using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Channels;
using System.Threading.Tasks;
using FluentAssertions;
using Xunit;
using ZB.MOM.WW.LmxProxy.Host.Domain;
using ZB.MOM.WW.LmxProxy.Host.Subscriptions;

namespace ZB.MOM.WW.LmxProxy.Host.Tests.Subscriptions
{
    public class SubscriptionManagerTests
    {
        /// <summary>Fake IScadaClient for testing (no COM dependency).</summary>
        private class FakeScadaClient : IScadaClient
        {
            public bool IsConnected => true;
            public ConnectionState ConnectionState => ConnectionState.Connected;
            public event EventHandler<ConnectionStateChangedEventArgs>? ConnectionStateChanged;
            public Task ConnectAsync(CancellationToken ct = default) => Task.CompletedTask;
            public Task DisconnectAsync(CancellationToken ct = default) => Task.CompletedTask;
            public Task<Vtq> ReadAsync(string address, CancellationToken ct = default) =>
                Task.FromResult(Vtq.Good(42.0));
            public Task<IReadOnlyDictionary<string, Vtq>> ReadBatchAsync(IEnumerable<string> addresses, CancellationToken ct = default) =>
                Task.FromResult<IReadOnlyDictionary<string, Vtq>>(new Dictionary<string, Vtq>());
            public Task WriteAsync(string address, object value, CancellationToken ct = default) => Task.CompletedTask;
            public Task WriteBatchAsync(IReadOnlyDictionary<string, object> values, CancellationToken ct = default) => Task.CompletedTask;
            public Task<(bool flagReached, int elapsedMs)> WriteBatchAndWaitAsync(
                IReadOnlyDictionary<string, object> values, string flagTag, object flagValue,
                int timeoutMs, int pollIntervalMs, CancellationToken ct = default) =>
                Task.FromResult((false, 0));
            public Task<ProbeResult> ProbeConnectionAsync(string testTagAddress, int timeoutMs, CancellationToken ct = default) =>
                Task.FromResult(ProbeResult.Healthy(Quality.Good, DateTime.UtcNow));
            public ValueTask DisposeAsync() => default;

            // Track subscribe/unsubscribe calls for assertions
            public List<List<string>> SubscribeCalls { get; } = new List<List<string>>();
            public List<List<string>> UnsubscribeCalls { get; } = new List<List<string>>();
            public List<Action<string, Vtq>> StoredCallbacks { get; } = new List<Action<string, Vtq>>();

            // When true, SubscribeAsync throws to simulate MxAccess being down
            public bool FailSubscriptions { get; set; }

            public Task UnsubscribeByAddressAsync(IEnumerable<string> addresses)
            {
                UnsubscribeCalls.Add(addresses.ToList());
                return Task.CompletedTask;
            }

            public Task<IAsyncDisposable> SubscribeAsync(IEnumerable<string> addresses, Action<string, Vtq> callback, CancellationToken ct = default)
            {
                var addressList = addresses.ToList();
                SubscribeCalls.Add(addressList);
                StoredCallbacks.Add(callback);

                if (FailSubscriptions)
                    throw new InvalidOperationException("Not connected to MxAccess");

                return Task.FromResult<IAsyncDisposable>(new FakeSubscriptionHandle());
            }

            // Suppress unused event warning
            internal void FireEvent() => ConnectionStateChanged?.Invoke(this, null!);

            private class FakeSubscriptionHandle : IAsyncDisposable
            {
                public ValueTask DisposeAsync() => default;
            }
        }

        [Fact]
        public async Task Subscribe_ReturnsChannelReader()
        {
            using var sm = new SubscriptionManager(new FakeScadaClient());
            using var cts = new CancellationTokenSource();
            var reader = await sm.SubscribeAsync("client1", new[] { "Tag1", "Tag2" }, cts.Token);
            reader.Should().NotBeNull();
        }

        [Fact]
        public async Task OnTagValueChanged_FansOutToSubscribedClients()
        {
            using var sm = new SubscriptionManager(new FakeScadaClient());
            using var cts = new CancellationTokenSource();
            var reader = await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);

            var vtq = Vtq.Good(42.0);
            sm.OnTagValueChanged("Motor.Speed", vtq);

            var result = await reader.ReadAsync(cts.Token);
            result.address.Should().Be("Motor.Speed");
            result.vtq.Value.Should().Be(42.0);
            result.vtq.Quality.Should().Be(Quality.Good);
        }

        [Fact]
        public async Task OnTagValueChanged_MultipleClients_BothReceive()
        {
            using var sm = new SubscriptionManager(new FakeScadaClient());
            using var cts = new CancellationTokenSource();
            var reader1 = await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);
            var reader2 = await sm.SubscribeAsync("client2", new[] { "Motor.Speed" }, cts.Token);

            sm.OnTagValueChanged("Motor.Speed", Vtq.Good(99.0));

            var r1 = await reader1.ReadAsync(cts.Token);
            var r2 = await reader2.ReadAsync(cts.Token);
            r1.vtq.Value.Should().Be(99.0);
            r2.vtq.Value.Should().Be(99.0);
        }

        [Fact]
        public async Task OnTagValueChanged_NonSubscribedTag_NoDelivery()
        {
            using var sm = new SubscriptionManager(new FakeScadaClient());
            using var cts = new CancellationTokenSource();
            var reader = await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);

            sm.OnTagValueChanged("Motor.Torque", Vtq.Good(10.0));

            // Channel should be empty
            reader.TryRead(out _).Should().BeFalse();
        }

        [Fact]
        public async Task UnsubscribeClient_CompletesChannel()
        {
            using var sm = new SubscriptionManager(new FakeScadaClient());
            using var cts = new CancellationTokenSource();
            var reader = await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);

            sm.UnsubscribeClient("client1");

            // Channel should be completed
            reader.Completion.IsCompleted.Should().BeTrue();
        }

        [Fact]
        public async Task UnsubscribeClient_RemovesFromTagSubscriptions()
        {
            using var sm = new SubscriptionManager(new FakeScadaClient());
            using var cts = new CancellationTokenSource();
            await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);

            sm.UnsubscribeClient("client1");

            var stats = sm.GetStats();
            stats.TotalClients.Should().Be(0);
            stats.TotalTags.Should().Be(0);
        }

        [Fact]
        public async Task RefCounting_LastClientUnsubscribeRemovesTag()
        {
            using var sm = new SubscriptionManager(new FakeScadaClient());
            using var cts = new CancellationTokenSource();
            await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);
            await sm.SubscribeAsync("client2", new[] { "Motor.Speed" }, cts.Token);

            sm.GetStats().TotalTags.Should().Be(1);

            sm.UnsubscribeClient("client1");
            sm.GetStats().TotalTags.Should().Be(1); // client2 still subscribed

            sm.UnsubscribeClient("client2");
            sm.GetStats().TotalTags.Should().Be(0); // last client gone
        }

        [Fact]
        public async Task NotifyDisconnection_SendsBadQualityToAll()
        {
            using var sm = new SubscriptionManager(new FakeScadaClient());
            using var cts = new CancellationTokenSource();
            var reader = await sm.SubscribeAsync("client1", new[] { "Motor.Speed", "Motor.Torque" }, cts.Token);

            sm.NotifyDisconnection();

            // Should receive 2 bad quality messages
            reader.TryRead(out var r1).Should().BeTrue();
            r1.vtq.Quality.Should().Be(Quality.Bad_NotConnected);
            reader.TryRead(out var r2).Should().BeTrue();
            r2.vtq.Quality.Should().Be(Quality.Bad_NotConnected);
        }

        [Fact]
        public async Task Backpressure_DropOldest_DropsWhenFull()
        {
            using var sm = new SubscriptionManager(new FakeScadaClient(), channelCapacity: 3);
            using var cts = new CancellationTokenSource();
            var reader = await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);

            // Fill the channel beyond capacity
            for (int i = 0; i < 10; i++)
            {
                sm.OnTagValueChanged("Motor.Speed", Vtq.Good((double)i));
            }

            // Should have exactly 3 messages (capacity limit)
            int count = 0;
            while (reader.TryRead(out _)) count++;
            count.Should().Be(3);
        }

        [Fact]
        public async Task GetStats_ReturnsCorrectCounts()
        {
            using var sm = new SubscriptionManager(new FakeScadaClient());
            using var cts = new CancellationTokenSource();
            await sm.SubscribeAsync("c1", new[] { "Tag1", "Tag2" }, cts.Token);
            await sm.SubscribeAsync("c2", new[] { "Tag2", "Tag3" }, cts.Token);

            var stats = sm.GetStats();
            stats.TotalClients.Should().Be(2);
            stats.TotalTags.Should().Be(3); // Tag1, Tag2, Tag3
            stats.ActiveSubscriptions.Should().Be(4); // c1:Tag1, c1:Tag2, c2:Tag2, c2:Tag3
        }

        // ── New tests for race condition fix ──────────────────────────

        [Fact]
        public async Task SubscribeAfterUnsubscribe_CreatesMxAccessSubscriptions()
        {
            // Verifies FIX 1: when a client disconnects and reconnects with the same tags,
            // the new subscribe must create fresh MxAccess subscriptions (not skip them
            // because old handles still exist).
            var fake = new FakeScadaClient();
            using var sm = new SubscriptionManager(fake);
            using var cts = new CancellationTokenSource();

            // First client subscribes
            await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);
            fake.SubscribeCalls.Should().HaveCount(1);
            fake.SubscribeCalls[0].Should().Contain("Motor.Speed");

            // Client disconnects — unsubscribe removes the tag (ref count → 0)
            sm.UnsubscribeClient("client1");
            fake.UnsubscribeCalls.Should().HaveCount(1);
            fake.UnsubscribeCalls[0].Should().Contain("Motor.Speed");

            // Same client reconnects — must create a NEW MxAccess subscription
            await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);
            fake.SubscribeCalls.Should().HaveCount(2, "new subscribe must create fresh MxAccess subscription");
            fake.SubscribeCalls[1].Should().Contain("Motor.Speed");
        }

        [Fact]
        public async Task SubscribeAfterUnsubscribe_SerializedByGate()
        {
            // Verifies FIX 1: subscribe and unsubscribe are serialized so they cannot
            // interleave and cause the race condition.
            var fake = new FakeScadaClient();
            using var sm = new SubscriptionManager(fake);
            using var cts = new CancellationTokenSource();

            var tags = new[] { "Tag.A", "Tag.B", "Tag.C" };

            // Subscribe, unsubscribe, re-subscribe in sequence
            await sm.SubscribeAsync("session1", tags, cts.Token);
            sm.UnsubscribeClient("session1");
            await sm.SubscribeAsync("session2", tags, cts.Token);

            // Both subscribes should have called SubscribeAsync on the scada client
            fake.SubscribeCalls.Should().HaveCount(2);
            // The unsubscribe in between should have cleaned up
            fake.UnsubscribeCalls.Should().HaveCount(1);

            // Data should flow to the new session
            var reader = await sm.SubscribeAsync("session3", tags, cts.Token);
            sm.OnTagValueChanged("Tag.A", Vtq.Good(1.0));
            var result = await reader.ReadAsync(cts.Token);
            result.vtq.Value.Should().Be(1.0);
        }

        [Fact]
        public async Task OnTagValueChanged_NoDuplicateDelivery()
        {
            // Verifies FIX 2: each OnDataChange produces exactly one VTQ per client,
            // not two (which happened when both stored callback and OnTagValueChanged
            // property were invoked).
            var fake = new FakeScadaClient();
            using var sm = new SubscriptionManager(fake);
            using var cts = new CancellationTokenSource();

            var reader = await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);

            // Deliver one update
            sm.OnTagValueChanged("Motor.Speed", Vtq.Good(42.0));

            // Should receive exactly one message
            reader.TryRead(out var msg).Should().BeTrue();
            msg.vtq.Value.Should().Be(42.0);

            // No duplicate
            reader.TryRead(out _).Should().BeFalse("each update should be delivered exactly once");
        }

        [Fact]
        public async Task FailedSubscription_StoredAsPending_RetriedOnReconnect()
        {
            // Verifies FIX 3: when MxAccess is down during subscribe, tags are stored
            // as pending and retried when NotifyReconnection is called.
            var fake = new FakeScadaClient();
            fake.FailSubscriptions = true;
            using var sm = new SubscriptionManager(fake);
            using var cts = new CancellationTokenSource();

            // Subscribe while MxAccess is "down" — should not throw (errors are logged)
            var reader = await sm.SubscribeAsync("client1", new[] { "Motor.Speed" }, cts.Token);
            reader.Should().NotBeNull();
            fake.SubscribeCalls.Should().HaveCount(1);

            // MxAccess comes back up
            fake.FailSubscriptions = false;
            sm.NotifyReconnection();

            // Give the async retry a moment to complete
            await Task.Delay(100);

            // Should have retried the subscription
            fake.SubscribeCalls.Should().HaveCount(2, "pending subscriptions should be retried on reconnect");
            fake.SubscribeCalls[1].Should().Contain("Motor.Speed");
        }
    }
}