using System.Diagnostics.Metrics;
using System.Threading.Channels;
using Akka.Actor;
using Akka.TestKit.Xunit2;
using Grpc.Core;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.Abstractions;
using NSubstitute;
using ZB.MOM.WW.ScadaBridge.Commons.Observability;
using ZB.MOM.WW.ScadaBridge.Communication.Grpc;

namespace ZB.MOM.WW.ScadaBridge.Communication.Tests.Grpc;

public class SiteStreamGrpcServerTests : TestKit
{
    private readonly ISiteStreamSubscriber _subscriber;
    private readonly ILogger<SiteStreamGrpcServer> _logger;

    public SiteStreamGrpcServerTests()
    {
        _subscriber = Substitute.For<ISiteStreamSubscriber>();
        _subscriber.Subscribe(Arg.Any<string>(), Arg.Any<IActorRef>())
            .Returns("sub-1");
        _subscriber.SubscribeSiteAlarms(Arg.Any<IActorRef>())
            .Returns("site-sub-1");
        _logger = NullLogger<SiteStreamGrpcServer>.Instance;
    }

    private SiteStreamGrpcServer CreateServer(int maxStreams = 100)
    {
        return new SiteStreamGrpcServer(_subscriber, _logger, maxStreams);
    }

    private static InstanceStreamRequest MakeRequest(string correlationId = "corr-1", string instance = "Site1.Pump01")
    {
        return new InstanceStreamRequest
        {
            CorrelationId = correlationId,
            InstanceUniqueName = instance
        };
    }

    [Fact]
    public async Task RejectsWhenNotReady()
    {
        var server = CreateServer();
        // Do NOT call SetReady()

        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
        var context = CreateMockContext();

        var ex = await Assert.ThrowsAsync<RpcException>(
            () => server.SubscribeInstance(MakeRequest(), writer, context));

        Assert.Equal(StatusCode.Unavailable, ex.StatusCode);
    }

    [Fact]
    public async Task RejectsWhenMaxStreamsReached()
    {
        var server = CreateServer(maxStreams: 1);
        server.SetReady(Sys);

        // Start one stream that blocks
        var cts1 = new CancellationTokenSource();
        var context1 = CreateMockContext(cts1.Token);
        var writer1 = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();

        var stream1Task = Task.Run(() => server.SubscribeInstance(
            MakeRequest("corr-1"), writer1, context1));

        // Wait for the first stream to register
        await WaitForConditionAsync(() => server.ActiveStreamCount == 1);

        // Second stream should be rejected
        var writer2 = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
        var context2 = CreateMockContext();

        var ex = await Assert.ThrowsAsync<RpcException>(
            () => server.SubscribeInstance(MakeRequest("corr-2"), writer2, context2));

        Assert.Equal(StatusCode.ResourceExhausted, ex.StatusCode);

        // Clean up first stream
        cts1.Cancel();
        await stream1Task;
    }

    [Fact]
    public async Task CancelsDuplicateCorrelationId()
    {
        var server = CreateServer();
        server.SetReady(Sys);

        var cts1 = new CancellationTokenSource();
        var context1 = CreateMockContext(cts1.Token);
        var writer1 = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();

        // Start first stream
        var stream1Task = Task.Run(() => server.SubscribeInstance(
            MakeRequest("corr-dup"), writer1, context1));

        await WaitForConditionAsync(() => server.ActiveStreamCount == 1);

        // Start second stream with same correlationId -- should cancel first
        var cts2 = new CancellationTokenSource();
        var context2 = CreateMockContext(cts2.Token);
        var writer2 = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();

        var stream2Task = Task.Run(() => server.SubscribeInstance(
            MakeRequest("corr-dup"), writer2, context2));

        // First stream should complete (cancelled by duplicate replacement)
        await stream1Task;

        // Second stream should be active
        await WaitForConditionAsync(() => server.ActiveStreamCount == 1);

        // Clean up
        cts2.Cancel();
        await stream2Task;
    }

    [Fact]
    public async Task CleansUpOnCancellation()
    {
        var server = CreateServer();
        server.SetReady(Sys);

        var cts = new CancellationTokenSource();
        var context = CreateMockContext(cts.Token);
        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();

        var streamTask = Task.Run(() => server.SubscribeInstance(
            MakeRequest("corr-cleanup"), writer, context));

        await WaitForConditionAsync(() => server.ActiveStreamCount == 1);

        cts.Cancel();
        await streamTask;

        Assert.Equal(0, server.ActiveStreamCount);
    }

    // --- Host-017 / REQ-HOST-7: site-shutdown ordering ---

    [Fact]
    public async Task Host017_CancelAllStreams_CancelsActiveStreamsAndRefusesNewOnes()
    {
        // REQ-HOST-7 step (1)+(2): on CoordinatedShutdown the gRPC server must
        // stop accepting new streams AND cancel every active stream so the
        // client observes a clean Cancelled (not a silent stream that only
        // times out via keepalive). Program.cs registers
        // ApplicationStopping → CancelAllStreams(); this test exercises the
        // server-side guarantee in isolation.
        var server = CreateServer();
        server.SetReady(Sys);

        var cts1 = new CancellationTokenSource();
        var context1 = CreateMockContext(cts1.Token);
        var writer1 = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();

        var stream1Task = Task.Run(() => server.SubscribeInstance(
            MakeRequest("corr-shutdown-1"), writer1, context1));

        await WaitForConditionAsync(() => server.ActiveStreamCount == 1);

        // Begin shutdown — flip the flag AND cancel the active stream.
        server.CancelAllStreams();

        Assert.True(server.IsShuttingDown);

        // Active stream's await foreach observes OCE and falls through finally
        // → entry is removed from _activeStreams.
        await stream1Task;
        Assert.Equal(0, server.ActiveStreamCount);

        // A second SubscribeInstance after shutdown is refused immediately
        // with Unavailable rather than allowed to register a new stream.
        var writer2 = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
        var context2 = CreateMockContext();
        var ex = await Assert.ThrowsAsync<RpcException>(
            () => server.SubscribeInstance(MakeRequest("corr-shutdown-2"), writer2, context2));
        Assert.Equal(StatusCode.Unavailable, ex.StatusCode);
        Assert.Contains("shutting", ex.Status.Detail, StringComparison.OrdinalIgnoreCase);
    }

    [Fact]
    public void Host017_CancelAllStreams_IsIdempotent()
    {
        // Repeated calls during a double-fire shutdown sequence must not throw.
        var server = CreateServer();
        server.SetReady(Sys);

        server.CancelAllStreams();
        server.CancelAllStreams();

        Assert.True(server.IsShuttingDown);
        Assert.Equal(0, server.ActiveStreamCount);
    }

    [Fact]
    public async Task SubscribesAndRemovesFromStreamManager()
    {
        var server = CreateServer();
        server.SetReady(Sys);

        var cts = new CancellationTokenSource();
        var context = CreateMockContext(cts.Token);
        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();

        var streamTask = Task.Run(() => server.SubscribeInstance(
            MakeRequest("corr-sub", "Site1.Motor01"), writer, context));

        await WaitForConditionAsync(() => server.ActiveStreamCount == 1);

        // Verify Subscribe was called
        _subscriber.Received(1).Subscribe("Site1.Motor01", Arg.Any<IActorRef>());

        cts.Cancel();
        await streamTask;

        // Verify RemoveSubscriber was called
        _subscriber.Received(1).RemoveSubscriber(Arg.Any<IActorRef>());
    }

    [Fact]
    public async Task WritesEventsToResponseStream()
    {
        var server = CreateServer();
        server.SetReady(Sys);

        // Capture the relay actor so we can send it events
        IActorRef? capturedActor = null;
        _subscriber.Subscribe(Arg.Any<string>(), Arg.Any<IActorRef>())
            .Returns(ci =>
            {
                capturedActor = ci.Arg<IActorRef>();
                return "sub-write";
            });

        var cts = new CancellationTokenSource();
        var context = CreateMockContext(cts.Token);
        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
        var writtenEvents = new List<SiteStreamEvent>();
        writer.WriteAsync(Arg.Any<SiteStreamEvent>(), Arg.Any<CancellationToken>())
            .Returns(Task.CompletedTask)
            .AndDoes(ci => writtenEvents.Add(ci.Arg<SiteStreamEvent>()));

        var streamTask = Task.Run(() => server.SubscribeInstance(
            MakeRequest("corr-write", "Site1.Pump01"), writer, context));

        await WaitForConditionAsync(() => capturedActor != null);

        // Send a domain event to the relay actor
        var ts = DateTimeOffset.UtcNow;
        capturedActor!.Tell(new Commons.Messages.Streaming.AttributeValueChanged(
            "Site1.Pump01", "Path", "Attr", 99.5, "Good", ts));

        // Wait for event to be written
        await WaitForConditionAsync(() => writtenEvents.Count >= 1);

        Assert.Single(writtenEvents);
        Assert.Equal("corr-write", writtenEvents[0].CorrelationId);
        Assert.Equal(SiteStreamEvent.EventOneofCase.AttributeChanged, writtenEvents[0].EventCase);

        cts.Cancel();
        await streamTask;
    }

    // --- SubscribeSite (site-wide, alarm-only aggregated stream, plan #10 T2) ---

    private static SiteStreamRequest MakeSiteRequest(string correlationId = "site-corr-1")
        => new() { CorrelationId = correlationId };

    [Fact]
    public async Task SubscribeSite_SubscribesSiteAlarmsAndRemovesOnCancel()
    {
        var server = CreateServer();
        server.SetReady(Sys);

        var cts = new CancellationTokenSource();
        var context = CreateMockContext(cts.Token);
        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();

        var streamTask = Task.Run(() => server.SubscribeSite(
            MakeSiteRequest("site-corr-sub"), writer, context));

        await WaitForConditionAsync(() => server.ActiveStreamCount == 1);

        // Site-wide handler must call SubscribeSiteAlarms (no instance filter),
        // never the per-instance Subscribe.
        _subscriber.Received(1).SubscribeSiteAlarms(Arg.Any<IActorRef>());
        _subscriber.DidNotReceive().Subscribe(Arg.Any<string>(), Arg.Any<IActorRef>());

        cts.Cancel();
        await streamTask;

        _subscriber.Received(1).RemoveSubscriber(Arg.Any<IActorRef>());
        Assert.Equal(0, server.ActiveStreamCount);
    }

    [Fact]
    public async Task SubscribeSite_RejectsUnsafeCorrelationId()
    {
        var server = CreateServer();
        server.SetReady(Sys);

        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
        var context = CreateMockContext();

        var ex = await Assert.ThrowsAsync<RpcException>(
            () => server.SubscribeSite(MakeSiteRequest("bad/id"), writer, context));

        Assert.Equal(StatusCode.InvalidArgument, ex.StatusCode);
        Assert.Equal(0, server.ActiveStreamCount);
    }

    [Fact]
    public async Task SubscribeSite_RelaysAlarmStateChangedAsAlarmStateUpdate()
    {
        var server = CreateServer();
        server.SetReady(Sys);

        // Capture the relay actor spawned for the site-wide subscription.
        IActorRef? capturedActor = null;
        _subscriber.SubscribeSiteAlarms(Arg.Any<IActorRef>())
            .Returns(ci =>
            {
                capturedActor = ci.Arg<IActorRef>();
                return "site-sub-relay";
            });

        var cts = new CancellationTokenSource();
        var context = CreateMockContext(cts.Token);
        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
        var writtenEvents = new List<SiteStreamEvent>();
        writer.WriteAsync(Arg.Any<SiteStreamEvent>(), Arg.Any<CancellationToken>())
            .Returns(Task.CompletedTask)
            .AndDoes(ci => writtenEvents.Add(ci.Arg<SiteStreamEvent>()));

        var streamTask = Task.Run(() => server.SubscribeSite(
            MakeSiteRequest("site-corr-write"), writer, context));

        await WaitForConditionAsync(() => capturedActor != null);

        // A real alarm transition for ANY instance must arrive as an AlarmStateUpdate.
        capturedActor!.Tell(new Commons.Messages.Streaming.AlarmStateChanged(
            "Site1.Pump01",
            "HighPressure",
            Commons.Types.Enums.AlarmState.Active,
            700,
            DateTimeOffset.UtcNow));

        await WaitForConditionAsync(() => writtenEvents.Count >= 1);

        Assert.Single(writtenEvents);
        Assert.Equal("site-corr-write", writtenEvents[0].CorrelationId);
        Assert.Equal(SiteStreamEvent.EventOneofCase.AlarmChanged, writtenEvents[0].EventCase);
        Assert.Equal("Site1.Pump01", writtenEvents[0].AlarmChanged.InstanceUniqueName);
        Assert.Equal("HighPressure", writtenEvents[0].AlarmChanged.AlarmName);

        cts.Cancel();
        await streamTask;
    }

    [Theory]
    [InlineData("corr/with/slash")]
    [InlineData("corr with space")]
    [InlineData("")]
    [InlineData("$weird")]
    public async Task RejectsCorrelationIdThatIsNotActorNameSafe(string badCorrelationId)
    {
        // Communication-014 regression: a public gRPC SubscribeInstance must not feed
        // an untrusted correlation_id straight into an Akka actor name. An unsafe id
        // must be rejected cleanly with InvalidArgument rather than escaping as an
        // unhandled InvalidActorNameException.
        var server = CreateServer();
        server.SetReady(Sys);

        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
        var context = CreateMockContext();

        var ex = await Assert.ThrowsAsync<RpcException>(
            () => server.SubscribeInstance(MakeRequest(badCorrelationId), writer, context));

        Assert.Equal(StatusCode.InvalidArgument, ex.StatusCode);
        Assert.Equal(0, server.ActiveStreamCount);
    }

    [Fact]
    public async Task AcceptsActorNameSafeCorrelationId()
    {
        // A normal GUID-style correlation id (what central always supplies) is accepted.
        var server = CreateServer();
        server.SetReady(Sys);

        var cts = new CancellationTokenSource();
        var context = CreateMockContext(cts.Token);
        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();

        var streamTask = Task.Run(() => server.SubscribeInstance(
            MakeRequest(Guid.NewGuid().ToString()), writer, context));

        await WaitForConditionAsync(() => server.ActiveStreamCount == 1);

        cts.Cancel();
        await streamTask;
    }

    [Fact]
    public async Task Comm021_SubscribeThrows_StopsRelayActorAndRemovesActiveStreamEntry()
    {
        // Communication-021 regression: SubscribeInstance creates a StreamRelayActor
        // and registers an _activeStreams entry BEFORE calling _streamSubscriber.Subscribe.
        // If Subscribe throws (e.g. stale instance, site runtime shutting down) and the
        // pre-fix code lets the throw escape without the wrapping try, the relay actor
        // and the activeStreams entry both leak. The fix wraps the Subscribe call so the
        // catch deterministically stops the actor and removes the entry before re-throw.
        var subscriber = Substitute.For<ISiteStreamSubscriber>();
        subscriber.Subscribe(Arg.Any<string>(), Arg.Any<IActorRef>())
            .Returns<string>(_ => throw new InvalidOperationException("instance not found"));

        var server = new SiteStreamGrpcServer(subscriber, _logger);
        server.SetReady(Sys);

        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
        var context = CreateMockContext();

        // The InvalidOperationException is expected to propagate (the gRPC stack maps
        // unhandled throws to Internal); the load-bearing assertion is the cleanup.
        await Assert.ThrowsAsync<InvalidOperationException>(
            () => server.SubscribeInstance(MakeRequest("corr-comm021"), writer, context));

        // _activeStreams entry was inserted before Subscribe was called; the catch
        // must remove it so a follow-up subscription with the same correlation id is
        // not blocked, and the relay actor must be stopped so it does not leak.
        Assert.Equal(0, server.ActiveStreamCount);

        // RemoveSubscriber must NOT have been called (Subscribe never returned a
        // subscription id) — verifying we hit the catch path, not the finally path.
        subscriber.DidNotReceive().RemoveSubscriber(Arg.Any<IActorRef>());
    }

    [Fact]
    public async Task SiteConnectionUpGauge_GoesToOneOnConnect_AndBackToZeroOnCancel()
    {
        // Telemetry follow-on: the scadabridge.site.connection.up gauge must read
        // exactly 1 while a site stream is established and return to 0 once the
        // stream terminates on the cancel path — proving SiteConnectionOpened() is
        // matched by exactly one SiteConnectionClosed() in the handler's finally.
        var server = CreateServer();
        server.SetReady(Sys);

        long ReadGauge()
        {
            long observed = 0;
            using var listener = new MeterListener();
            listener.InstrumentPublished = (instrument, l) =>
            {
                if (instrument.Meter.Name == ScadaBridgeTelemetry.MeterName &&
                    instrument.Name == "scadabridge.site.connection.up")
                {
                    l.EnableMeasurementEvents(instrument);
                }
            };
            listener.SetMeasurementEventCallback<long>((_, measurement, _, _) => observed = measurement);
            listener.Start();
            listener.RecordObservableInstruments();
            return observed;
        }

        var baseline = ReadGauge();

        var cts = new CancellationTokenSource();
        var context = CreateMockContext(cts.Token);
        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();

        var streamTask = Task.Run(() => server.SubscribeInstance(
            MakeRequest("corr-gauge", "Site1.Pump01"), writer, context));

        await WaitForConditionAsync(() => server.ActiveStreamCount == 1);

        // While the stream is up the gauge is one above whatever baseline other
        // (possibly parallel) tests left behind — read relative so the assertion
        // is robust to test interleaving on the process-wide static counter.
        //
        // SiteConnectionOpened() runs AFTER the _activeStreams insertion that
        // WaitForConditionAsync(ActiveStreamCount == 1) keys off (see
        // SiteStreamGrpcServer.SubscribeInstance), so a one-shot ReadGauge() here
        // can race the increment under full-suite CPU oversubscription. Poll the
        // gauge with a generous timeout until it reaches baseline + 1.
        AwaitAssert(() => Assert.Equal(baseline + 1, ReadGauge()), TimeSpan.FromSeconds(5));

        cts.Cancel();
        await streamTask;
        await WaitForConditionAsync(() => server.ActiveStreamCount == 0);

        // After the cancel path runs the finally, the gauge is balanced back to
        // the baseline — no leaked "up" count. Poll here too: the gauge decrement
        // and the stream-count drop are independent observations, so under load
        // give the finally's SiteConnectionClosed() time to land.
        AwaitAssert(() => Assert.Equal(baseline, ReadGauge()), TimeSpan.FromSeconds(5));
    }

    [Fact]
    public void SetReady_AllowsStreamCreation()
    {
        var server = CreateServer();
        // Initially not ready -- just verify the property works
        server.SetReady(Sys);
        // No assertion needed -- the other tests verify that SetReady enables streaming
        Assert.Equal(0, server.ActiveStreamCount);
    }

    private static ServerCallContext CreateMockContext(CancellationToken cancellationToken = default)
    {
        var context = Substitute.For<ServerCallContext>();
        context.CancellationToken.Returns(cancellationToken);
        return context;
    }

    private static async Task WaitForConditionAsync(Func<bool> condition, int timeoutMs = 5000)
    {
        var deadline = DateTime.UtcNow.AddMilliseconds(timeoutMs);
        while (!condition() && DateTime.UtcNow < deadline)
        {
            await Task.Delay(25);
        }

        Assert.True(condition(), $"Condition not met within {timeoutMs}ms");
    }

    // ── WP2.3: the site-wide alarm feed gets its OWN, larger send channel ──

    [Fact]
    public void SiteAlarmStream_HasItsOwnLargerChannel_ThanTheDebugView()
    {
        // Sharing the Debug View's 1000-slot DropOldest channel meant an alarm burst during
        // a WAN stall silently evicted operator-visible transitions to make room for
        // diagnostics traffic. The two feeds are now sized independently.
        var options = Microsoft.Extensions.Options.Options.Create(new CommunicationOptions());
        var server = new SiteStreamGrpcServer(_subscriber, _logger, options);

        Assert.Equal(1000, server.InstanceChannelCapacity);
        Assert.Equal(20_000, server.SiteAlarmChannelCapacity);
        Assert.True(server.SiteAlarmChannelCapacity > server.InstanceChannelCapacity);
    }

    [Fact]
    public void ChannelCapacities_AreBoundFromOptions_AndFloorAtOne()
    {
        var options = Microsoft.Extensions.Options.Options.Create(new CommunicationOptions
        {
            GrpcInstanceStreamChannelCapacity = 42,
            GrpcSiteAlarmStreamChannelCapacity = 4242,
        });
        var server = new SiteStreamGrpcServer(_subscriber, _logger, options);

        Assert.Equal(42, server.InstanceChannelCapacity);
        Assert.Equal(4242, server.SiteAlarmChannelCapacity);

        // A misconfigured zero/negative capacity must not throw at channel-construction
        // time deep inside a live RPC — it floors at one instead.
        var degenerate = new SiteStreamGrpcServer(_subscriber, _logger,
            Microsoft.Extensions.Options.Options.Create(new CommunicationOptions
            {
                GrpcInstanceStreamChannelCapacity = 0,
                GrpcSiteAlarmStreamChannelCapacity = -5,
            }));
        Assert.Equal(1, degenerate.InstanceChannelCapacity);
        Assert.Equal(1, degenerate.SiteAlarmChannelCapacity);
    }

    [Fact]
    public void DroppedStreamEventCount_StartsAtZero()
    {
        // The raw counter behind scadabridge.site.stream.events_dropped — a fresh node has
        // evicted nothing.
        var server = CreateServer();
        Assert.Equal(0, server.DroppedStreamEventCount);
    }

    // ── R2: gRPC event batching, and its negotiation ────────────────────────────

    [Fact]
    public void BatchOptions_AreBoundFromOptions_AndClampDegenerateValues()
    {
        var options = Microsoft.Extensions.Options.Options.Create(new CommunicationOptions());
        var server = new SiteStreamGrpcServer(_subscriber, _logger, options);

        Assert.Equal(100, server.StreamBatchMaxEvents);
        Assert.Equal(TimeSpan.FromMilliseconds(25), server.StreamBatchWindow);

        // CommunicationOptionsValidator fails the boot on these, but a host composed
        // without validation must not blow up deep inside a live RPC.
        var degenerate = new SiteStreamGrpcServer(_subscriber, _logger,
            Microsoft.Extensions.Options.Options.Create(new CommunicationOptions
            {
                GrpcStreamBatchMaxEvents = 0,
                GrpcStreamBatchWindow = TimeSpan.FromMilliseconds(-5),
            }));
        Assert.Equal(1, degenerate.StreamBatchMaxEvents);
        Assert.Equal(TimeSpan.Zero, degenerate.StreamBatchWindow);
    }

    [Fact]
    public async Task UnnegotiatedSubscription_NeverEmitsABatchFrame()
    {
        // OLD-CENTRAL ↔ NEW-SITE skew. proto3 defaults batching_supported to false, which
        // is exactly what a central built before R2 sends. The site must then keep to one
        // event per frame — a Batch frame would arrive at that central as
        // EventOneofCase.None and be silently dropped by its ConvertToDomainEvent.
        var (server, capture, cts, streamTask, relay) =
            await StartCapturingStreamAsync(batchingSupported: false);

        for (var i = 0; i < 50; i++)
        {
            relay.Tell(new Commons.Messages.Streaming.AttributeValueChanged(
                "Site1.Pump01", "Path", "Attr", i, "Good", DateTimeOffset.UtcNow));
        }

        await WaitForConditionAsync(() => CountEvents(capture) >= 50, 10_000);

        cts.Cancel();
        await streamTask;

        lock (capture)
        {
            Assert.All(capture, f => Assert.NotEqual(SiteStreamEvent.EventOneofCase.Batch, f.EventCase));
            Assert.Equal(50, capture.Count);
        }

        GC.KeepAlive(server);
    }

    [Fact]
    public async Task NegotiatedSubscription_CoalescesABacklogIntoFewerFramesThanEvents()
    {
        // NEW-CENTRAL ↔ NEW-SITE. A burst pushed at the relay faster than the pump drains
        // it must come out in strictly fewer frames than events, with every event
        // preserved in order.
        const int burst = 400;
        var (server, capture, cts, streamTask, relay) =
            await StartCapturingStreamAsync(batchingSupported: true);

        for (var i = 0; i < burst; i++)
        {
            relay.Tell(new Commons.Messages.Streaming.AttributeValueChanged(
                "Site1.Pump01", "Path", "Attr", i, "Good", DateTimeOffset.UtcNow));
        }

        await WaitForConditionAsync(() => CountEvents(capture) >= burst, 15_000);

        cts.Cancel();
        await streamTask;

        List<SiteStreamEvent> frames;
        lock (capture) { frames = [.. capture]; }

        Assert.Equal(burst, frames.Sum(CountFrameEvents));
        Assert.True(frames.Count < burst,
            $"batching produced {frames.Count} frames for {burst} events — no coalescing happened");
        Assert.Contains(frames, f => f.EventCase == SiteStreamEvent.EventOneofCase.Batch);

        // Order is preserved end to end: the values arrive 0..burst-1 exactly once each.
        var values = frames.SelectMany(FlattenAttributeValues).ToArray();
        Assert.Equal(Enumerable.Range(0, burst).Select(i => i.ToString()), values);

        GC.KeepAlive(server);
    }

    [Fact]
    public async Task BatchSizeHistogram_IsRecordedOnlyForNegotiatedStreams()
    {
        // scadabridge.site.stream.batch_size rides ScadaBridgeTelemetry.MeterName, which is
        // already in SiteServiceRegistration.ObservedMeters — an unlisted meter exports
        // nothing, silently. Assert the instrument actually fires, and that it does NOT
        // fire on an un-negotiated stream (where it would degenerate into a per-event
        // instrument on the hottest path in the product).
        var measurements = new List<int>();
        using var listener = new MeterListener();
        listener.InstrumentPublished = (instrument, l) =>
        {
            if (instrument.Meter.Name == ScadaBridgeTelemetry.MeterName &&
                instrument.Name == "scadabridge.site.stream.batch_size")
            {
                l.EnableMeasurementEvents(instrument);
            }
        };
        listener.SetMeasurementEventCallback<int>((_, m, _, _) =>
        {
            lock (measurements) { measurements.Add(m); }
        });
        listener.Start();

        // Un-negotiated: no measurements at all.
        var (_, plainCapture, plainCts, plainTask, plainRelay) =
            await StartCapturingStreamAsync(batchingSupported: false, correlationId: "corr-hist-off");
        plainRelay.Tell(new Commons.Messages.Streaming.AttributeValueChanged(
            "Site1.Pump01", "Path", "Attr", 1, "Good", DateTimeOffset.UtcNow));
        await WaitForConditionAsync(() => CountEvents(plainCapture) >= 1);
        plainCts.Cancel();
        await plainTask;

        lock (measurements) { Assert.Empty(measurements); }

        // Negotiated: one measurement per emitted frame, each within the size cap.
        var (_, capture, cts, streamTask, relay) =
            await StartCapturingStreamAsync(batchingSupported: true, correlationId: "corr-hist-on");
        for (var i = 0; i < 20; i++)
        {
            relay.Tell(new Commons.Messages.Streaming.AttributeValueChanged(
                "Site1.Pump01", "Path", "Attr", i, "Good", DateTimeOffset.UtcNow));
        }
        await WaitForConditionAsync(() => CountEvents(capture) >= 20, 10_000);
        cts.Cancel();
        await streamTask;

        lock (measurements)
        {
            Assert.NotEmpty(measurements);
            Assert.Equal(20, measurements.Sum());
            Assert.All(measurements, m => Assert.InRange(m, 1, SiteStreamGrpcServer.DefaultStreamBatchMaxEvents));
        }
    }

    /// <summary>Total events carried across all captured frames (unpacking batch frames).</summary>
    private static int CountEvents(List<SiteStreamEvent> capture)
    {
        lock (capture) { return capture.Sum(CountFrameEvents); }
    }

    private static int CountFrameEvents(SiteStreamEvent frame) =>
        frame.EventCase == SiteStreamEvent.EventOneofCase.Batch ? frame.Batch.Events.Count : 1;

    private static IEnumerable<string> FlattenAttributeValues(SiteStreamEvent frame)
    {
        if (frame.EventCase == SiteStreamEvent.EventOneofCase.Batch)
        {
            foreach (var inner in frame.Batch.Events)
                yield return inner.AttributeChanged.Value;
            yield break;
        }

        yield return frame.AttributeChanged.Value;
    }

    /// <summary>
    /// Starts a SubscribeInstance stream with the given batch negotiation, capturing every
    /// written frame and handing back the relay actor so the test can drive domain events.
    /// </summary>
    private async Task<(SiteStreamGrpcServer Server, List<SiteStreamEvent> Capture,
        CancellationTokenSource Cts, Task StreamTask, IActorRef Relay)>
        StartCapturingStreamAsync(bool batchingSupported, string correlationId = "corr-batch")
    {
        IActorRef? capturedActor = null;
        var subscriber = Substitute.For<ISiteStreamSubscriber>();
        subscriber.Subscribe(Arg.Any<string>(), Arg.Any<IActorRef>())
            .Returns(ci =>
            {
                capturedActor = ci.Arg<IActorRef>();
                return "sub-batch";
            });

        var server = new SiteStreamGrpcServer(subscriber, _logger,
            Microsoft.Extensions.Options.Options.Create(new CommunicationOptions()));
        server.SetReady(Sys);

        var capture = new List<SiteStreamEvent>();
        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
        writer.WriteAsync(Arg.Any<SiteStreamEvent>(), Arg.Any<CancellationToken>())
            .Returns(Task.CompletedTask)
            .AndDoes(ci =>
            {
                var frame = ci.Arg<SiteStreamEvent>();
                lock (capture) { capture.Add(frame); }
            });

        var cts = new CancellationTokenSource();
        var context = CreateMockContext(cts.Token);

        var request = new InstanceStreamRequest
        {
            CorrelationId = correlationId,
            InstanceUniqueName = "Site1.Pump01",
            BatchingSupported = batchingSupported
        };

        var streamTask = Task.Run(() => server.SubscribeInstance(request, writer, context));
        await WaitForConditionAsync(() => capturedActor != null);

        return (server, capture, cts, streamTask, capturedActor!);
    }
}