using System.Text.Json;
using Moq;
using Opc.Ua;
using Opc.Ua.Client;
using Shouldly;
using Xunit;
using ZB.MOM.WW.OtOpcUa.Core.Abstractions;

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

/// <summary>
///     Gitea #670 — upstream session loss must reach the driver's health and its subscribers. Before the fix a bad
///     keep-alive only flipped the host status: the subscribed values stayed Good forever, health stayed Healthy (so the
///     #527 comms-loss watchdog could never trip — its first conjunct is "non-Healthy"), and <c>LastSuccessfulRead</c>
///     only moved on init / <c>ReadAsync</c>, which a subscription-only driver never calls.
/// </summary>
[Trait("Category", "Unit")]
public sealed class OpcUaClientSessionLossTests
{
    private const uint BadNoCommunication = 0x80310000u;
    private const string SpeedRef = "Plant/dev/Speed";
    private const string CounterRef = "Plant/dev/Counter";

    private sealed class ManualClock(DateTimeOffset start) : TimeProvider
    {
        public DateTimeOffset Now { get; set; } = start;
        public override DateTimeOffset GetUtcNow() => Now;
        public void Advance(TimeSpan by) => Now += by;
    }

    private static RawTagEntry Tag(string rawPath, string nodeId) =>
        new(rawPath, JsonSerializer.Serialize(new { nodeId }), WriteIdempotent: true);

    private static readonly IReadOnlyList<RawTagEntry> Tags =
    [
        Tag(SpeedRef, "ns=0;s=Speed"),
        Tag(CounterRef, "ns=0;s=Counter"),
    ];

    private static Mock<ISession> NewSession()
    {
        var mock = new Mock<ISession>(MockBehavior.Loose);
        mock.SetupGet(s => s.MessageContext)
            .Returns(new ServiceMessageContext(SdkTelemetry.Context));
        mock.SetupGet(s => s.NamespaceUris).Returns(new NamespaceTable());
        return mock;
    }

    private static (OpcUaClientDriver Driver, Mock<ISession> Session, ManualClock Clock) NewConnectedDriver(
        string driverInstanceId = "opcua-670")
    {
        var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions { RawTags = Tags }, driverInstanceId);
        var session = NewSession();
        var clock = new ManualClock(new DateTimeOffset(2026, 10, 11, 12, 0, 0, TimeSpan.Zero));
        drv.Clock = clock;
        drv.SetSessionForTest(session.Object);
        drv.SetNamespaceMapForTest(NamespaceMap.FromTable(new NamespaceTable(), Tags));
        drv.SetHealthForTest(new DriverHealth(DriverState.Healthy, clock.Now.UtcDateTime, null));
        return (drv, session, clock);
    }

    private static DataValue Good(object value, DateTime ts) =>
        new(new Variant(value), StatusCodes.Good, ts, ts);

    private static void SetupRead(Mock<ISession> session, params object[] values) =>
        session.Setup(s => s.ReadAsync(
                It.IsAny<RequestHeader>(), It.IsAny<double>(), It.IsAny<TimestampsToReturn>(),
                It.IsAny<ReadValueIdCollection>(), It.IsAny<CancellationToken>()))
            .ReturnsAsync((RequestHeader _, double _, TimestampsToReturn _, ReadValueIdCollection nodes, CancellationToken _) =>
                new ReadResponse
                {
                    ResponseHeader = new ResponseHeader(),
                    Results = [.. nodes.Select((_, i) => new DataValue(new Variant(values[i]), StatusCodes.Good, DateTime.UtcNow, DateTime.UtcNow))],
                    DiagnosticInfos = [],
                });

    private static async Task WaitForAsync(Func<bool> condition, string because)
    {
        var deadline = DateTime.UtcNow + TimeSpan.FromSeconds(10);
        while (DateTime.UtcNow < deadline)
        {
            if (condition()) return;
            await Task.Delay(10);
        }
        condition().ShouldBeTrue(because);
    }

    [Fact]
    public void A_bad_keep_alive_degrades_health_and_preserves_the_last_successful_read()
    {
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        var before = drv.GetHealth().LastSuccessfulRead;

        clock.Advance(TimeSpan.FromSeconds(5));
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);

        var h = drv.GetHealth();
        h.State.ShouldNotBe(DriverState.Healthy, "a lost session must be visible to the #527 watchdog's first conjunct");
        h.State.ShouldBe(DriverState.Degraded);
        h.LastSuccessfulRead.ShouldBe(before, "the watchdog measures silence from this stamp, so loss must not move it");
        h.LastError.ShouldNotBeNull().ShouldContain("keep-alive");
    }

    [Fact]
    public void A_bad_keep_alive_publishes_BadNoCommunication_once_per_ref_retaining_the_value()
    {
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef, CounterRef]);
        var t0 = clock.Now.UtcDateTime;
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(42.5f, t0));
        drv.DeliverNotificationForTest(handle, CounterRef, Good(7, t0));

        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);

        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);

        seen.Count.ShouldBe(2, "exactly one Bad per subscribed ref");
        var speed = seen.Single(e => e.FullReference == SpeedRef);
        speed.Snapshot.StatusCode.ShouldBe(BadNoCommunication);
        speed.Snapshot.Value.ShouldBe(42.5f, "the value is retained — only the quality changes (as the host does)");
        speed.SubscriptionHandle.ShouldBe(handle);
        seen.Single(e => e.FullReference == CounterRef).Snapshot.Value.ShouldBe(7);

        // The SDK re-raises a bad keep-alive every interval while the channel stays down: no further publishes.
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);
        drv.RaiseKeepAliveForTest(StatusCodes.BadConnectionClosed);
        seen.Count.ShouldBe(2, "a second bad keep-alive in the same outage must publish nothing");
    }

    [Fact]
    public void Good_keep_alives_and_notifications_advance_the_last_successful_read()
    {
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);

        clock.Advance(TimeSpan.FromSeconds(30));
        drv.RaiseKeepAliveForTest(StatusCodes.Good);
        drv.GetHealth().LastSuccessfulRead.ShouldBe(clock.Now.UtcDateTime);

        clock.Advance(TimeSpan.FromSeconds(30));
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(1.0f, clock.Now.UtcDateTime));
        drv.GetHealth().LastSuccessfulRead.ShouldBe(clock.Now.UtcDateTime);
        drv.GetHealth().State.ShouldBe(DriverState.Healthy);
    }

    /// <summary>
    ///     Falsification control for the R2 half of the fix: a long-running, idle, subscription-only driver whose
    ///     server is perfectly alive must never look dead to the #527 watchdog (non-Healthy AND silent ≥ 90 s).
    ///     Without good keep-alives advancing <c>LastSuccessfulRead</c>, fixing R1 alone would trip it on the first
    ///     bad tick of any connection older than 90 s.
    /// </summary>
    [Fact]
    public void An_idle_healthy_subscription_driver_never_looks_dead_to_the_watchdog()
    {
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        drv.AddSubscriptionForTest([SpeedRef]); // static value — no notifications ever arrive

        for (var i = 0; i < 60; i++) // 5 minutes of 5 s keep-alives
        {
            clock.Advance(TimeSpan.FromSeconds(5));
            drv.RaiseKeepAliveForTest(StatusCodes.Good);
            var h = drv.GetHealth();
            h.State.ShouldBe(DriverState.Healthy);
            (clock.Now.UtcDateTime - h.LastSuccessfulRead!.Value).ShouldBeLessThan(TimeSpan.FromSeconds(90));
        }
    }

    [Fact]
    public async Task Recovery_on_the_same_session_republishes_the_identical_value_Good()
    {
        var (drv, session, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(42.5f, clock.Now.UtcDateTime));
        SetupRead(session, 42.5f); // the server still holds the SAME value — it will not resend it

        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => { lock (seen) seen.Add(e); };

        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);
        clock.Advance(TimeSpan.FromSeconds(20));
        drv.RaiseKeepAliveForTest(StatusCodes.Good);

        drv.GetHealth().State.ShouldBe(DriverState.Healthy);
        drv.GetHealth().LastSuccessfulRead.ShouldBe(clock.Now.UtcDateTime);
        await WaitForAsync(() => { lock (seen) return seen.Count >= 2; }, "the recovery read must publish");

        lock (seen)
        {
            seen.Count.ShouldBe(2);
            seen[0].Snapshot.StatusCode.ShouldBe(BadNoCommunication);
            seen[1].FullReference.ShouldBe(SpeedRef);
            seen[1].SubscriptionHandle.ShouldBe(handle);
            seen[1].Snapshot.StatusCode.ShouldBe(0u);
            seen[1].Snapshot.Value.ShouldBe(42.5f, "the IDENTICAL value must come back Good — a (value,status) dedup lets it through");
        }
    }

    [Fact]
    public async Task A_notification_during_recovery_wins_over_the_recovery_read()
    {
        var (drv, session, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(1.0f, clock.Now.UtcDateTime));
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);

        // A fresh notification arrives before the keep-alive recovers: the read must not overwrite it.
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(2.0f, clock.Now.UtcDateTime));
        SetupRead(session, 1.0f);
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => { lock (seen) seen.Add(e); };
        drv.RaiseKeepAliveForTest(StatusCodes.Good);

        await Task.Delay(200, TestContext.Current.CancellationToken);
        lock (seen) seen.ShouldBeEmpty();
    }

    [Fact]
    public void A_bad_keep_alive_annotates_subscribed_alarm_conditions_quality_only_once()
    {
        var (drv, _, _) = NewConnectedDriver();
        using var _ = drv;
        drv.AddAlarmSubscriptionForTest([SpeedRef]);
        var seen = new List<AlarmEventArgs>();
        drv.OnAlarmEvent += (_, e) => seen.Add(e);

        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);

        var e = seen.ShouldHaveSingleItem();
        e.ConditionId.ShouldBe(SpeedRef);
        e.IsQualityOnly.ShouldBeTrue("an outage says nothing about the alarm state — quality only, no transition");
        e.QualityStatusCode.ShouldBe(BadNoCommunication);
    }

    [Fact]
    public async Task Recovery_restores_alarm_condition_quality_Good()
    {
        var (drv, session, _) = NewConnectedDriver();
        using var _ = drv;
        SetupRead(session);
        drv.AddAlarmSubscriptionForTest([SpeedRef]);
        var seen = new List<AlarmEventArgs>();
        drv.OnAlarmEvent += (_, e) => { lock (seen) seen.Add(e); };

        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);
        drv.RaiseKeepAliveForTest(StatusCodes.Good);

        await WaitForAsync(() => { lock (seen) return seen.Count >= 2; }, "recovery must re-annotate Good");
        lock (seen)
        {
            seen[1].IsQualityOnly.ShouldBeTrue();
            seen[1].QualityStatusCode.ShouldBe(0u);
        }
    }

    [Fact]
    public void A_keep_alive_from_a_superseded_session_is_ignored()
    {
        var (drv, _, _) = NewConnectedDriver();
        using var _ = drv;
        var stale = NewSession();

        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication, stale.Object);

        drv.GetHealth().State.ShouldBe(DriverState.Healthy);
    }

    [Fact]
    public async Task Shutdown_clears_the_outage_so_a_fresh_session_reports_its_own_loss()
    {
        var (drv, _, _) = NewConnectedDriver();
        using var _ = drv;
        drv.AddSubscriptionForTest([SpeedRef]);
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);

        await drv.ShutdownAsync(TestContext.Current.CancellationToken);

        // As after a watchdog reinit: a new session + subscription, and a new outage must be reported again.
        var fresh = NewSession();
        drv.SetSessionForTest(fresh.Object);
        drv.SetNamespaceMapForTest(NamespaceMap.FromTable(new NamespaceTable(), Tags));
        drv.AddSubscriptionForTest([SpeedRef]);
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);

        seen.ShouldHaveSingleItem().Snapshot.StatusCode.ShouldBe(BadNoCommunication);
    }

    /// <summary>
    ///     Review M2: a recovery overtaken by a re-initialise (watchdog Shutdown + Initialize) must not leak its
    ///     outcome onto the NEW session — previously its failed read re-armed the outage latch, so the new session's
    ///     first real outage was swallowed.
    /// </summary>
    [Fact]
    public async Task A_recovery_overtaken_by_a_reinit_does_not_latch_the_new_session()
    {
        var ct = TestContext.Current.CancellationToken;
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(1.0f, clock.Now.UtcDateTime));
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);

        await drv.Gate.WaitAsync(ct);           // park the recovery read on the gate
        drv.RaiseKeepAliveForTest(StatusCodes.Good);
        await drv.ShutdownAsync(ct);            // the watchdog's re-initialise overtakes it …
        var fresh = NewSession();               // … and a new session is up (its reads would fail)
        drv.SetSessionForTest(fresh.Object);
        drv.SetNamespaceMapForTest(NamespaceMap.FromTable(new NamespaceTable(), Tags));
        drv.SetHealthForTest(new DriverHealth(DriverState.Healthy, clock.Now.UtcDateTime, null));
        drv.AddSubscriptionForTest([SpeedRef]);
        drv.Gate.Release();
        await drv.RecoveryTaskForTest;

        drv.GetHealth().State.ShouldBe(DriverState.Healthy, "the stale recovery must not degrade the new session");
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);
        seen.ShouldHaveSingleItem("the new session's first outage must be published, not swallowed by a stale latch")
            .Snapshot.StatusCode.ShouldBe(BadNoCommunication);
    }

    /// <summary>
    ///     Review M1: the subscription dies while the session lives (an outage longer than the subscription lifetime
    ///     but shorter than the session timeout). Keep-alives stay Good, so without detection the driver ends Healthy
    ///     with fresh stamps and frozen values forever. It must go Degraded, publish the loss once, and STOP advancing
    ///     the read stamp — so the watchdog's reinit recreates the subscription.
    /// </summary>
    [Fact]
    public void A_subscription_that_stops_publishing_while_the_session_lives_freezes_the_stamp_and_goes_Bad()
    {
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        var otherHandle = drv.AddSubscriptionForTest([CounterRef]);
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(3.0f, clock.Now.UtcDateTime));
        var stalled = false;
        drv.IsPublishingStopped = sub => stalled && sub.DisplayName == "opcua-sub-1";
        drv.PublishingStallGrace = _ => TimeSpan.FromSeconds(10);
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);

        stalled = true;
        clock.Advance(TimeSpan.FromSeconds(5));
        drv.RaiseKeepAliveForTest(StatusCodes.Good);   // first observation — within grace
        drv.GetHealth().State.ShouldBe(DriverState.Healthy);
        clock.Advance(TimeSpan.FromSeconds(10));
        drv.RaiseKeepAliveForTest(StatusCodes.Good);   // grace elapsed

        var h = drv.GetHealth();
        h.State.ShouldBe(DriverState.Degraded);
        h.LastError.ShouldNotBeNull().ShouldContain("subscription lost");
        var frozen = h.LastSuccessfulRead;
        seen.Count(e => e.Snapshot.StatusCode == BadNoCommunication).ShouldBe(2, "every subscribed ref published Bad once");
        seen.Single(e => e.FullReference == SpeedRef).Snapshot.Value.ShouldBe(3.0f);

        // Nothing may revive the stamp: not good keep-alives, not a bad→good keep-alive cycle, not notifications
        // from a still-healthy sibling subscription.
        for (var i = 0; i < 30; i++)
        {
            clock.Advance(TimeSpan.FromSeconds(5));
            drv.RaiseKeepAliveForTest(StatusCodes.Good);
            drv.DeliverNotificationForTest(otherHandle, CounterRef, Good(i, clock.Now.UtcDateTime));
        }
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);
        drv.RaiseKeepAliveForTest(StatusCodes.Good);

        h = drv.GetHealth();
        h.State.ShouldBe(DriverState.Degraded);
        h.LastSuccessfulRead.ShouldBe(frozen, "the #527 watchdog needs a frozen stamp to re-initialise the driver");
        seen.Count(e => e.Snapshot.StatusCode == BadNoCommunication).ShouldBe(2, "the loss is published once per connection");
    }

    /// <summary>Control for M1: a subscription that is briefly silent (e.g. right after an outage) and then resumes
    /// within its grace is NOT declared dead.</summary>
    [Fact]
    public void A_subscription_silent_for_less_than_its_grace_is_not_declared_lost()
    {
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        drv.AddSubscriptionForTest([SpeedRef]);
        var stalled = true;
        drv.IsPublishingStopped = _ => stalled;
        drv.PublishingStallGrace = _ => TimeSpan.FromSeconds(10);

        drv.RaiseKeepAliveForTest(StatusCodes.Good);
        clock.Advance(TimeSpan.FromSeconds(9));
        drv.RaiseKeepAliveForTest(StatusCodes.Good);
        stalled = false;
        for (var i = 0; i < 10; i++)
        {
            clock.Advance(TimeSpan.FromSeconds(5));
            drv.RaiseKeepAliveForTest(StatusCodes.Good);
        }
        stalled = true; // a NEW stall restarts the grace rather than inheriting the old one
        clock.Advance(TimeSpan.FromSeconds(5));
        drv.RaiseKeepAliveForTest(StatusCodes.Good);

        drv.GetHealth().State.ShouldBe(DriverState.Healthy);
        drv.GetHealth().LastSuccessfulRead.ShouldBe(clock.Now.UtcDateTime);
    }

    [Fact]
    public void A_BadNoSubscription_publish_error_with_tracked_subscriptions_declares_the_subscription_lost()
    {
        var (drv, _, _) = NewConnectedDriver();
        using var _ = drv;
        drv.AddSubscriptionForTest([SpeedRef]);
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);

        drv.RaisePublishErrorForTest(StatusCodes.BadSubscriptionIdInvalid, subscriptionId: 4242); // not ours
        drv.GetHealth().State.ShouldBe(DriverState.Healthy);

        drv.RaisePublishErrorForTest(StatusCodes.BadNoSubscription);
        drv.GetHealth().State.ShouldBe(DriverState.Degraded);
        seen.ShouldHaveSingleItem().Snapshot.StatusCode.ShouldBe(BadNoCommunication);
    }

    [Fact]
    public void A_BadNoSubscription_with_nothing_tracked_is_not_a_loss()
    {
        var (drv, _, _) = NewConnectedDriver();
        using var _ = drv;
        drv.RaisePublishErrorForTest(StatusCodes.BadNoSubscription);
        drv.GetHealth().State.ShouldBe(DriverState.Healthy);
    }

    [Fact]
    public async Task Shutdown_clears_a_subscription_loss()
    {
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        drv.AddSubscriptionForTest([SpeedRef]);
        drv.RaisePublishErrorForTest(StatusCodes.BadNoSubscription);

        await drv.ShutdownAsync(TestContext.Current.CancellationToken);
        drv.SetSessionForTest(NewSession().Object);
        drv.SetHealthForTest(new DriverHealth(DriverState.Healthy, clock.Now.UtcDateTime, null));
        clock.Advance(TimeSpan.FromSeconds(5));
        drv.RaiseKeepAliveForTest(StatusCodes.Good);

        drv.GetHealth().LastSuccessfulRead.ShouldBe(clock.Now.UtcDateTime, "the re-subscribed connection starts clean");
    }

    [Fact]
    public void Sdk_log_forwarding_is_rate_limited()
    {
        var target = new CountingLogger();
        var limited = new SdkTelemetry.RateLimitedLogger(target, max: 3, window: TimeSpan.FromHours(1));
        for (var i = 0; i < 50; i++)
            Microsoft.Extensions.Logging.LoggerExtensions.LogWarning(limited, "sdk warning {I}", i);
        target.Count.ShouldBe(3);
    }

    private sealed class CountingLogger : Microsoft.Extensions.Logging.ILogger
    {
        public int Count;
        public IDisposable? BeginScope<TState>(TState state) where TState : notnull => null;
        public bool IsEnabled(Microsoft.Extensions.Logging.LogLevel logLevel) => true;
        public void Log<TState>(Microsoft.Extensions.Logging.LogLevel logLevel, Microsoft.Extensions.Logging.EventId eventId,
            TState state, Exception? exception, Func<TState, Exception?, string> formatter) => Count++;
    }

    /// <summary>
    ///     #670 follow-up F1 (live gate): the outage Bad carried the last Good sample's SOURCE timestamp, so the
    ///     recorder (which historizes <c>SourceTimestampUtc ?? ServerTimestampUtc</c>) wrote it at the Good row's key —
    ///     the historian's insert-if-absent (TagId, SourceTimeUtc) key dropped it and no outage marker was ever stored.
    ///     The Bad is a fresh observation: it is stamped with the detection time, like the host's own comms-loss fan.
    /// </summary>
    [Fact]
    public void The_outage_Bad_is_stamped_with_the_detection_time_not_the_last_good_source_time()
    {
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        var goodAt = clock.Now.UtcDateTime;
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(42.5f, goodAt));
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);

        clock.Advance(TimeSpan.FromSeconds(37));
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);

        var bad = seen.ShouldHaveSingleItem().Snapshot;
        bad.StatusCode.ShouldBe(BadNoCommunication);
        bad.Value.ShouldBe(42.5f, "the value is still retained");
        bad.SourceTimestampUtc.ShouldBe(clock.Now.UtcDateTime, "a Bad at the Good row's source time collides with it in history");
        bad.ServerTimestampUtc.ShouldBe(clock.Now.UtcDateTime);
    }

    private static void SetupReadAt(Mock<ISession> session, object value, DateTime sourceTime) =>
        session.Setup(s => s.ReadAsync(
                It.IsAny<RequestHeader>(), It.IsAny<double>(), It.IsAny<TimestampsToReturn>(),
                It.IsAny<ReadValueIdCollection>(), It.IsAny<CancellationToken>()))
            .ReturnsAsync(new ReadResponse
            {
                ResponseHeader = new ResponseHeader(),
                Results = [new DataValue(new Variant(value), StatusCodes.Good, sourceTime, sourceTime)],
                DiagnosticInfos = [],
            });

    /// <summary>Drives a same-session outage + recovery (bad → good keep-alive) and waits for the recovery read.</summary>
    private static async Task RecoverInPlaceAsync(OpcUaClientDriver drv)
    {
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);
        drv.RaiseKeepAliveForTest(StatusCodes.Good);
        await drv.RecoveryTaskForTest;
    }

    /// <summary>
    ///     #670 follow-up F2 (live gate): after a same-session recovery the SDK delivered one notification late with
    ///     an OLDER source time, and the node regressed to it. Inside the post-recovery replay window an older
    ///     notification is dropped, and so is one carrying the exact source time already accepted (OPC UA moves the
    ///     source timestamp on every value OR status change, so an equal one is a replay, not a status-only change).
    /// </summary>
    [Fact]
    public async Task After_a_recovery_a_late_older_or_equal_notification_is_dropped()
    {
        var (drv, session, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        var t0 = clock.Now.UtcDateTime;
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(668, t0.AddSeconds(2)));
        SetupReadAt(session, 668, t0.AddSeconds(2));
        await RecoverInPlaceAsync(drv);
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);

        drv.DeliverNotificationForTest(handle, SpeedRef, Good(667, t0.AddSeconds(1)));          // late replay
        drv.DeliverNotificationForTest(handle, SpeedRef,
            new DataValue(new Variant(668), StatusCodes.UncertainLastUsableValue, t0.AddSeconds(2), t0.AddSeconds(2))); // same ts
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(669, t0.AddSeconds(3)));
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(665, t0));                        // still guarded

        seen.Select(e => e.Snapshot.Value).ShouldBe(new object[] { 669 });
    }

    /// <summary>
    ///     #670 F2 re-gate (live gate FAILED on the per-ref disarm): fresh notifications arrived ~1 s after recovery
    ///     and disarmed each ref, then the SDK's late replay arrived 4–6 s later and regressed the node (value 670 with
    ///     a pre-outage source time delivered after newer values). The guard must hold for the WHOLE window against the
    ///     newest ACCEPTED source time per ref — older, equal-to-the-recovery-read and equal-to-the-last-accepted
    ///     replays are all dropped, and counted.
    /// </summary>
    [Fact]
    public async Task A_late_replay_after_a_fresh_in_order_notification_is_still_dropped_inside_the_window()
    {
        var instance = $"opcua-670-regate-{Guid.NewGuid():N}";
        using var drops = new OpcUaClientDiagnosticsTests.Capture(instance);
        var (drv, session, clock) = NewConnectedDriver(instance);
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        var t0 = clock.Now.UtcDateTime;
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(100, t0.AddSeconds(-5)));
        SetupReadAt(session, 105, t0);                                // recovery read seeds T0
        await RecoverInPlaceAsync(drv);
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);

        clock.Advance(TimeSpan.FromSeconds(1));
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(116, t0.AddSeconds(6)));   // fresh, in order: T1
        clock.Advance(TimeSpan.FromSeconds(5));
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(109, t0.AddSeconds(3)));   // late replay, older than T1
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(105, t0));                 // replay == T0 (recovery read)
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(116, t0.AddSeconds(6)));   // replay == T1
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(670, t0.AddSeconds(-10))); // pre-outage replay
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(117, t0.AddSeconds(7)));   // fresh again

        seen.Select(e => e.Snapshot.Value).ShouldBe(new object[] { 116, 117 });
        drops[OpcUaClientDiagnostics.StaleNotificationDropped].ShouldBe(4);
    }

    /// <summary>
    ///     The re-gated guard still expires: after the window an older source time (an upstream clock step-back) is
    ///     accepted, even though the ref accepted newer notifications inside the window.
    /// </summary>
    [Fact]
    public async Task After_the_window_a_clock_step_back_is_accepted_even_after_in_window_notifications()
    {
        var (drv, session, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        var t0 = clock.Now.UtcDateTime;
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(1, t0.AddSeconds(-5)));
        SetupReadAt(session, 2, t0);
        await RecoverInPlaceAsync(drv);
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);

        drv.DeliverNotificationForTest(handle, SpeedRef, Good(3, t0.AddSeconds(20)));
        clock.Advance(TimeSpan.FromSeconds(31));
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(4, t0.AddSeconds(1)));     // clock stepped back
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(5, t0));                   // and again

        seen.Select(e => e.Snapshot.Value).ShouldBe(new object[] { 3, 4, 5 });
    }

    /// <summary>A notification with no source timestamp passes through the open guard and does not move it.</summary>
    [Fact]
    public async Task A_notification_without_a_source_time_passes_the_guard_and_does_not_advance_it()
    {
        var (drv, session, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        var t0 = clock.Now.UtcDateTime;
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(1, t0.AddSeconds(-5)));
        SetupReadAt(session, 2, t0);
        await RecoverInPlaceAsync(drv);
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);

        drv.DeliverNotificationForTest(handle, SpeedRef,
            new DataValue(new Variant(3), StatusCodes.Good, DateTime.MinValue, t0));      // no source time
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(4, t0.AddSeconds(-1)));     // still guarded at T0
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(5, t0.AddSeconds(1)));

        seen.Select(e => e.Snapshot.Value).ShouldBe(new object[] { 3, 5 });
    }

    /// <summary>
    ///     Coordinator review of F2: with no recovery in play, an upstream clock that steps BACKWARDS (NTP correction,
    ///     a PLC with a reset clock) must not freeze the node — every notification is published, whatever its time.
    /// </summary>
    [Fact]
    public void In_steady_state_a_clock_step_backwards_still_publishes()
    {
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        var t0 = clock.Now.UtcDateTime;
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);

        drv.DeliverNotificationForTest(handle, SpeedRef, Good(1, t0.AddHours(1)));
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(2, t0));              // clock stepped back an hour
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(3, t0.AddSeconds(1)));

        seen.Select(e => e.Snapshot.Value).ShouldBe(new object[] { 1, 2, 3 });
    }

    /// <summary>The replay guard is bounded in time: once its window has passed, an older notification is accepted
    /// even for a ref that never saw an in-order one.</summary>
    [Fact]
    public async Task The_replay_guard_expires()
    {
        var (drv, session, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        var t0 = clock.Now.UtcDateTime;
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(10, t0.AddSeconds(5)));
        SetupReadAt(session, 10, t0.AddSeconds(5));
        await RecoverInPlaceAsync(drv);
        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);

        clock.Advance(TimeSpan.FromSeconds(29));
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(9, t0));
        seen.ShouldBeEmpty("still inside the 30 s window");

        clock.Advance(TimeSpan.FromSeconds(2));
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(8, t0));
        seen.ShouldHaveSingleItem().Snapshot.Value.ShouldBe(8);
    }

    [Fact]
    public void The_outage_Bad_does_not_poison_the_ordering_check()
    {
        var (drv, _, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        var t0 = clock.Now.UtcDateTime;
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(1, t0));
        clock.Advance(TimeSpan.FromSeconds(60));
        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication); // stamped t0+60 s

        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);
        // An upstream sample produced during the outage (server-side) and delivered on recovery: older than the Bad's
        // detection stamp, newer than the last Good — it is real data and must pass.
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(2, t0.AddSeconds(30)));

        seen.ShouldHaveSingleItem().Snapshot.Value.ShouldBe(2);
    }

    [Fact]
    public async Task A_late_notification_older_than_the_recovery_read_is_dropped()
    {
        var (drv, session, clock) = NewConnectedDriver();
        using var _ = drv;
        var handle = drv.AddSubscriptionForTest([SpeedRef]);
        var t0 = clock.Now.UtcDateTime;
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(666, t0));
        var readAt = t0.AddSeconds(10);
        session.Setup(s => s.ReadAsync(
                It.IsAny<RequestHeader>(), It.IsAny<double>(), It.IsAny<TimestampsToReturn>(),
                It.IsAny<ReadValueIdCollection>(), It.IsAny<CancellationToken>()))
            .ReturnsAsync(new ReadResponse
            {
                ResponseHeader = new ResponseHeader(),
                Results = [new DataValue(new Variant(670), StatusCodes.Good, readAt, readAt)],
                DiagnosticInfos = [],
            });

        drv.RaiseKeepAliveForTest(StatusCodes.BadNoCommunication);
        drv.RaiseKeepAliveForTest(StatusCodes.Good);
        await drv.RecoveryTaskForTest;

        var seen = new List<DataChangeEventArgs>();
        drv.OnDataChange += (_, e) => seen.Add(e);
        drv.DeliverNotificationForTest(handle, SpeedRef, Good(667, t0.AddSeconds(5))); // replayed, older than the read
        seen.ShouldBeEmpty();
    }
}