using System.Threading.Channels;
using Microsoft.Extensions.Logging.Abstractions;
using Microsoft.Extensions.Time.Testing;
using ZB.MOM.WW.MxGateway.Contracts.Proto;
using ZB.MOM.WW.MxGateway.Server.Sessions;

namespace ZB.MOM.WW.MxGateway.Tests.Gateway.Sessions;

/// <summary>
///     Concurrency and fan-out tests for <see cref="SessionEventDistributor"/>, the
///     Session Resilience epic's per-session event pump. One pump drains the source
///     exactly once and fans every event to N independent per-subscriber channels.
///     Every async wait is bounded so a fan-out or shutdown deadlock fails fast.
/// </summary>
public sealed class SessionEventDistributorTests
{
    private static readonly TimeSpan ReadTimeout = TimeSpan.FromSeconds(5);

    [Fact]
    public async Task TwoSubscribers_BothReceiveFannedEventsInOrder()
    {
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(source.Reader);
        await distributor.StartAsync(CancellationToken.None);

        using IEventSubscriberLease leaseA = distributor.Register();
        using IEventSubscriberLease leaseB = distributor.Register();

        source.Writer.TryWrite(Event(1));
        source.Writer.TryWrite(Event(2));

        MxEvent a1 = await ReadOneAsync(leaseA.Reader);
        MxEvent a2 = await ReadOneAsync(leaseA.Reader);
        MxEvent b1 = await ReadOneAsync(leaseB.Reader);
        MxEvent b2 = await ReadOneAsync(leaseB.Reader);

        Assert.Equal(1ul, a1.WorkerSequence);
        Assert.Equal(2ul, a2.WorkerSequence);
        Assert.Equal(1ul, b1.WorkerSequence);
        Assert.Equal(2ul, b2.WorkerSequence);
    }

    [Fact]
    public async Task DisposingOneLease_StopsItsDelivery_OtherKeepsReceiving()
    {
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(source.Reader);
        await distributor.StartAsync(CancellationToken.None);

        IEventSubscriberLease leaseA = distributor.Register();
        using IEventSubscriberLease leaseB = distributor.Register();

        source.Writer.TryWrite(Event(1));
        _ = await ReadOneAsync(leaseA.Reader);
        _ = await ReadOneAsync(leaseB.Reader);

        leaseA.Dispose();

        // A's reader must complete (no more delivery) after dispose.
        await AssertCompletedAsync(leaseA.Reader);

        // B still receives subsequent events.
        source.Writer.TryWrite(Event(2));
        MxEvent b2 = await ReadOneAsync(leaseB.Reader);
        Assert.Equal(2ul, b2.WorkerSequence);
    }

    [Fact]
    public async Task SubscriberRegisteredAfterStart_ReceivesEventsEmittedAfterRegistration()
    {
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(source.Reader);
        await distributor.StartAsync(CancellationToken.None);

        using IEventSubscriberLease leaseA = distributor.Register();
        source.Writer.TryWrite(Event(1));
        _ = await ReadOneAsync(leaseA.Reader);

        // Late subscriber: only sees events emitted after it registered.
        using IEventSubscriberLease leaseB = distributor.Register();
        source.Writer.TryWrite(Event(2));

        MxEvent b = await ReadOneAsync(leaseB.Reader);
        Assert.Equal(2ul, b.WorkerSequence);
    }

    [Fact]
    public async Task DisposingDistributor_CompletesAllSubscriberChannels_AndStopsPump()
    {
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        SessionEventDistributor distributor = CreateDistributor(source.Reader);
        await distributor.StartAsync(CancellationToken.None);

        using IEventSubscriberLease leaseA = distributor.Register();
        using IEventSubscriberLease leaseB = distributor.Register();

        // Bounded so a shutdown hang fails fast.
        await distributor.DisposeAsync().AsTask().WaitAsync(ReadTimeout);

        await AssertCompletedAsync(leaseA.Reader);
        await AssertCompletedAsync(leaseB.Reader);
    }

    [Fact]
    public async Task Register_AfterDispose_ThrowsObjectDisposedException()
    {
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        SessionEventDistributor distributor = CreateDistributor(source.Reader);
        await distributor.StartAsync(CancellationToken.None);

        await distributor.DisposeAsync().AsTask().WaitAsync(ReadTimeout);

        Assert.Throws<ObjectDisposedException>(() => distributor.Register());
    }

    [Fact]
    public async Task ReplayBuffer_OverCapacity_EvictsOldestFirst_AndReportsGap()
    {
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(
            source.Reader,
            replayBufferCapacity: 3,
            replayRetentionSeconds: 0);
        await distributor.StartAsync(CancellationToken.None);

        // A live subscriber forces the pump to fan (and thereby retain) each event,
        // and gives us a deterministic point to know the pump has processed event 5.
        using IEventSubscriberLease lease = distributor.Register();
        for (ulong sequence = 1; sequence <= 5; sequence++)
        {
            source.Writer.TryWrite(Event(sequence));
        }

        for (ulong sequence = 1; sequence <= 5; sequence++)
        {
            MxEvent e = await ReadOneAsync(lease.Reader);
            Assert.Equal(sequence, e.WorkerSequence);
        }

        // Capacity 3 retains only the newest three: sequences 3, 4, 5. Events 1 and 2
        // were evicted, so a caller asking from 0 missed events => gap=true, and it
        // gets only the retained tail.
        bool found = distributor.TryGetReplayFrom(0, out IReadOnlyList<MxEvent> replay, out bool gap);

        Assert.True(found);
        Assert.True(gap);
        Assert.Equal(new ulong[] { 3, 4, 5 }, replay.Select(e => e.WorkerSequence));
    }

    [Fact]
    public async Task ReplayBuffer_WithinRetainedWindow_ReturnsNewerEvents_NoGap()
    {
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(
            source.Reader,
            replayBufferCapacity: 10,
            replayRetentionSeconds: 0);
        await distributor.StartAsync(CancellationToken.None);

        using IEventSubscriberLease lease = distributor.Register();
        for (ulong sequence = 1; sequence <= 5; sequence++)
        {
            source.Writer.TryWrite(Event(sequence));
            _ = await ReadOneAsync(lease.Reader);
        }

        // afterSequence 2 is still inside the retained window [1..5], so no gap and
        // exactly the newer events 3, 4, 5 come back.
        bool found = distributor.TryGetReplayFrom(2, out IReadOnlyList<MxEvent> replay, out bool gap);

        Assert.True(found);
        Assert.False(gap);
        Assert.Equal(new ulong[] { 3, 4, 5 }, replay.Select(e => e.WorkerSequence));
    }

    [Fact]
    public async Task ReplayBuffer_AgedEntries_AreEvictedAfterRetentionElapses()
    {
        FakeTimeProvider time = new(new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero));
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(
            source.Reader,
            replayBufferCapacity: 100,
            replayRetentionSeconds: 30,
            timeProvider: time);
        await distributor.StartAsync(CancellationToken.None);

        using IEventSubscriberLease lease = distributor.Register();

        // Two old events, then advance the clock well past the retention window.
        source.Writer.TryWrite(Event(1));
        source.Writer.TryWrite(Event(2));
        _ = await ReadOneAsync(lease.Reader);
        _ = await ReadOneAsync(lease.Reader);

        time.Advance(TimeSpan.FromSeconds(60));

        // A fresh event triggers age-eviction of the now-stale entries 1 and 2.
        source.Writer.TryWrite(Event(3));
        _ = await ReadOneAsync(lease.Reader);

        bool found = distributor.TryGetReplayFrom(0, out IReadOnlyList<MxEvent> replay, out bool gap);

        Assert.True(found);
        // Events 1 and 2 aged out; only 3 remains, and 0 predates the oldest retained.
        Assert.Equal(new ulong[] { 3 }, replay.Select(e => e.WorkerSequence));
        Assert.True(gap);
    }

    [Fact]
    public async Task ReplayBuffer_AfterSequenceNewerThanAllRetained_ReturnsEmpty_NoGap()
    {
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(
            source.Reader,
            replayBufferCapacity: 10,
            replayRetentionSeconds: 0);
        await distributor.StartAsync(CancellationToken.None);

        using IEventSubscriberLease lease = distributor.Register();
        for (ulong sequence = 1; sequence <= 3; sequence++)
        {
            source.Writer.TryWrite(Event(sequence));
            _ = await ReadOneAsync(lease.Reader);
        }

        // afterSequence 3 is at/after the newest retained; nothing newer, and the
        // caller is fully caught up => empty list, gap=false.
        bool found = distributor.TryGetReplayFrom(3, out IReadOnlyList<MxEvent> replay, out bool gap);

        Assert.True(found);
        Assert.False(gap);
        Assert.Empty(replay);
    }

    [Fact]
    public async Task ReplayBuffer_Capacity0_AfterSequenceBelowHighestSeen_ReportsGap_NoEvents()
    {
        // Disabled buffer: events are tracked for the highest-seen counter but not
        // retained. A caller behind the highest-seen sequence must be told to re-snapshot.
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(
            source.Reader,
            replayBufferCapacity: 0,
            replayRetentionSeconds: 0);
        await distributor.StartAsync(CancellationToken.None);

        using IEventSubscriberLease lease = distributor.Register();
        for (ulong sequence = 1; sequence <= 3; sequence++)
        {
            source.Writer.TryWrite(Event(sequence));
            _ = await ReadOneAsync(lease.Reader);
        }

        // afterSequence=1 is below highestSeen=3 — gap, nothing to replay.
        bool found = distributor.TryGetReplayFrom(1, out IReadOnlyList<MxEvent> replay, out bool gap);

        Assert.True(found);
        Assert.True(gap);
        Assert.Empty(replay);
    }

    [Fact]
    public async Task ReplayBuffer_Capacity0_AfterSequenceAtOrAboveHighestSeen_NoGap_NoEvents()
    {
        // Disabled buffer: caller is already caught up — no gap, nothing to replay.
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(
            source.Reader,
            replayBufferCapacity: 0,
            replayRetentionSeconds: 0);
        await distributor.StartAsync(CancellationToken.None);

        using IEventSubscriberLease lease = distributor.Register();
        for (ulong sequence = 1; sequence <= 3; sequence++)
        {
            source.Writer.TryWrite(Event(sequence));
            _ = await ReadOneAsync(lease.Reader);
        }

        // afterSequence=3 equals highestSeen — caller is fully caught up.
        bool found = distributor.TryGetReplayFrom(3, out IReadOnlyList<MxEvent> replay, out bool gap);

        Assert.True(found);
        Assert.False(gap);
        Assert.Empty(replay);
    }

    [Fact]
    public async Task ReplayBuffer_NoEventsSeen_AnyAfterSequence_NoGap_NoEvents()
    {
        // No events ever seen: nothing can have been missed, so gap must be false.
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(
            source.Reader,
            replayBufferCapacity: 0,
            replayRetentionSeconds: 0);
        // Pump not started — no events arrive.

        bool found = distributor.TryGetReplayFrom(0, out IReadOnlyList<MxEvent> replay, out bool gap);

        Assert.True(found);
        Assert.False(gap);
        Assert.Empty(replay);
    }

    [Fact]
    public async Task ReplayBuffer_AfterSequenceMaxValue_WithRetainedEvents_NoGap_NoNewEvents()
    {
        // ulong.MaxValue as afterSequence: afterSequence + 1 would wrap to 0, which the
        // old code used to compare against oldestRetained, falsely reporting gap=true.
        // The corrected formula must yield gap=false and an empty replay list.
        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
        await using SessionEventDistributor distributor = CreateDistributor(
            source.Reader,
            replayBufferCapacity: 10,
            replayRetentionSeconds: 0);
        await distributor.StartAsync(CancellationToken.None);

        using IEventSubscriberLease lease = distributor.Register();
        source.Writer.TryWrite(Event(1));
        _ = await ReadOneAsync(lease.Reader);

        bool found = distributor.TryGetReplayFrom(ulong.MaxValue, out IReadOnlyList<MxEvent> replay, out bool gap);

        Assert.True(found);
        Assert.False(gap);
        Assert.Empty(replay);
    }

    private static SessionEventDistributor CreateDistributor(ChannelReader<MxEvent> source)
        => CreateDistributor(source, replayBufferCapacity: 1024, replayRetentionSeconds: 300);

    private static SessionEventDistributor CreateDistributor(
        ChannelReader<MxEvent> source,
        int replayBufferCapacity,
        double replayRetentionSeconds,
        TimeProvider? timeProvider = null)
        => new(
            "session-test",
            ct => source.ReadAllAsync(ct),
            subscriberQueueCapacity: 64,
            replayBufferCapacity: replayBufferCapacity,
            replayRetentionSeconds: replayRetentionSeconds,
            NullLogger<SessionEventDistributor>.Instance,
            timeProvider ?? TimeProvider.System);

    private static MxEvent Event(ulong sequence)
        => new() { SessionId = "session-test", WorkerSequence = sequence };

    private static async Task<MxEvent> ReadOneAsync(ChannelReader<MxEvent> reader)
    {
        await reader.WaitToReadAsync().AsTask().WaitAsync(ReadTimeout);
        Assert.True(reader.TryRead(out MxEvent? value));
        return value!;
    }

    private static async Task AssertCompletedAsync(ChannelReader<MxEvent> reader)
    {
        // Drain anything still buffered, then assert the channel is completed
        // (no further events). Bounded so a never-completing channel fails fast.
        await reader.Completion.WaitAsync(ReadTimeout);
    }
}