using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.Abstractions;
using MxGateway.Client;
using MxGateway.Contracts.Proto;
using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Config;
using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Health;
using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;

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

/// <summary>
///     In-process .NET 10 Galaxy driver — the v2 replacement for the Galaxy.Host /
///     Galaxy.Proxy pair. PR 4.0 ships the project skeleton with <see cref="IDriver"/>
///     bodies that wire to a future <c>IGalaxyGatewayClient</c> abstraction. Capability
///     interfaces (browse, read, write, subscribe, history routing, host probes) land in
///     PRs 4.1–4.7; the wiring sequence keeps every intermediate state buildable so the
///     <c>Galaxy:Backend</c> flag (PR 4.W) can flip between legacy-host and mxgateway
///     for parity testing.
/// </summary>
/// <remarks>
///     This driver is registered as a Tier A in-process driver alongside Modbus / S7 / etc.
///     The legacy <c>GalaxyProxyDriver</c> (Driver.Galaxy.Proxy) coexists until PR 7.2;
///     <see cref="GalaxyDriverFactoryExtensions"/> registers under driver-type name
///     "GalaxyMxGateway" so both paths can be live simultaneously during parity testing.
/// </remarks>
public sealed class GalaxyDriver
    : IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IRediscoverable, IHostConnectivityProbe, IDisposable
{
    private readonly string _driverInstanceId;
    private readonly GalaxyDriverOptions _options;
    private readonly ILogger<GalaxyDriver> _logger;

    // PR 4.1 — IGalaxyHierarchySource is the test seam for browse. When null, the driver
    // lazily builds a GatewayGalaxyHierarchySource around a GalaxyRepositoryClient on
    // first DiscoverAsync. Tests inject a fake source via the internal ctor to exercise
    // GalaxyDiscoverer's translation logic without a real gRPC channel.
    private IGalaxyHierarchySource? _hierarchySource;
    private GalaxyRepositoryClient? _ownedRepositoryClient;

    // PR 4.2 — IGalaxyDataReader is the test seam for IReadable. PR 4.4 supplies the
    // production implementation that wraps GalaxyMxSession's SubscribeBulk + StreamEvents
    // pump; until then ReadAsync throws NotSupportedException when the reader is null
    // (legacy-host backend handles reads in production via DriverNodeManager's
    // capability-routing).
    private IGalaxyDataReader? _dataReader;

    // PR 4.3 — IGalaxyDataWriter is the test seam for IWritable. Production wraps
    // GalaxyMxSession via GatewayGalaxyDataWriter (Write / WriteSecured routing). The
    // per-tag SecurityClassification map is populated during ITagDiscovery and consumed
    // here at write time.
    private IGalaxyDataWriter? _dataWriter;
    private readonly System.Collections.Concurrent.ConcurrentDictionary<string, SecurityClassification>
        _securityByFullRef = new(StringComparer.OrdinalIgnoreCase);

    // PR 4.4 — subscription lifecycle. The pump consumes the gw event stream and fans
    // out OnDataChange events to every registered driver subscription via the registry's
    // reverse map. The subscriber is the test seam — production uses
    // GatewayGalaxySubscriber over a connected GalaxyMxSession.
    private IGalaxySubscriber? _subscriber;
    private readonly SubscriptionRegistry _subscriptions = new();
    private EventPump? _eventPump;
    private readonly Lock _pumpLock = new();

    // PR 4.W — production runtime owned by InitializeAsync. The driver builds these
    // when it opens a real gw session; tests bypass them by injecting seams via the
    // internal ctor.
    private GalaxyMxSession? _ownedMxSession;
    private MxGatewayClient? _ownedMxClient;

    // PR 4.5 — reconnect supervisor. Reflects in DriverState.Degraded while not Healthy.
    private ReconnectSupervisor? _supervisor;

    // PR 4.6 — IRediscoverable plumbing.
    private DeployWatcher? _deployWatcher;

    // PR 4.7 — IHostConnectivityProbe plumbing. The aggregator owns the merged
    // transport+per-platform view; the forwarder is fed from the supervisor on
    // transport state transitions; the probe watcher subscribes ScanState attributes
    // for every discovered platform and pushes value changes to the aggregator.
    private readonly HostStatusAggregator _hostStatuses = new();
    private HostConnectivityForwarder? _transportForwarder;
    private PerPlatformProbeWatcher? _probeWatcher;

    private DriverHealth _health = new(DriverState.Unknown, null, null);
    private bool _disposed;

    /// <summary>
    ///     Server-pushed data-change notification. Fires from the
    ///     <see cref="EventPump"/>'s background loop; handlers should be cheap (or queue
    ///     onto another thread) to avoid blocking the gw event stream.
    /// </summary>
    public event EventHandler<DataChangeEventArgs>? OnDataChange;

    /// <summary>Fires when the gateway signals a deploy-time change (PR 4.6 DeployWatcher).</summary>
    public event EventHandler<RediscoveryEventArgs>? OnRediscoveryNeeded;

    /// <summary>Fires when a host transitions Running ↔ Stopped (PR 4.7 HostStatusAggregator).</summary>
    public event EventHandler<HostStatusChangedEventArgs>? OnHostStatusChanged;

    public GalaxyDriver(
        string driverInstanceId,
        GalaxyDriverOptions options,
        ILogger<GalaxyDriver>? logger = null)
        : this(driverInstanceId, options,
              hierarchySource: null, dataReader: null, dataWriter: null, subscriber: null, logger)
    {
    }

    /// <summary>
    ///     Test-visible ctor — inject custom seams so <see cref="DiscoverAsync"/>,
    ///     <see cref="ReadAsync"/>, <see cref="WriteAsync"/>, and
    ///     <see cref="SubscribeAsync"/> can be exercised against canned data without
    ///     building real gRPC channels.
    /// </summary>
    internal GalaxyDriver(
        string driverInstanceId,
        GalaxyDriverOptions options,
        IGalaxyHierarchySource? hierarchySource,
        IGalaxyDataReader? dataReader = null,
        IGalaxyDataWriter? dataWriter = null,
        IGalaxySubscriber? subscriber = null,
        ILogger<GalaxyDriver>? logger = null)
    {
        _driverInstanceId = !string.IsNullOrWhiteSpace(driverInstanceId)
            ? driverInstanceId
            : throw new ArgumentException("Driver instance id required.", nameof(driverInstanceId));
        _options = options ?? throw new ArgumentNullException(nameof(options));
        _logger = logger ?? NullLogger<GalaxyDriver>.Instance;
        _hierarchySource = hierarchySource;
        _dataReader = dataReader;
        _dataWriter = dataWriter;
        _subscriber = subscriber;

        // Forward the aggregator's transitions through IHostConnectivityProbe.
        _hostStatuses.OnHostStatusChanged += (_, args) => OnHostStatusChanged?.Invoke(this, args);
    }

    /// <inheritdoc />
    public string DriverInstanceId => _driverInstanceId;

    /// <inheritdoc />
    public string DriverType => GalaxyDriverFactoryExtensions.DriverTypeName;

    /// <summary>Test-visible options snapshot.</summary>
    internal GalaxyDriverOptions Options => _options;

    /// <inheritdoc />
    public async Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
    {
        ObjectDisposedException.ThrowIf(_disposed, this);

        // Tests inject seams via the internal ctor; production InitializeAsync builds
        // the gateway client + session + per-capability runtime components from
        // GalaxyDriverOptions. When seams are pre-injected we leave them alone (the
        // test exercises the wired surface without a real gw round-trip).
        if (_subscriber is null && _dataWriter is null && _hierarchySource is null)
        {
            await BuildProductionRuntimeAsync(cancellationToken).ConfigureAwait(false);
        }
        else
        {
            _logger.LogDebug(
                "GalaxyDriver {InstanceId} initializing with pre-injected seams — production runtime build skipped",
                _driverInstanceId);
        }

        StartDeployWatcher();
        _logger.LogInformation(
            "GalaxyDriver {InstanceId} initialized — endpoint={Endpoint} clientName={ClientName}",
            _driverInstanceId, _options.Gateway.Endpoint, _options.MxAccess.ClientName);
        _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
    }

    /// <summary>
    ///     Build the production gw client + session + per-capability runtime components
    ///     from <c>_options</c>. Sets up the reconnect supervisor's reopen / replay
    ///     callbacks so a transport drop replays every active subscription on the
    ///     restored session.
    /// </summary>
    private async Task BuildProductionRuntimeAsync(CancellationToken cancellationToken)
    {
        var clientOptions = BuildClientOptions(_options.Gateway);
        _ownedMxClient = MxGatewayClient.Create(clientOptions);
        _ownedMxSession = new GalaxyMxSession(_options.MxAccess, _logger);
        await _ownedMxSession.ConnectAsync(clientOptions, cancellationToken).ConfigureAwait(false);

        // PR 6.1 — wrap the gw-facing seams in tracing decorators so every Subscribe /
        // Unsubscribe / Write / StreamEvents call emits a span on the
        // "ZB.MOM.WW.OtOpcUa.Driver.Galaxy" ActivitySource. The host process's tracing
        // listener (OTLP exporter, dotnet-trace, etc.) consumes these without the driver
        // taking a dependency on the OpenTelemetry packages.
        _subscriber = new TracedGalaxySubscriber(
            new GatewayGalaxySubscriber(_ownedMxSession), _options.MxAccess.ClientName);
        _dataWriter = new TracedGalaxyDataWriter(
            new GatewayGalaxyDataWriter(_ownedMxSession, _options.MxAccess.WriteUserId, _logger),
            _options.MxAccess.ClientName);

        _supervisor = new ReconnectSupervisor(
            reopen: ReopenAsync,
            replay: ReplayAsync,
            options: new ReconnectOptions(
                InitialBackoffOverride: TimeSpan.FromMilliseconds(_options.Reconnect.InitialBackoffMs),
                MaxBackoffOverride: TimeSpan.FromMilliseconds(_options.Reconnect.MaxBackoffMs)),
            logger: _logger);

        _transportForwarder = new HostConnectivityForwarder(_options.MxAccess.ClientName, _hostStatuses, _logger);
        _transportForwarder.SetTransport(HostState.Running); // initial state — we just connected

        _supervisor.StateChanged += OnSupervisorStateChanged;

        _probeWatcher = new PerPlatformProbeWatcher(
            _subscriber, _hostStatuses, _logger,
            bufferedUpdateIntervalMs: _options.MxAccess.PublishingIntervalMs);
    }

    /// <summary>
    ///     Reopen callback for <see cref="ReconnectSupervisor"/>: re-Register the gw session.
    ///     If the session never connected, this is a fresh ConnectAsync; otherwise it's a
    ///     reconnect against the existing client.
    /// </summary>
    private async Task ReopenAsync(CancellationToken cancellationToken)
    {
        if (_ownedMxSession is null) return;
        var clientOptions = BuildClientOptions(_options.Gateway);
        await _ownedMxSession.ConnectAsync(clientOptions, cancellationToken).ConfigureAwait(false);
    }

    /// <summary>
    ///     Replay callback. Walks every active subscription's bindings and re-issues
    ///     SubscribeBulk for the tag list. PR 6.x can swap this for the gw's batched
    ///     <c>ReplaySubscriptionsCommand</c> once it ships.
    /// </summary>
    private async Task ReplayAsync(CancellationToken cancellationToken)
    {
        if (_subscriber is null) return;
        var bindings = _subscriptions.SnapshotAllBindings();
        if (bindings.Count == 0) return;

        var refs = bindings.Select(b => b.FullReference).Distinct(StringComparer.OrdinalIgnoreCase).ToArray();
        await _subscriber.SubscribeBulkAsync(
            refs, _options.MxAccess.PublishingIntervalMs, cancellationToken).ConfigureAwait(false);
        _logger.LogInformation(
            "GalaxyDriver {InstanceId} replay completed — {Count} tags re-subscribed",
            _driverInstanceId, refs.Length);
    }

    private void OnSupervisorStateChanged(object? sender, StateTransition transition)
    {
        // Reflect supervisor state in DriverHealth + transport forwarder.
        _health = transition.Next switch
        {
            ReconnectSupervisor.State.Healthy => new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null),
            _ => new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, transition.Cause),
        };
        if (_transportForwarder is not null)
        {
            var hostState = transition.Next == ReconnectSupervisor.State.Healthy
                ? HostState.Running
                : HostState.Stopped;
            _transportForwarder.SetTransport(hostState);
        }
    }

    /// <summary>
    ///     Resolves <c>Gateway.ApiKeySecretRef</c> to the actual API-key bytes. Three
    ///     forms supported, evaluated in order:
    ///     <list type="number">
    ///       <item><c>env:NAME</c> — reads <c>Environment.GetEnvironmentVariable(NAME)</c>.
    ///         Throws when the variable is unset, so a misconfigured deployment fails
    ///         fast at InitializeAsync rather than silently sending an empty key.</item>
    ///       <item><c>file:PATH</c> — reads UTF-8 text from <c>PATH</c>, trimming
    ///         whitespace. Lets operators stash the key in an ACL'd file outside the
    ///         repo (the same pattern as the legacy <c>.local/galaxy-host-secret.txt</c>).</item>
    ///       <item>Anything else — used as the literal API key. Convenient for dev,
    ///         and avoids breaking existing configs that pre-date this resolver.</item>
    ///     </list>
    ///     A future PR can swap any of these arms for a DPAPI-backed lookup without
    ///     changing the call site.
    /// </summary>
    internal static string ResolveApiKey(string secretRef)
    {
        ArgumentException.ThrowIfNullOrEmpty(secretRef);

        if (secretRef.StartsWith("env:", StringComparison.OrdinalIgnoreCase))
        {
            var name = secretRef[4..];
            var value = Environment.GetEnvironmentVariable(name);
            return !string.IsNullOrEmpty(value)
                ? value
                : throw new InvalidOperationException(
                    $"Galaxy.Gateway.ApiKeySecretRef='{secretRef}' resolves to env var '{name}', but it is unset.");
        }

        if (secretRef.StartsWith("file:", StringComparison.OrdinalIgnoreCase))
        {
            var path = secretRef[5..];
            if (!File.Exists(path))
            {
                throw new InvalidOperationException(
                    $"Galaxy.Gateway.ApiKeySecretRef='{secretRef}' points at '{path}', which doesn't exist.");
            }
            var contents = File.ReadAllText(path).Trim();
            return !string.IsNullOrEmpty(contents)
                ? contents
                : throw new InvalidOperationException(
                    $"Galaxy.Gateway.ApiKeySecretRef='{secretRef}' file '{path}' is empty.");
        }

        return secretRef;
    }

    private static MxGatewayClientOptions BuildClientOptions(GalaxyGatewayOptions gw) => new()
    {
        Endpoint = new Uri(gw.Endpoint, UriKind.Absolute),
        ApiKey = ResolveApiKey(gw.ApiKeySecretRef),
        UseTls = gw.UseTls,
        CaCertificatePath = gw.CaCertificatePath,
        ConnectTimeout = TimeSpan.FromSeconds(gw.ConnectTimeoutSeconds),
        DefaultCallTimeout = TimeSpan.FromSeconds(gw.DefaultCallTimeoutSeconds),
        StreamTimeout = gw.StreamTimeoutSeconds > 0 ? TimeSpan.FromSeconds(gw.StreamTimeoutSeconds) : null,
    };

    private void StartDeployWatcher()
    {
        if (!_options.Repository.WatchDeployEvents) return;
        if (_ownedRepositoryClient is null && _hierarchySource is null) return;

        // Reuse the lazily-built repository client (DiscoverAsync constructs it on demand).
        // If discovery hasn't run yet, build the client here so the watcher has a target.
        if (_ownedRepositoryClient is null)
        {
            _ownedRepositoryClient = MxGateway.Client.GalaxyRepositoryClient.Create(
                BuildClientOptions(_options.Gateway));
        }

        var source = new GatewayGalaxyDeployWatchSource(_ownedRepositoryClient);
        _deployWatcher = new DeployWatcher(source, _logger);
        _deployWatcher.OnRediscoveryNeeded += (_, args) => OnRediscoveryNeeded?.Invoke(this, args);

        _ = _deployWatcher.StartAsync(CancellationToken.None);
    }

    /// <inheritdoc />
    public Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
    {
        // In-place config reapply. PR 4.5's reconnect supervisor will swap the
        // gateway-client options under the lock; for the skeleton we just refresh health.
        ObjectDisposedException.ThrowIf(_disposed, this);
        _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
        return Task.CompletedTask;
    }

    /// <inheritdoc />
    public Task ShutdownAsync(CancellationToken cancellationToken)
    {
        if (_disposed) return Task.CompletedTask;
        _logger.LogInformation("GalaxyDriver {InstanceId} shutting down", _driverInstanceId);
        _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
        return Task.CompletedTask;
    }

    /// <inheritdoc />
    public DriverHealth GetHealth()
    {
        // Reconnect supervisor wins when degraded — the cached _health reflects the last
        // successful operation, but ongoing recovery should surface as Degraded.
        if (_supervisor?.IsDegraded == true)
        {
            return new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, _supervisor.LastError);
        }
        return _health;
    }

    // ===== IHostConnectivityProbe (PR 4.7 wire-up) =====

    /// <inheritdoc />
    public IReadOnlyList<HostConnectivityStatus> GetHostStatuses() => _hostStatuses.Snapshot();

    /// <inheritdoc />
    public long GetMemoryFootprint() => 0; // PR 4.4 sets this from SubscriptionRegistry size.

    /// <inheritdoc />
    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;

    // ===== ITagDiscovery (PR 4.1) =====

    /// <inheritdoc />
    public async Task DiscoverAsync(IAddressSpaceBuilder builder, CancellationToken cancellationToken)
    {
        ObjectDisposedException.ThrowIf(_disposed, this);
        ArgumentNullException.ThrowIfNull(builder);

        // PR 4.3 — capture SecurityClassification per attribute. PR 4.W — also refresh
        // the per-platform probe watcher's membership after discovery so newly-added
        // $WinPlatform / $AppEngine objects start advising their ScanState attribute.
        var capturingBuilder = new SecurityCapturingBuilder(builder, _securityByFullRef);
        var source = _hierarchySource ??= BuildDefaultHierarchySource();
        var discoverer = new GalaxyDiscoverer(source);
        await discoverer.DiscoverAsync(capturingBuilder, cancellationToken).ConfigureAwait(false);

        if (_probeWatcher is not null)
        {
            var hierarchy = await source.GetHierarchyAsync(cancellationToken).ConfigureAwait(false);
            var platforms = hierarchy
                .Where(o => o.TemplateChain.Any(t =>
                    string.Equals(t, "$WinPlatform", StringComparison.OrdinalIgnoreCase)
                    || string.Equals(t, "$AppEngine", StringComparison.OrdinalIgnoreCase)))
                .Select(o => o.TagName)
                .Where(name => !string.IsNullOrEmpty(name));
            await _probeWatcher.SyncPlatformsAsync(platforms, cancellationToken).ConfigureAwait(false);
        }
    }

    private SecurityClassification ResolveSecurity(string fullReference) =>
        _securityByFullRef.TryGetValue(fullReference, out var sec) ? sec : SecurityClassification.FreeAccess;

    // ===== IReadable =====

    /// <inheritdoc />
    public Task<IReadOnlyList<DataValueSnapshot>> ReadAsync(
        IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
    {
        ObjectDisposedException.ThrowIf(_disposed, this);
        ArgumentNullException.ThrowIfNull(fullReferences);
        if (fullReferences.Count == 0) return Task.FromResult<IReadOnlyList<DataValueSnapshot>>([]);

        if (_dataReader is not null)
        {
            // Test-only path — tests inject a canned reader via the internal ctor.
            return _dataReader.ReadAsync(fullReferences, cancellationToken);
        }

        if (_subscriber is null)
        {
            throw new NotSupportedException(
                "GalaxyDriver.ReadAsync requires a connected GalaxyMxSession (production runtime not built). " +
                "Either inject a test seam via the internal ctor or call InitializeAsync against a real gateway.");
        }

        return ReadViaSubscribeOnceAsync(fullReferences, cancellationToken);
    }

    /// <summary>
    ///     Production read path. MxAccess has no one-shot Read RPC — every value comes
    ///     through the event stream. We synthesise a Read by:
    ///     <list type="number">
    ///       <item>Subscribing the requested tags through the existing
    ///         <see cref="SubscriptionRegistry"/> + <see cref="EventPump"/>.</item>
    ///       <item>Waiting for the first <c>OnDataChange</c> per item handle (the gateway
    ///         pushes the current value as the initial event after a SubscribeBulk).</item>
    ///       <item>Unsubscribing.</item>
    ///     </list>
    ///     Tags the gw rejects at SubscribeBulk time, or that never publish before the
    ///     caller's cancellation token fires, return a Bad-status snapshot in input order
    ///     so the caller still sees one snapshot per requested reference.
    /// </summary>
    private async Task<IReadOnlyList<DataValueSnapshot>> ReadViaSubscribeOnceAsync(
        IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
    {
        var pump = EnsureEventPumpStarted();
        var subscriptionId = _subscriptions.NextSubscriptionId();

        // Pre-allocate one TaskCompletionSource per full-reference so the OnDataChange
        // handler can complete them out-of-order as events arrive. Wired BEFORE the
        // SubscribeBulk call so we don't race with the first event the gw pushes.
        var pendingByRef = new Dictionary<string, TaskCompletionSource<DataValueSnapshot>>(
            StringComparer.OrdinalIgnoreCase);
        foreach (var fullRef in fullReferences.Distinct(StringComparer.OrdinalIgnoreCase))
        {
            pendingByRef[fullRef] = new TaskCompletionSource<DataValueSnapshot>(
                TaskCreationOptions.RunContinuationsAsynchronously);
        }

        EventHandler<DataChangeEventArgs> handler = (_, args) =>
        {
            // Filter to OUR subscription — the pump's OnDataChange fans out across all
            // subscriptions on the driver, and we don't want a parallel ISubscribable
            // caller's events to leak into our read.
            if (args.SubscriptionHandle is GalaxySubscriptionHandle gsh
                && gsh.SubscriptionId == subscriptionId
                && pendingByRef.TryGetValue(args.FullReference, out var tcs))
            {
                tcs.TrySetResult(args.Snapshot);
            }
        };
        pump.OnDataChange += handler;

        var bufferedIntervalMs = _options.MxAccess.PublishingIntervalMs;
        IReadOnlyList<SubscribeResult> results;
        try
        {
            results = await _subscriber!
                .SubscribeBulkAsync(fullReferences, bufferedIntervalMs, cancellationToken)
                .ConfigureAwait(false);
        }
        catch
        {
            pump.OnDataChange -= handler;
            throw;
        }

        // Register bindings so the pump knows to dispatch events for these handles.
        var bindings = new List<TagBinding>(fullReferences.Count);
        for (var i = 0; i < fullReferences.Count; i++)
        {
            var fullRef = fullReferences[i];
            var match = results.FirstOrDefault(r => string.Equals(r.TagAddress, fullRef, StringComparison.OrdinalIgnoreCase));
            var itemHandle = match is { WasSuccessful: true } ? match.ItemHandle : 0;
            bindings.Add(new TagBinding(fullRef, itemHandle));

            // Tags the gw rejected up front — complete with Bad status now so the
            // wait below doesn't time out on them.
            if (itemHandle <= 0
                && pendingByRef.TryGetValue(fullRef, out var rejectedTcs))
            {
                rejectedTcs.TrySetResult(new DataValueSnapshot(
                    Value: null,
                    StatusCode: 0x80000000u, // Bad
                    SourceTimestampUtc: null,
                    ServerTimestampUtc: DateTime.UtcNow));
            }
        }
        _subscriptions.Register(subscriptionId, bindings);

        try
        {
            // Wait for every pending TCS to complete or the caller's CT to fire. When the
            // CT fires before all values arrive, fill the still-pending entries with a
            // Bad-status snapshot rather than throwing — Read semantics let callers see
            // partial results.
            using var registration = cancellationToken.Register(() =>
            {
                foreach (var tcs in pendingByRef.Values)
                {
                    tcs.TrySetResult(new DataValueSnapshot(
                        Value: null,
                        StatusCode: 0x800B0000u, // BadTimeout
                        SourceTimestampUtc: null,
                        ServerTimestampUtc: DateTime.UtcNow));
                }
            });

            var snapshots = new DataValueSnapshot[fullReferences.Count];
            for (var i = 0; i < fullReferences.Count; i++)
            {
                snapshots[i] = await pendingByRef[fullReferences[i]].Task.ConfigureAwait(false);
            }
            return snapshots;
        }
        finally
        {
            pump.OnDataChange -= handler;
            // Drop the bindings + unsubscribe the live handles. UnsubscribeBulkAsync's
            // failure isn't fatal — the registry is already cleared, so any straggling
            // event from the gw would be a no-op fan-out.
            _subscriptions.Remove(subscriptionId);
            var liveHandles = bindings.Where(b => b.ItemHandle > 0).Select(b => b.ItemHandle).ToArray();
            if (liveHandles.Length > 0)
            {
                try
                {
                    await _subscriber!.UnsubscribeBulkAsync(liveHandles, CancellationToken.None)
                        .ConfigureAwait(false);
                }
                catch (Exception ex)
                {
                    _logger.LogWarning(ex,
                        "GalaxyDriver.ReadViaSubscribeOnceAsync UnsubscribeBulk failed for {Count} handle(s) — registry already cleared.",
                        liveHandles.Length);
                }
            }
        }
    }

    // ===== IWritable (PR 4.3) =====

    /// <inheritdoc />
    public Task<IReadOnlyList<WriteResult>> WriteAsync(
        IReadOnlyList<WriteRequest> writes, CancellationToken cancellationToken)
    {
        ObjectDisposedException.ThrowIf(_disposed, this);
        ArgumentNullException.ThrowIfNull(writes);
        if (writes.Count == 0) return Task.FromResult<IReadOnlyList<WriteResult>>([]);

        if (_dataWriter is null)
        {
            // Mirror the IReadable fallback: production write path runs on top of
            // GalaxyMxSession (PR 4.2 skeleton; PR 4.4 wires the live session). Until
            // that lands, deployments selecting Galaxy:Backend=mxgateway can't write.
            throw new NotSupportedException(
                "GalaxyDriver.WriteAsync requires GatewayGalaxyDataWriter wired against a connected " +
                "GalaxyMxSession (PR 4.4). Until that lands, route writes through the legacy-host " +
                "backend (Galaxy:Backend=legacy-host).");
        }

        return _dataWriter.WriteAsync(writes, ResolveSecurity, cancellationToken);
    }

    // ===== ISubscribable (PR 4.4) =====

    /// <inheritdoc />
    public async Task<ISubscriptionHandle> SubscribeAsync(
        IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken)
    {
        ObjectDisposedException.ThrowIf(_disposed, this);
        ArgumentNullException.ThrowIfNull(fullReferences);

        if (_subscriber is null)
        {
            throw new NotSupportedException(
                "GalaxyDriver.SubscribeAsync requires a connected GalaxyMxSession + GatewayGalaxySubscriber. " +
                "PR 4.W wires the production session; until then route subscriptions through the legacy-host backend.");
        }

        var pump = EnsureEventPumpStarted();
        var subscriptionId = _subscriptions.NextSubscriptionId();

        if (fullReferences.Count == 0)
        {
            // Empty subscriptions register but never bind anything — keeps Unsubscribe
            // symmetric for callers that conditionally add tags later.
            _subscriptions.Register(subscriptionId, []);
            return new GalaxySubscriptionHandle(subscriptionId);
        }

        // PR 6.3 — when the caller doesn't set a publishing interval (TimeSpan.Zero or
        // negative), fall back to the configured MxAccess.PublishingIntervalMs. The
        // server's UA subscription publishingInterval drives this in production; tests
        // and infrastructure callers (probe watcher, deploy watcher) hit the fallback.
        var requested = (int)Math.Max(0, publishingInterval.TotalMilliseconds);
        var bufferedIntervalMs = requested > 0 ? requested : _options.MxAccess.PublishingIntervalMs;
        var results = await _subscriber
            .SubscribeBulkAsync(fullReferences, bufferedIntervalMs, cancellationToken)
            .ConfigureAwait(false);

        // Build the binding list in input order. Failed entries (gw rejected the tag) are
        // recorded with a non-positive ItemHandle so the caller can detect partial failure
        // by inspecting the returned handle's diagnostic context — full per-tag error
        // surface lands in PR 5.3's parity tests.
        var bindings = new List<TagBinding>(fullReferences.Count);
        for (var i = 0; i < fullReferences.Count; i++)
        {
            var fullRef = fullReferences[i];
            var match = results.FirstOrDefault(r => string.Equals(r.TagAddress, fullRef, StringComparison.OrdinalIgnoreCase));
            var itemHandle = match is { WasSuccessful: true } ? match.ItemHandle : 0;
            bindings.Add(new TagBinding(fullRef, itemHandle));
            if (match is null || !match.WasSuccessful)
            {
                _logger.LogWarning(
                    "Galaxy subscribe for {FullRef} failed: {Error}",
                    fullRef, match?.ErrorMessage ?? "<no result returned>");
            }
        }

        _subscriptions.Register(subscriptionId, bindings);
        _ = pump; // keep the pump alive for the subscription's lifetime
        return new GalaxySubscriptionHandle(subscriptionId);
    }

    /// <inheritdoc />
    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
    {
        ObjectDisposedException.ThrowIf(_disposed, this);
        ArgumentNullException.ThrowIfNull(handle);
        if (handle is not GalaxySubscriptionHandle gsh)
        {
            throw new ArgumentException(
                $"Subscription handle was not issued by this driver (expected GalaxySubscriptionHandle, got {handle.GetType().Name}).",
                nameof(handle));
        }

        var bindings = _subscriptions.Remove(gsh.SubscriptionId);
        if (bindings is null) return; // already removed or never registered

        var liveItemHandles = bindings.Where(b => b.ItemHandle > 0).Select(b => b.ItemHandle).ToArray();
        if (liveItemHandles.Length == 0 || _subscriber is null) return;

        try
        {
            await _subscriber.UnsubscribeBulkAsync(liveItemHandles, cancellationToken).ConfigureAwait(false);
        }
        catch (Exception ex)
        {
            _logger.LogWarning(ex,
                "Galaxy UnsubscribeBulk failed for subscription {SubscriptionId} — registry already cleared on driver side.",
                gsh.SubscriptionId);
        }
    }

    /// <summary>
    ///     Lazily start the <see cref="EventPump"/> on the first subscribe. The pump is
    ///     shared across every subscription on this driver — fan-out happens through the
    ///     <see cref="SubscriptionRegistry"/> reverse map, not by spinning a pump per
    ///     subscription.
    /// </summary>
    private EventPump EnsureEventPumpStarted()
    {
        lock (_pumpLock)
        {
            if (_eventPump is not null) return _eventPump;
            _eventPump = new EventPump(
                _subscriber!, _subscriptions, _logger,
                channelCapacity: _options.MxAccess.EventPumpChannelCapacity,
                clientName: _options.MxAccess.ClientName);
            _eventPump.OnDataChange += OnPumpDataChange;
            _eventPump.Start();
            return _eventPump;
        }
    }

    /// <summary>
    ///     Forwards every fan-out event to the public <see cref="OnDataChange"/> for
    ///     ISubscribable consumers, AND routes ScanState changes to the per-platform
    ///     probe watcher (PR 4.7) so platform health entries update without the watcher
    ///     consuming the event stream itself.
    /// </summary>
    private void OnPumpDataChange(object? sender, DataChangeEventArgs args)
    {
        OnDataChange?.Invoke(this, args);

        if (_probeWatcher is not null
            && args.FullReference.EndsWith(PerPlatformProbeWatcher.ProbeSuffix, StringComparison.OrdinalIgnoreCase))
        {
            // The probe decoder takes a raw quality byte; recover it from the StatusCode
            // top byte (Good=0x00 → byte 192, Uncertain=0x40 → byte 64, Bad=0x80 → byte 0).
            var qualityByte = (byte)((args.Snapshot.StatusCode >> 30) & 0x3) switch
            {
                0 => 192,
                1 => 64,
                _ => 0,
            };
            _probeWatcher.OnProbeValueChanged(args.FullReference, args.Snapshot.Value, (byte)qualityByte);
        }
    }

    /// <summary>
    ///     Lazily builds the default <see cref="IGalaxyHierarchySource"/> from
    ///     <c>_options.Gateway</c>. Owned <see cref="GalaxyRepositoryClient"/> is disposed in
    ///     <see cref="Dispose"/>. Tests bypass this by injecting their own source via the
    ///     internal ctor.
    /// </summary>
    private IGalaxyHierarchySource BuildDefaultHierarchySource()
    {
        var gw = _options.Gateway;
        var clientOptions = new MxGatewayClientOptions
        {
            Endpoint = new Uri(gw.Endpoint, UriKind.Absolute),
            ApiKey = ResolveApiKey(gw.ApiKeySecretRef),
            UseTls = gw.UseTls,
            CaCertificatePath = gw.CaCertificatePath,
            ConnectTimeout = TimeSpan.FromSeconds(gw.ConnectTimeoutSeconds),
            DefaultCallTimeout = TimeSpan.FromSeconds(gw.DefaultCallTimeoutSeconds),
            StreamTimeout = gw.StreamTimeoutSeconds > 0
                ? TimeSpan.FromSeconds(gw.StreamTimeoutSeconds)
                : null,
        };
        _ownedRepositoryClient = GalaxyRepositoryClient.Create(clientOptions);
        return new TracedGalaxyHierarchySource(
            new GatewayGalaxyHierarchySource(_ownedRepositoryClient), _options.MxAccess.ClientName);
    }

    public void Dispose()
    {
        if (_disposed) return;
        _disposed = true;

        // Order: stop deploy watcher, supervisor, probe watcher, pump, then sessions and
        // clients. Each step is best-effort — disposal during a faulted state shouldn't
        // throw and prevent the rest of the cleanup.
        try { _deployWatcher?.Dispose(); } catch (Exception ex) { _logger.LogWarning(ex, "DeployWatcher dispose failed"); }
        try { _supervisor?.Dispose(); } catch (Exception ex) { _logger.LogWarning(ex, "ReconnectSupervisor dispose failed"); }
        try { _probeWatcher?.Dispose(); } catch (Exception ex) { _logger.LogWarning(ex, "ProbeWatcher dispose failed"); }
        try { _transportForwarder?.Dispose(); } catch (Exception ex) { _logger.LogWarning(ex, "Transport forwarder dispose failed"); }

        EventPump? pump;
        lock (_pumpLock) { pump = _eventPump; _eventPump = null; }
        pump?.DisposeAsync().AsTask().GetAwaiter().GetResult();

        _ownedMxSession?.DisposeAsync().AsTask().GetAwaiter().GetResult();
        _ownedMxSession = null;

        _ownedMxClient?.DisposeAsync().AsTask().GetAwaiter().GetResult();
        _ownedMxClient = null;

        _ownedRepositoryClient?.DisposeAsync().AsTask().GetAwaiter().GetResult();
        _ownedRepositoryClient = null;
        _hierarchySource = null;
    }

    /// <summary>
    ///     Address-space builder wrapper that records each variable's
    ///     <see cref="DriverAttributeInfo.SecurityClass"/> into the supplied dictionary
    ///     before delegating to the inner builder. Used by <see cref="DiscoverAsync"/>
    ///     to capture per-tag classifications for the IWritable routing decision —
    ///     PR 4.3 needs the data, but the discoverer itself doesn't (and shouldn't)
    ///     know about the driver's internal state.
    /// </summary>
    private sealed class SecurityCapturingBuilder(
        IAddressSpaceBuilder inner,
        System.Collections.Concurrent.ConcurrentDictionary<string, SecurityClassification> map)
        : IAddressSpaceBuilder
    {
        public IAddressSpaceBuilder Folder(string browseName, string displayName)
            => new SecurityCapturingBuilder(inner.Folder(browseName, displayName), map);

        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo attributeInfo)
        {
            map[attributeInfo.FullName] = attributeInfo.SecurityClass;
            return inner.Variable(browseName, displayName, attributeInfo);
        }

        public void AddProperty(string browseName, DriverDataType dataType, object? value)
            => inner.AddProperty(browseName, dataType, value);
    }
}