            // probe loop (PR 69).
            return;
        }

        foreach (var rf in refs)
        {
            if (discovered() >= _options.MaxDiscoveredNodes) break;

            var childId = ExpandedNodeId.ToNodeId(rf.NodeId, session.NamespaceUris);
            if (NodeId.IsNull(childId)) continue;

            var browseName = rf.BrowseName?.Name ?? childId.ToString();
            var displayName = rf.DisplayName?.Text ?? browseName;

            if (rf.NodeClass == NodeClass.Object)
            {
                var subFolder = folder.Folder(browseName, displayName);
                increment();
                await BrowseRecursiveAsync(session, childId, subFolder, visited,
                    depth + 1, discovered, increment, pendingVariables, ct).ConfigureAwait(false);
            }
            else if (rf.NodeClass == NodeClass.Variable)
            {
                pendingVariables.Add(new PendingVariable(folder, browseName, displayName, childId));
                increment();
            }
        }
    }

    /// <summary>
    ///     Pass 2 of discovery: batch-read DataType + ValueRank + AccessLevel + Historizing
    ///     for every collected variable in one Session.ReadAsync (the SDK chunks internally
    ///     to respect the server's per-request limits). Then register each variable on its
    ///     parent folder with the real <see cref="DriverAttributeInfo"/>.
    /// </summary>
    /// <remarks>
    ///     <para>
    ///         Attributes read: <c>DataType</c> (NodeId of the value type),
    ///         <c>ValueRank</c> (-1 = scalar, 1 = array), <c>UserAccessLevel</c> (the
    ///         effective access mask for our session — more accurate than AccessLevel which
    ///         is the server-side configured mask before user filtering), and
    ///         <c>Historizing</c> (server flags whether historian data is available).
    ///     </para>
    ///     <para>
    ///         When the upstream server returns Bad on any attribute, the variable falls back
    ///         to safe defaults (Int32 / ViewOnly / not-array / not-historized) and is still
    ///         registered — a partial enrichment failure shouldn't drop entire variables from
    ///         the address space. Operators reading the Admin dashboard see the variable
    ///         with conservative metadata which is obviously wrong and easy to triage.
    ///     </para>
    /// </remarks>
    private async Task EnrichAndRegisterVariablesAsync(
        ISession session, IReadOnlyList<PendingVariable> pending, CancellationToken ct)
    {
        if (pending.Count == 0) return;

        // 4 attributes per variable: DataType, ValueRank, UserAccessLevel, Historizing.
        var nodesToRead = new ReadValueIdCollection(pending.Count * 4);
        foreach (var pv in pending)
        {
            nodesToRead.Add(new ReadValueId { NodeId = pv.NodeId, AttributeId = Attributes.DataType });
            nodesToRead.Add(new ReadValueId { NodeId = pv.NodeId, AttributeId = Attributes.ValueRank });
            nodesToRead.Add(new ReadValueId { NodeId = pv.NodeId, AttributeId = Attributes.UserAccessLevel });
            nodesToRead.Add(new ReadValueId { NodeId = pv.NodeId, AttributeId = Attributes.Historizing });
        }

        DataValueCollection values;
        try
        {
            var resp = await session.ReadAsync(
                requestHeader: null,
                maxAge: 0,
                timestampsToReturn: TimestampsToReturn.Neither,
                nodesToRead: nodesToRead,
                ct: ct).ConfigureAwait(false);
            values = resp.Results;
        }
        catch
        {
            // Enrichment-read failed wholesale (server unreachable mid-browse). Register the
            // pending variables with conservative defaults rather than dropping them — the
            // downstream catalog is still useful for reading via IReadable.
            foreach (var pv in pending)
                RegisterFallback(pv);
            return;
        }

        for (var i = 0; i < pending.Count; i++)
        {
            var pv = pending[i];
            var baseIdx = i * 4;
            var dataTypeDv = values[baseIdx];
            var valueRankDv = values[baseIdx + 1];
            var accessDv = values[baseIdx + 2];
            var histDv = values[baseIdx + 3];

            var dataType = StatusCode.IsGood(dataTypeDv.StatusCode) && dataTypeDv.Value is NodeId dtId
                ? MapUpstreamDataType(dtId)
                : DriverDataType.Int32;
            var valueRank = StatusCode.IsGood(valueRankDv.StatusCode) && valueRankDv.Value is int vr ? vr : -1;
            // OPC UA Part 3 ValueRank constants: -3 = ScalarOrOneDimension, -2 = Any,
            // -1 = Scalar, 0 = OneOrMoreDimensions, 1 = OneDimension, >1 = N specific dimensions.
            // Deliberate choice: treat anything >= 0 as an array (the spec guarantees -3/-2/-1
            // are the only negative values, and any non-negative rank denotes at least one
            // array dimension). -3 ScalarOrOneDimension and -2 Any are conservatively treated
            // as scalar — array-of-one is exposed as scalar to the local address space until
            // the upstream variable carries a concrete dimensioned rank.
            var isArray = valueRank >= 0;
            var access = StatusCode.IsGood(accessDv.StatusCode) && accessDv.Value is byte ab ? ab : (byte)0;
            var securityClass = MapAccessLevelToSecurityClass(access);
            var historizing = StatusCode.IsGood(histDv.StatusCode) && histDv.Value is bool b && b;

            pv.ParentFolder.Variable(pv.BrowseName, pv.DisplayName, new DriverAttributeInfo(
                FullName: StableReference(pv.NodeId),
                DriverDataType: dataType,
                IsArray: isArray,
                ArrayDim: null,
                SecurityClass: securityClass,
                IsHistorized: historizing,
                IsAlarm: false));
        }

        void RegisterFallback(PendingVariable pv)
        {
            pv.ParentFolder.Variable(pv.BrowseName, pv.DisplayName, new DriverAttributeInfo(
                FullName: StableReference(pv.NodeId),
                DriverDataType: DriverDataType.Int32,
                IsArray: false,
                ArrayDim: null,
                SecurityClass: SecurityClassification.ViewOnly,
                IsHistorized: false,
                IsAlarm: false));
        }
    }

    /// <summary>
    ///     Map an upstream OPC UA built-in DataType NodeId (via <c>DataTypeIds.*</c>) to a
    ///     <see cref="DriverDataType"/>. Unknown / custom types fall through to
    ///     <see cref="DriverDataType.String"/> which is the safest passthrough for
    ///     Variant-wrapped structs + enums + extension objects; downstream clients see a
    ///     string rendering but the cascading-quality path still preserves upstream
    ///     StatusCode + timestamps.
    /// </summary>
    /// <param name="dataType">The OPC UA data type NodeId to map.</param>
    internal static DriverDataType MapUpstreamDataType(NodeId dataType)
    {
        if (dataType == DataTypeIds.Boolean) return DriverDataType.Boolean;
        // SByte (signed 8-bit) shares Int16 — DriverDataType has no narrower signed type.
        // Byte (unsigned 8-bit) belongs in the unsigned family → UInt16, not Int16
        // (Driver.OpcUaClient-010: mapping an unsigned 0-255 type onto Int16 misrepresents
        // type metadata and confuses range/validation logic keyed off DriverDataType).
        if (dataType == DataTypeIds.SByte || dataType == DataTypeIds.Int16) return DriverDataType.Int16;
        if (dataType == DataTypeIds.Byte || dataType == DataTypeIds.UInt16) return DriverDataType.UInt16;
        if (dataType == DataTypeIds.Int32) return DriverDataType.Int32;
        if (dataType == DataTypeIds.UInt32) return DriverDataType.UInt32;
        if (dataType == DataTypeIds.Int64) return DriverDataType.Int64;
        if (dataType == DataTypeIds.UInt64) return DriverDataType.UInt64;
        if (dataType == DataTypeIds.Float) return DriverDataType.Float32;
        if (dataType == DataTypeIds.Double) return DriverDataType.Float64;
        if (dataType == DataTypeIds.String) return DriverDataType.String;
        if (dataType == DataTypeIds.DateTime || dataType == DataTypeIds.UtcTime)
            return DriverDataType.DateTime;
        return DriverDataType.String;
    }

    /// <summary>
    ///     Map an OPC UA AccessLevel/UserAccessLevel attribute value (<c>AccessLevels</c>
    ///     bitmask) to a <see cref="SecurityClassification"/> the local node-manager's ACL
    ///     layer can gate writes off. CurrentWrite-capable variables surface as
    ///     <see cref="SecurityClassification.Operate"/>; read-only as <see cref="SecurityClassification.ViewOnly"/>.
    /// </summary>
    /// <param name="accessLevel">The OPC UA access level bitmask.</param>
    internal static SecurityClassification MapAccessLevelToSecurityClass(byte accessLevel)
    {
        const byte CurrentWrite = 2; // AccessLevels.CurrentWrite = 0x02
        return (accessLevel & CurrentWrite) != 0
            ? SecurityClassification.Operate
            : SecurityClassification.ViewOnly;
    }

    // ---- ISubscribable ----

    /// <summary>Subscribes to monitored value changes on the specified nodes from the remote OPC UA server.</summary>
    /// <param name="fullReferences">Fully-qualified node identifiers to monitor.</param>
    /// <param name="publishingInterval">Desired minimum interval between publish cycles.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task<ISubscriptionHandle> SubscribeAsync(
        IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken)
    {
        var session = RequireSession();
        var id = Interlocked.Increment(ref _nextSubscriptionId);
        var handle = new OpcUaSubscriptionHandle(id);

        // Floor the publishing interval at 50ms — OPC UA servers routinely negotiate
        // minimum-supported intervals up anyway, but sending sub-50ms wastes negotiation
        // bandwidth on every subscription create.
        var intervalMs = publishingInterval < TimeSpan.FromMilliseconds(50)
            ? 50
            : (int)publishingInterval.TotalMilliseconds;

        var subscription = new Subscription(telemetry: null!, new SubscriptionOptions
        {
            DisplayName = $"opcua-sub-{id}",
            PublishingInterval = intervalMs,
            KeepAliveCount = 10,
            LifetimeCount = 1000,
            MaxNotificationsPerPublish = 0,
            PublishingEnabled = true,
            Priority = 0,
            TimestampsToReturn = TimestampsToReturn.Both,
        });

        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            session.AddSubscription(subscription);
            await subscription.CreateAsync(cancellationToken).ConfigureAwait(false);

            // Track each (MonitoredItem, handler) pair so UnsubscribeAsync / ShutdownAsync
            // can detach the Notification delegate before disposing the session
            // (Driver.OpcUaClient-014). The lambda captures `handle`, so we must hold the
            // exact delegate instance returned by `+=` to be able to remove it.
            var itemHandlers = new List<MonitoredItemNotificationHandle>();
            foreach (var fullRef in fullReferences)
            {
                if (!TryParseNodeId(session, fullRef, out var nodeId)) continue;
                // The tag string is routed through MonitoredItem.Handle so the Notification
                // handler can identify which tag changed without an extra lookup.
                var item = new MonitoredItem(telemetry: null!, new MonitoredItemOptions
                {
                    DisplayName = fullRef,
                    StartNodeId = nodeId,
                    AttributeId = Attributes.Value,
                    MonitoringMode = MonitoringMode.Reporting,
                    SamplingInterval = intervalMs,
                    QueueSize = 1,
                    DiscardOldest = true,
                })
                {
                    Handle = fullRef,
                };
                MonitoredItemNotificationEventHandler notifHandler = (mi, args) =>
                    OnMonitoredItemNotification(handle, mi, args);
                item.Notification += notifHandler;
                itemHandlers.Add(new MonitoredItemNotificationHandle(item, notifHandler));
                subscription.AddItem(item);
            }

            await subscription.CreateItemsAsync(cancellationToken).ConfigureAwait(false);
            _subscriptions[id] = new RemoteSubscription(subscription, handle, itemHandlers);
        }
        finally { _gate.Release(); }

        return handle;
    }

    /// <summary>Unsubscribes from monitored value changes for the specified subscription handle.</summary>
    /// <param name="handle">The subscription handle to unsubscribe.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
    {
        if (handle is not OpcUaSubscriptionHandle h) return;
        if (!_subscriptions.TryRemove(h.Id, out var rs)) return;

        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            // Detach Notification handlers BEFORE deleting the subscription so the SDK's
            // MonitoredItem.Notification multicast invocation list no longer holds a
            // closure that captures the driver instance (Driver.OpcUaClient-014). The
            // delegate stored on RemoteSubscription is the exact instance that was added,
            // so `-=` removes it cleanly.
            DetachNotificationHandlers(rs.ItemHandlers);
            try { await rs.Subscription.DeleteAsync(silent: true, cancellationToken).ConfigureAwait(false); }
            catch { /* best-effort — the subscription may already be gone on reconnect */ }
        }
        finally { _gate.Release(); }
    }

    private static void DetachNotificationHandlers(IReadOnlyList<MonitoredItemNotificationHandle> items)
    {
        for (var i = 0; i < items.Count; i++)
        {
            var pair = items[i];
            try { pair.Item.Notification -= pair.Handler; }
            catch { /* best-effort — SDK may have already cleared its invocation list on session loss */ }
        }
    }

    private void OnMonitoredItemNotification(OpcUaSubscriptionHandle handle, MonitoredItem item, MonitoredItemNotificationEventArgs args)
    {
        // args.NotificationValue arrives as a MonitoredItemNotification for value-change
        // subscriptions; extract its DataValue. The Handle property carries our tag string.
        if (args.NotificationValue is not MonitoredItemNotification mn) return;
        var dv = mn.Value;
        if (dv is null) return;
        var fullRef = (item.Handle as string) ?? item.DisplayName ?? string.Empty;
        var snapshot = new DataValueSnapshot(
            Value: dv.Value,
            StatusCode: dv.StatusCode.Code,
            SourceTimestampUtc: dv.SourceTimestamp == DateTime.MinValue ? null : dv.SourceTimestamp,
            ServerTimestampUtc: dv.ServerTimestamp == DateTime.MinValue ? DateTime.UtcNow : dv.ServerTimestamp);
        OnDataChange?.Invoke(this, new DataChangeEventArgs(handle, fullRef, snapshot));
    }

    /// <summary>
    ///     Live data-change subscription bookkeeping. Holds the SDK <see cref="Subscription"/>,
    ///     the local handle, and the per-MonitoredItem (item, handler) pairs so
    ///     <see cref="UnsubscribeAsync"/> / <see cref="ShutdownAsync"/> can detach the
    ///     Notification delegates before the SDK disposes the subscription
    ///     (Driver.OpcUaClient-014).
    /// </summary>
    private sealed record RemoteSubscription(
        Subscription Subscription,
        OpcUaSubscriptionHandle Handle,
        IReadOnlyList<MonitoredItemNotificationHandle> ItemHandlers);

    /// <summary>
    ///     One (MonitoredItem, handler-delegate-instance) pair captured at subscribe time so
    ///     the same delegate instance can be `-=` removed at unsubscribe time. The lambda
    ///     captures the local <c>OpcUaSubscriptionHandle</c>, which is what makes detach
    ///     necessary — without it the SDK's multicast invocation list holds the driver
    ///     through the closure until the session itself is disposed.
    /// </summary>
    private sealed record MonitoredItemNotificationHandle(
        MonitoredItem Item,
        MonitoredItemNotificationEventHandler Handler);

    private sealed record OpcUaSubscriptionHandle(long Id) : ISubscriptionHandle
    {
        /// <summary>Gets the diagnostic identifier for this subscription.</summary>
        public string DiagnosticId => $"opcua-sub-{Id}";
    }

    // ---- IAlarmSource ----

    /// <summary>
    ///     Field positions in the EventFilter SelectClauses below. Used to index into the
    ///     <c>EventFieldList.EventFields</c> Variant collection when an event arrives.
    /// </summary>
    private const int AlarmFieldEventId = 0;
    private const int AlarmFieldEventType = 1;
    private const int AlarmFieldSourceNode = 2;
    private const int AlarmFieldMessage = 3;
    private const int AlarmFieldSeverity = 4;
    private const int AlarmFieldTime = 5;
    private const int AlarmFieldConditionId = 6;

    /// <summary>Subscribes to alarm and event notifications from the remote OPC UA server.</summary>
    /// <param name="sourceNodeIds">Source node identifiers to subscribe alarms from.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task<IAlarmSubscriptionHandle> SubscribeAlarmsAsync(
        IReadOnlyList<string> sourceNodeIds, CancellationToken cancellationToken)
    {
        var session = RequireSession();
        var id = Interlocked.Increment(ref _nextAlarmSubscriptionId);
        var handle = new OpcUaAlarmSubscriptionHandle(id);

        // Pre-resolve the source-node filter set so the per-event notification handler can
        // match in O(1) without re-parsing on every event.
        var sourceFilter = new HashSet<string>(sourceNodeIds, StringComparer.Ordinal);

        var subscription = new Subscription(telemetry: null!, new SubscriptionOptions
        {
            DisplayName = $"opcua-alarm-sub-{id}",
            PublishingInterval = 500,   // 500ms — alarms don't need fast polling; the server pushes
            KeepAliveCount = 10,
            LifetimeCount = 1000,
            MaxNotificationsPerPublish = 0,
            PublishingEnabled = true,
            Priority = 0,
            TimestampsToReturn = TimestampsToReturn.Both,
        });

        // EventFilter SelectClauses — pick the standard BaseEventType fields we need to
        // materialize an AlarmEventArgs. Field positions are indexed by the AlarmField*
        // constants so the notification handler indexes in O(1) without re-examining the
        // QualifiedName BrowsePaths.
        var filter = new EventFilter();
        void AddField(string browseName) => filter.SelectClauses.Add(new SimpleAttributeOperand
        {
            TypeDefinitionId = ObjectTypeIds.BaseEventType,
            BrowsePath = [new QualifiedName(browseName)],
            AttributeId = Attributes.Value,
        });
        AddField("EventId");
        AddField("EventType");
        AddField("SourceNode");
        AddField("Message");
        AddField("Severity");
        AddField("Time");
        // ConditionId on ConditionType nodes is the branch identifier for
        // acknowledgeable conditions. Not a BaseEventType field — reach it via the typed path.
        filter.SelectClauses.Add(new SimpleAttributeOperand
        {
            TypeDefinitionId = ObjectTypeIds.ConditionType,
            BrowsePath = [], // empty path = the condition node itself
            AttributeId = Attributes.NodeId,
        });

        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            session.AddSubscription(subscription);
            await subscription.CreateAsync(cancellationToken).ConfigureAwait(false);

            var eventItem = new MonitoredItem(telemetry: null!, new MonitoredItemOptions
            {
                DisplayName = "Server/Events",
                StartNodeId = ObjectIds.Server,
                AttributeId = Attributes.EventNotifier,
                MonitoringMode = MonitoringMode.Reporting,
                QueueSize = 1000, // deep queue — a server can fire many alarms in bursts
                DiscardOldest = false,
                Filter = filter,
            })
            {
                Handle = handle,
            };
            // Capture the exact delegate instance so UnsubscribeAlarmsAsync / ShutdownAsync
            // can `-=` it later (Driver.OpcUaClient-014). The lambda captures `handle` and
            // `sourceFilter`, so without the explicit detach the SDK's invocation list keeps
            // the driver instance alive until the session itself is disposed.
            MonitoredItemNotificationEventHandler notifHandler = (mi, args) =>
                OnEventNotification(handle, sourceFilter, mi, args);
            eventItem.Notification += notifHandler;
            subscription.AddItem(eventItem);
            await subscription.CreateItemsAsync(cancellationToken).ConfigureAwait(false);

            _alarmSubscriptions[id] = new RemoteAlarmSubscription(subscription, handle, eventItem, notifHandler);
        }
        finally { _gate.Release(); }

        return handle;
    }

    /// <summary>Unsubscribes from alarm and event notifications for the specified alarm subscription handle.</summary>
    /// <param name="handle">The alarm subscription handle to unsubscribe.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task UnsubscribeAlarmsAsync(IAlarmSubscriptionHandle handle, CancellationToken cancellationToken)
    {
        if (handle is not OpcUaAlarmSubscriptionHandle h) return;
        if (!_alarmSubscriptions.TryRemove(h.Id, out var rs)) return;

        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            // Detach the Notification handler before deleting the subscription so the SDK's
            // multicast invocation list no longer holds the driver instance through the
            // closure (Driver.OpcUaClient-014).
            try { rs.EventItem.Notification -= rs.Handler; }
            catch { /* best-effort */ }
            try { await rs.Subscription.DeleteAsync(silent: true, cancellationToken).ConfigureAwait(false); }
            catch { /* best-effort — session may already be gone across a reconnect */ }
        }
        finally { _gate.Release(); }
    }

    /// <summary>Acknowledges multiple alarms by calling the remote OPC UA server's Acknowledge method.</summary>
    /// <param name="acknowledgements">List of alarm acknowledgement requests.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task AcknowledgeAsync(
        IReadOnlyList<AlarmAcknowledgeRequest> acknowledgements, CancellationToken cancellationToken)
    {
        // Short-circuit empty batch BEFORE touching the session so callers can pass an empty
        // list without guarding the size themselves — e.g. a bulk-ack UI that built an empty
        // list because the filter matched nothing.
        if (acknowledgements.Count == 0) return;
        var session = RequireSession();

        // OPC UA A&C: call the AcknowledgeableConditionType.Acknowledge method on each
        // condition node with EventId + Comment arguments. CallAsync accepts a batch —
        // one CallMethodRequest per ack.
        var callRequests = new CallMethodRequestCollection();
        foreach (var ack in acknowledgements)
        {
            if (!TryParseNodeId(session, ack.ConditionId, out var conditionId)) continue;
            callRequests.Add(new CallMethodRequest
            {
                ObjectId = conditionId,
                MethodId = MethodIds.AcknowledgeableConditionType_Acknowledge,
                InputArguments = [
                    new Variant(Array.Empty<byte>()),  // EventId — server-side best-effort; empty resolves to 'most recent'
                    new Variant(new LocalizedText(ack.Comment ?? string.Empty)),
                ],
            });
        }

        if (callRequests.Count == 0) return;

        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            try
            {
                var resp = await session.CallAsync(
                    requestHeader: null,
                    methodsToCall: callRequests,
                    ct: cancellationToken).ConfigureAwait(false);

                // Inspect per-ack results — the upstream server can reject individual acks
                // (BadConditionAlreadyAcked, BadNodeIdUnknown, BadUserAccessDenied) even when
                // the batch transport succeeds. Operators acking a critical alarm deserve to
                // know if the ack didn't take (Driver.OpcUaClient-008).
                if (resp?.Results is not null)
                {
                    for (var i = 0; i < resp.Results.Count; i++)
                    {
                        var result = resp.Results[i];
                        if (StatusCode.IsBad(result.StatusCode))
                        {
                            _logger.LogWarning(
                                "OpcUaClientDriver '{DriverInstanceId}': AcknowledgeAsync ack[{Index}] " +
                                "rejected by upstream server with StatusCode {StatusCode:X8}. " +
                                "The acknowledgement may not have been applied.",
                                _driverInstanceId, i, result.StatusCode.Code);
                        }
                    }
                }
            }
            catch (OperationCanceledException ex)
            {
                // Transport-level timeout / cancellation — propagate so the caller's
                // retry / re-ack mechanism can decide what to do.
                _logger.LogWarning(ex,
                    "OpcUaClientDriver '{DriverInstanceId}': AcknowledgeAsync transport error.",
                    _driverInstanceId);
                throw;
            }
            catch (Exception ex)
            {
                // Log genuine transport failures rather than swallowing them silently.
                _logger.LogWarning(ex,
                    "OpcUaClientDriver '{DriverInstanceId}': AcknowledgeAsync failed; " +
                    "acknowledgements may not have been applied.",
                    _driverInstanceId);
            }
        }
        finally { _gate.Release(); }
    }

    private void OnEventNotification(
        OpcUaAlarmSubscriptionHandle handle,
        HashSet<string> sourceFilter,
        MonitoredItem item,
        MonitoredItemNotificationEventArgs args)
    {
        if (args.NotificationValue is not EventFieldList efl) return;
        if (efl.EventFields.Count <= AlarmFieldConditionId) return;

        var sourceNode = efl.EventFields[AlarmFieldSourceNode].Value?.ToString() ?? string.Empty;
        if (sourceFilter.Count > 0 && !sourceFilter.Contains(sourceNode)) return;

        var eventType = efl.EventFields[AlarmFieldEventType].Value?.ToString() ?? "BaseEventType";
        var message = (efl.EventFields[AlarmFieldMessage].Value as LocalizedText)?.Text ?? string.Empty;
        var severity = efl.EventFields[AlarmFieldSeverity].Value is ushort sev ? sev : (ushort)0;
        var time = efl.EventFields[AlarmFieldTime].Value is DateTime t ? t : DateTime.UtcNow;
        var conditionId = efl.EventFields[AlarmFieldConditionId].Value?.ToString() ?? string.Empty;

        OnAlarmEvent?.Invoke(this, new AlarmEventArgs(
            SubscriptionHandle: handle,
            SourceNodeId: sourceNode,
            ConditionId: conditionId,
            AlarmType: eventType,
            Message: message,
            Severity: MapSeverity(severity),
            SourceTimestampUtc: time));
    }

    /// <summary>
    ///     Map an OPC UA <c>BaseEventType.Severity</c> (1..1000) to our coarse-grained
    ///     <see cref="AlarmSeverity"/> bucket. Thresholds match the OPC UA A&amp;C Part 9
    ///     guidance: 1-200 Low, 201-500 Medium, 501-800 High, 801-1000 Critical.
    /// </summary>
    /// <param name="opcSeverity">The OPC UA severity value (1-1000).</param>
    internal static AlarmSeverity MapSeverity(ushort opcSeverity) => opcSeverity switch
    {
        <= 200 => AlarmSeverity.Low,
        <= 500 => AlarmSeverity.Medium,
        <= 800 => AlarmSeverity.High,
        _ => AlarmSeverity.Critical,
    };

    /// <summary>
    ///     Live alarm-event subscription bookkeeping. Holds the SDK <see cref="Subscription"/>,
    ///     the local handle, the single event-MonitoredItem (`Server/Events`), and the exact
    ///     handler delegate instance so unsubscribe / shutdown can detach the Notification
    ///     event before the SDK disposes the subscription (Driver.OpcUaClient-014).
    /// </summary>
    private sealed record RemoteAlarmSubscription(
        Subscription Subscription,
        OpcUaAlarmSubscriptionHandle Handle,
        MonitoredItem EventItem,
        MonitoredItemNotificationEventHandler Handler);

    private sealed record OpcUaAlarmSubscriptionHandle(long Id) : IAlarmSubscriptionHandle
    {
        /// <summary>Gets the diagnostic identifier for this alarm subscription.</summary>
        public string DiagnosticId => $"opcua-alarm-sub-{Id}";
    }

    // ---- IHistoryProvider (passthrough to upstream server) ----

    /// <summary>Reads raw historical data from the remote OPC UA server.</summary>
    /// <param name="fullReference">Fully-qualified node identifier to read history for.</param>
    /// <param name="startUtc">Start time in UTC for the history query.</param>
    /// <param name="endUtc">End time in UTC for the history query.</param>
    /// <param name="maxValuesPerNode">Maximum number of values to return.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task<Core.Abstractions.HistoryReadResult> ReadRawAsync(
        string fullReference, DateTime startUtc, DateTime endUtc, uint maxValuesPerNode,
        CancellationToken cancellationToken)
    {
        var details = new ReadRawModifiedDetails
        {
            IsReadModified = false,
            StartTime = startUtc,
            EndTime = endUtc,
            NumValuesPerNode = maxValuesPerNode,
            ReturnBounds = false,
        };
        return await ExecuteHistoryReadAsync(fullReference, new ExtensionObject(details), cancellationToken)
            .ConfigureAwait(false);
    }

    /// <summary>Reads processed (aggregated) historical data from the remote OPC UA server.</summary>
    /// <param name="fullReference">Fully-qualified node identifier to read history for.</param>
    /// <param name="startUtc">Start time in UTC for the history query.</param>
    /// <param name="endUtc">End time in UTC for the history query.</param>
    /// <param name="interval">Time interval for aggregation.</param>
    /// <param name="aggregate">The aggregation function to apply.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task<Core.Abstractions.HistoryReadResult> ReadProcessedAsync(
        string fullReference, DateTime startUtc, DateTime endUtc, TimeSpan interval,
        HistoryAggregateType aggregate, CancellationToken cancellationToken)
    {
        var aggregateId = MapAggregateToNodeId(aggregate);
        var details = new ReadProcessedDetails
        {
            StartTime = startUtc,
            EndTime = endUtc,
            ProcessingInterval = interval.TotalMilliseconds,
            AggregateType = [aggregateId],
        };
        return await ExecuteHistoryReadAsync(fullReference, new ExtensionObject(details), cancellationToken)
            .ConfigureAwait(false);
    }

    /// <summary>Reads historical data at specific timestamps from the remote OPC UA server.</summary>
    /// <param name="fullReference">Fully-qualified node identifier to read history for.</param>
    /// <param name="timestampsUtc">List of specific timestamps to read values at.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    public async Task<Core.Abstractions.HistoryReadResult> ReadAtTimeAsync(
        string fullReference, IReadOnlyList<DateTime> timestampsUtc, CancellationToken cancellationToken)
    {
        var reqTimes = new DateTimeCollection(timestampsUtc);
        var details = new ReadAtTimeDetails
        {
            ReqTimes = reqTimes,
            UseSimpleBounds = true,
        };
        return await ExecuteHistoryReadAsync(fullReference, new ExtensionObject(details), cancellationToken)
            .ConfigureAwait(false);
    }

    /// <summary>
    ///     Shared HistoryRead wire path — used by Raw/Processed/AtTime. Handles NodeId parse,
    ///     Session.HistoryReadAsync call, Bad-StatusCode passthrough (no translation per §8
    ///     cascading-quality rule), and HistoryData unwrap into <see cref="DataValueSnapshot"/>.
    /// </summary>
    private async Task<Core.Abstractions.HistoryReadResult> ExecuteHistoryReadAsync(
        string fullReference, ExtensionObject historyReadDetails, CancellationToken ct)
    {
        var session = RequireSession();
        if (!TryParseNodeId(session, fullReference, out var nodeId))
        {
            return new Core.Abstractions.HistoryReadResult([], null);
        }

        var nodesToRead = new HistoryReadValueIdCollection
        {
            new HistoryReadValueId { NodeId = nodeId },
        };

        await _gate.WaitAsync(ct).ConfigureAwait(false);
        try
        {
            var resp = await session.HistoryReadAsync(
                requestHeader: null,
                historyReadDetails: historyReadDetails,
                timestampsToReturn: TimestampsToReturn.Both,
                releaseContinuationPoints: false,
                nodesToRead: nodesToRead,
                ct: ct).ConfigureAwait(false);

            if (resp.Results.Count == 0) return new Core.Abstractions.HistoryReadResult([], null);
            var r = resp.Results[0];

            // Unwrap HistoryData from the ExtensionObject-encoded payload the SDK returns.
            // Samples stay in chronological order per OPC UA Part 11; cascading-quality
            // rule: preserve each DataValue's upstream StatusCode + timestamps verbatim.
            var samples = new List<DataValueSnapshot>();
            if (r.HistoryData?.Body is HistoryData hd)
            {
                var now = DateTime.UtcNow;
                foreach (var dv in hd.DataValues)
                {
                    samples.Add(new DataValueSnapshot(
                        Value: dv.Value,
                        StatusCode: dv.StatusCode.Code,
                        SourceTimestampUtc: dv.SourceTimestamp == DateTime.MinValue ? null : dv.SourceTimestamp,
                        ServerTimestampUtc: dv.ServerTimestamp == DateTime.MinValue ? now : dv.ServerTimestamp));
                }
            }

            var contPt = r.ContinuationPoint is { Length: > 0 } ? r.ContinuationPoint : null;
            return new Core.Abstractions.HistoryReadResult(samples, contPt);
        }
        finally { _gate.Release(); }
    }

    /// <summary>Map <see cref="HistoryAggregateType"/> to the OPC UA Part 13 standard aggregate NodeId.</summary>
    /// <param name="aggregate">The aggregation function type to map.</param>
    internal static NodeId MapAggregateToNodeId(HistoryAggregateType aggregate) => aggregate switch
    {
        HistoryAggregateType.Average => ObjectIds.AggregateFunction_Average,
        HistoryAggregateType.Minimum => ObjectIds.AggregateFunction_Minimum,
        HistoryAggregateType.Maximum => ObjectIds.AggregateFunction_Maximum,
        HistoryAggregateType.Total   => ObjectIds.AggregateFunction_Total,
        HistoryAggregateType.Count   => ObjectIds.AggregateFunction_Count,
        _ => throw new ArgumentOutOfRangeException(nameof(aggregate), aggregate, null),
    };

    // ReadEventsAsync stays at the interface default (throws NotSupportedException) per
    // IHistoryProvider contract -- the OPC UA Client driver CAN forward HistoryReadEvents,
    // but the call-site needs an EventFilter SelectClauses surface which the interface
    // doesn't carry. Landing the event-history passthrough requires extending
    // IHistoryProvider.ReadEventsAsync with a filter-spec parameter; out of scope for this PR.

    // ---- IHostConnectivityProbe ----

    /// <summary>
    ///     Endpoint-URL-keyed host identity for the Admin /hosts dashboard. Reflects the
    ///     endpoint the driver actually connected to after the failover sweep — not the
    ///     first URL in the candidate list — so operators see which of the configured
    ///     endpoints is currently serving traffic. Falls back to the first configured URL
    ///     pre-init so the dashboard has something to render before the first connect.
    /// </summary>
    public string HostName => _connectedEndpointUrl
        ?? ResolveEndpointCandidates(_options).FirstOrDefault()
        ?? _options.EndpointUrl;

    /// <summary>Gets the current connectivity status of the remote OPC UA server host.</summary>
    public IReadOnlyList<HostConnectivityStatus> GetHostStatuses()
    {
        lock (_probeLock)
            return [new HostConnectivityStatus(HostName, _hostState, _hostStateChangedUtc)];
    }

    /// <summary>
    ///     Session keep-alive handler. On a healthy ping, bumps HostState back to Running
    ///     (typical bounce after a transient network blip). On a bad ping, starts the SDK's
    ///     <see cref="SessionReconnectHandler"/> which retries on the configured period +
    ///     fires <see cref="OnReconnectComplete"/> when it lands a new session.
    /// </summary>
    private void OnKeepAlive(ISession sender, KeepAliveEventArgs e)
    {
        if (!ServiceResult.IsBad(e.Status))
        {
            TransitionTo(HostState.Running);
            return;
        }

        TransitionTo(HostState.Stopped);

        // Kick off the SDK's reconnect loop exactly once per drop. Keep-alive callbacks fire
        // from the SDK keep-alive timer thread and the SDK can fire this handler repeatedly
        // while the channel stays down — the check-then-set must be atomic, otherwise two
        // callbacks both observe null, both construct a SessionReconnectHandler, and the
        // second assignment leaks the first (its retry loop keeps running, unreferenced and
        // never disposed). Guard with _probeLock (Driver.OpcUaClient-005).
        SessionReconnectHandler handler;
        lock (_probeLock)
        {
            if (_reconnectHandler is not null || _disposed) return;
            handler = new SessionReconnectHandler(telemetry: null!,
                reconnectAbort: false,
                maxReconnectPeriod: (int)TimeSpan.FromMinutes(2).TotalMilliseconds);
            _reconnectHandler = handler;
        }

        // BeginReconnect is started outside the lock — it does no _reconnectHandler mutation
        // and we don't want to hold _probeLock across an SDK call.
        handler.BeginReconnect(
            sender,
            (int)_options.ReconnectPeriod.TotalMilliseconds,
            OnReconnectComplete);
    }

    /// <summary>
    ///     Called by <see cref="SessionReconnectHandler"/> when its retry loop has either
    ///     successfully swapped to a new session or given up. Reads the new session off
    ///     <c>handler.Session</c>, unwires the old keep-alive hook, rewires for the new
    ///     one, and tears down the handler. Subscription migration is already handled
    ///     inside the SDK via <c>TransferSubscriptions</c> (the SDK calls it automatically
    ///     when <see cref="Session.TransferSubscriptionsOnReconnect"/> is <c>true</c>,
    ///     which is the default).
    /// </summary>
    private void OnReconnectComplete(object? sender, EventArgs e)
    {
        if (sender is not SessionReconnectHandler handler) return;
        var newSession = handler.Session;

        // All mutations to Session and _reconnectHandler run under _probeLock so
        // OnReconnectComplete, OnKeepAlive, and ShutdownAsync cannot race each other:
        // a session swap visible to concurrent ReadAsync/WriteAsync/DiscoverAsync callers
        // (which re-read Session inside _gate) must be atomic w.r.t. disposal and
        // re-arming (Driver.OpcUaClient-006).
        ISession? oldSession;
        lock (_probeLock)
        {
            oldSession = Session;

            // Rewire keep-alive before swapping the reference so a hot keep-alive can't
            // fire against the old session after we've already assigned the new one.
            if (oldSession is not null && _keepAliveHandler is not null)
            {
                try { oldSession.KeepAlive -= _keepAliveHandler; } catch { }
            }
            if (newSession is not null && _keepAliveHandler is not null)
            {
                newSession.KeepAlive += _keepAliveHandler;
            }

            Session = newSession;

            // Retire the handler that just finished.
            if (ReferenceEquals(_reconnectHandler, handler))
            {
                _reconnectHandler.Dispose();
                _reconnectHandler = null;
            }
        }

        if (newSession is not null)
        {
            // Reconnect succeeded. Rebuild the namespace map from the *new* session: the
            // remote server may have reordered its namespace table across the restart that
            // caused the drop (driver-specs.md §8). Stable nsu= references stored in the
            // address space re-resolve correctly against this fresh map.
            _namespaceMap = NamespaceMap.FromSession(newSession);
            TransitionTo(HostState.Running);
            _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
            return;
        }

        // The reconnect handler gave up — its retry loop exhausted the 2-minute
        // maxReconnectPeriod and invoked the callback with a null Session. Without an
        // explicit Faulted signal the driver is permanently wedged: no session, no live
        // keep-alive to re-trigger OnKeepAlive, and the Core never learns it must offer an
        // operator reinitialize (Driver.OpcUaClient-002). Surface Faulted so the Core fans
        // out Bad quality and ReinitializeAsync becomes available, and arm a fresh reconnect
        // attempt against the last-known session for an always-on gateway rather than
        // abandoning recovery entirely.
        TransitionTo(HostState.Faulted);
        _health = new DriverHealth(
            DriverState.Faulted, _health.LastSuccessfulRead,
            "OPC UA session reconnect exhausted its retry window without recovering. " +
            "The remote server is unreachable; reinitialize the driver once it is back.");

        if (oldSession is not null && !_disposed)
            TryRearmReconnect(handler, oldSession);
    }

    /// <summary>
    ///     Arm a fresh reconnect attempt after a previous handler gave up. The OPC UA Client
    ///     driver gateways an always-on remote server, so abandoning recovery permanently is
    ///     the wrong default — a new <see cref="SessionReconnectHandler"/> keeps retrying so
    ///     the driver self-heals when the remote returns, while the Faulted health set by the
    ///     caller still lets an operator force a clean reinitialize in the meantime.
    /// </summary>
    private void TryRearmReconnect(SessionReconnectHandler exhausted, ISession lastSession)
    {
        SessionReconnectHandler handler;
        lock (_probeLock)
        {
            // Only re-arm if no other handler took over and we aren't shutting down.
            if (_disposed || (_reconnectHandler is not null && !ReferenceEquals(_reconnectHandler, exhausted)))
                return;
            handler = new SessionReconnectHandler(telemetry: null!,
                reconnectAbort: false,
                maxReconnectPeriod: (int)TimeSpan.FromMinutes(2).TotalMilliseconds);
            _reconnectHandler = handler;
        }

        try
        {
            handler.BeginReconnect(
                lastSession,
                (int)_options.ReconnectPeriod.TotalMilliseconds,
                OnReconnectComplete);
        }
        catch
        {
            // If the SDK refuses to re-arm (e.g. the last session is fully torn down), drop
            // the handler so a later operator ReinitializeAsync isn't blocked by a stale one.
            lock (_probeLock)
            {
                if (ReferenceEquals(_reconnectHandler, handler))
                {
                    _reconnectHandler.Dispose();
                    _reconnectHandler = null;
                }
            }
        }
    }

    private void TransitionTo(HostState newState)
    {
        HostState old;
        lock (_probeLock)
        {
            old = _hostState;
            if (old == newState) return;
            _hostState = newState;
            _hostStateChangedUtc = DateTime.UtcNow;
        }
        OnHostStatusChanged?.Invoke(this, new HostStatusChangedEventArgs(HostName, old, newState));
    }

    /// <summary>
    ///     Synchronous disposal. Cancels the reconnect handler and detaches the keep-alive
    ///     hook synchronously (no async work on this hot path), then fires the cert-validation
    ///     handler detach. The async session-close is intentionally skipped — it requires a
    ///     live session + network round-trip and is unsafe to block-on from a potentially
    ///     single-threaded context (OPC UA stack thread). The session will be cleaned up by
    ///     the SDK's own finalizer on GC (Driver.OpcUaClient-007: no sync-over-async).
    /// </summary>
    public void Dispose()
    {
        if (_disposed) return;
        _disposed = true;

        // Cancel any in-flight reconnect handler.
        SessionReconnectHandler? handlerToCancel;
        lock (_probeLock)
        {
            handlerToCancel = _reconnectHandler;
            _reconnectHandler = null;
            // Detach keep-alive and null Session so in-flight gated callers see null
            // after their next _gate.WaitAsync — they return BadCommunicationError cleanly.
            if (_keepAliveHandler is not null && Session is not null)
            {
                try { Session.KeepAlive -= _keepAliveHandler; } catch { }
            }
            _keepAliveHandler = null;
            Session = null;
        }
        try { handlerToCancel?.CancelReconnect(); } catch { }
        handlerToCancel?.Dispose();

        // Detach the cert-validation handler registered during InitializeAsync so the
        // CertificateValidator (which may be process-shared) doesn't hold a reference to
        // a disposed driver (Driver.OpcUaClient-012).
        if (_certValidationHandler is not null && _certValidatorRef is not null)
        {
            try { _certValidatorRef.CertificateValidation -= _certValidationHandler; } catch { }
            _certValidationHandler = null;
            _certValidatorRef = null;
        }

        // Acquire the gate once so any in-flight gated operation (ReadAsync / WriteAsync /
        // DiscoverAsync) has definitely released before we dispose the gate. Without this
        // drain, a background read that calls _gate.Release() after Dispose throws
        // ObjectDisposedException (Driver.OpcUaClient-007).
        try
        {
            if (_gate.Wait(TimeSpan.FromSeconds(2)))
                _gate.Release();
        }
        catch { /* timeout or already disposed — proceed */ }
        _gate.Dispose();
    }

    /// <summary>Asynchronously disposes the driver and releases all associated resources.</summary>
    public async ValueTask DisposeAsync()
    {
        if (_disposed) return;
        _disposed = true;
        try { await ShutdownAsync(CancellationToken.None).ConfigureAwait(false); }
        catch { /* disposal is best-effort */ }

        // Detach the cert-validation handler (Driver.OpcUaClient-012).
        if (_certValidationHandler is not null && _certValidatorRef is not null)
        {
            try { _certValidatorRef.CertificateValidation -= _certValidationHandler; } catch { }
            _certValidationHandler = null;
            _certValidatorRef = null;
        }

        // Drain the gate before disposal so no in-flight _gate.Release() fires after
        // Dispose (Driver.OpcUaClient-007).
        try
        {
            await _gate.WaitAsync(TimeSpan.FromSeconds(2)).ConfigureAwait(false);
            _gate.Release();
        }
        catch { /* timeout or already disposed */ }
        _gate.Dispose();
    }
}