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");
        _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;
    }

    [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.
        Assert.Equal(baseline + 1, ReadGauge());

        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.
        Assert.Equal(baseline, ReadGauge());
    }

    [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");
    }
}