using System.Buffers;
using System.Net;
using System.Net.Sockets;
using System.Text;
using NATS.Server.Mqtt;

namespace NATS.Server.Mqtt.Tests;

/// <summary>
/// Tests for the binary MQTT 3.1.1 wire protocol implementation.
/// Covers: TryRead, ParseUnsubscribe, new WriteXxx methods, PipeReader-based
/// connection handling, and MQTT 3.1.1 compliance rules.
/// </summary>
public class MqttBinaryProtocolTests
{
    // -----------------------------------------------------------------------
    // MqttPacketReader.TryRead tests
    // -----------------------------------------------------------------------

    [Fact]
    public void TryRead_complete_connect_packet_succeeds()
    {
        // Build a CONNECT packet
        var connectPayload = BuildConnectPayload("test-client");
        var raw = MqttPacketWriter.Write(MqttControlPacketType.Connect, connectPayload);
        var seq = new ReadOnlySequence<byte>(raw);

        MqttPacketReader.TryRead(seq, out var packet, out var consumed).ShouldBeTrue();
        packet.ShouldNotBeNull();
        packet.Type.ShouldBe(MqttControlPacketType.Connect);
        seq.GetOffset(consumed).ShouldBe(raw.Length);
    }

    [Fact]
    public void TryRead_returns_false_on_partial_fixed_header()
    {
        var seq = new ReadOnlySequence<byte>([0x10]); // just first byte, no remaining length
        MqttPacketReader.TryRead(seq, out var packet, out _).ShouldBeFalse();
        packet.ShouldBeNull();
    }

    [Fact]
    public void TryRead_returns_false_on_partial_payload()
    {
        // CONNECT with remaining length indicating 10 bytes but only 3 present
        var raw = new byte[] { 0x10, 10, 0x00, 0x04, 0x4D }; // truncated
        var seq = new ReadOnlySequence<byte>(raw);
        MqttPacketReader.TryRead(seq, out var packet, out _).ShouldBeFalse();
        packet.ShouldBeNull();
    }

    [Fact]
    public void TryRead_handles_multi_byte_remaining_length()
    {
        // Create a packet with remaining length = 200 (requires 2 bytes to encode)
        var payload = new byte[200];
        var raw = MqttPacketWriter.Write(MqttControlPacketType.Publish, payload, flags: 0x00);
        var seq = new ReadOnlySequence<byte>(raw);

        MqttPacketReader.TryRead(seq, out var packet, out var consumed).ShouldBeTrue();
        packet.ShouldNotBeNull();
        packet.Type.ShouldBe(MqttControlPacketType.Publish);
        packet.RemainingLength.ShouldBe(200);
    }

    [Fact]
    public void TryRead_handles_segmented_sequence()
    {
        // Simulate a split packet across two segments
        var connectPayload = BuildConnectPayload("seg-client");
        var raw = MqttPacketWriter.Write(MqttControlPacketType.Connect, connectPayload);
        var mid = raw.Length / 2;

        var first = new ReadOnlyMemory<byte>(raw, 0, mid);
        var second = new ReadOnlyMemory<byte>(raw, mid, raw.Length - mid);

        var firstSegment = new MemorySegment<byte>(first);
        var lastSegment = firstSegment.Append(second);
        var seq = new ReadOnlySequence<byte>(firstSegment, 0, lastSegment, second.Length);

        MqttPacketReader.TryRead(seq, out var packet, out _).ShouldBeTrue();
        packet.ShouldNotBeNull();
        packet.Type.ShouldBe(MqttControlPacketType.Connect);
    }

    [Fact]
    public void TryRead_reads_multiple_packets_from_buffer()
    {
        var ping = MqttPacketWriter.Write(MqttControlPacketType.PingReq, []);
        var combined = new byte[ping.Length * 3];
        ping.CopyTo(combined, 0);
        ping.CopyTo(combined, ping.Length);
        ping.CopyTo(combined, ping.Length * 2);

        var seq = new ReadOnlySequence<byte>(combined);
        var count = 0;

        while (MqttPacketReader.TryRead(seq, out var packet, out var consumed))
        {
            packet!.Type.ShouldBe(MqttControlPacketType.PingReq);
            seq = seq.Slice(consumed);
            count++;
        }

        count.ShouldBe(3);
    }

    [Fact]
    public void TryRead_zero_remaining_length_packet()
    {
        // PINGREQ has 0 remaining length
        var raw = MqttPacketWriter.Write(MqttControlPacketType.PingReq, []);
        raw.Length.ShouldBe(2); // 1 byte header + 1 byte remaining length (0)

        var seq = new ReadOnlySequence<byte>(raw);
        MqttPacketReader.TryRead(seq, out var packet, out _).ShouldBeTrue();
        packet!.Type.ShouldBe(MqttControlPacketType.PingReq);
        packet.RemainingLength.ShouldBe(0);
    }

    // -----------------------------------------------------------------------
    // MqttBinaryDecoder.ParseUnsubscribe tests
    // -----------------------------------------------------------------------

    [Fact]
    public void ParseUnsubscribe_single_filter()
    {
        var payload = new List<byte>();
        // Packet ID
        payload.Add(0x00);
        payload.Add(0x0A); // 10
        // Topic filter
        var filter = Encoding.UTF8.GetBytes("sensor/temp");
        payload.Add((byte)(filter.Length >> 8));
        payload.Add((byte)(filter.Length & 0xFF));
        payload.AddRange(filter);

        var result = MqttBinaryDecoder.ParseUnsubscribe([.. payload]);
        result.PacketId.ShouldBe((ushort)10);
        result.Filters.Count.ShouldBe(1);
        result.Filters[0].ShouldBe("sensor/temp");
    }

    [Fact]
    public void ParseUnsubscribe_multiple_filters()
    {
        var payload = new List<byte>();
        payload.Add(0x00);
        payload.Add(0x01); // Packet ID = 1
        foreach (var topic in new[] { "a/b", "c/d", "e/f" })
        {
            var bytes = Encoding.UTF8.GetBytes(topic);
            payload.Add((byte)(bytes.Length >> 8));
            payload.Add((byte)(bytes.Length & 0xFF));
            payload.AddRange(bytes);
        }

        var result = MqttBinaryDecoder.ParseUnsubscribe([.. payload]);
        result.PacketId.ShouldBe((ushort)1);
        result.Filters.Count.ShouldBe(3);
        result.Filters[0].ShouldBe("a/b");
        result.Filters[1].ShouldBe("c/d");
        result.Filters[2].ShouldBe("e/f");
    }

    [Fact]
    public void ParseUnsubscribe_rejects_invalid_flags()
    {
        var payload = new byte[] { 0x00, 0x01, 0x00, 0x01, (byte)'a' };
        Should.Throw<FormatException>(() => MqttBinaryDecoder.ParseUnsubscribe(payload, flags: 0x00));
    }

    [Fact]
    public void ParseUnsubscribe_rejects_empty_filter_list()
    {
        // Just packet ID, no filters
        var payload = new byte[] { 0x00, 0x01 };
        Should.Throw<FormatException>(() => MqttBinaryDecoder.ParseUnsubscribe(payload));
    }

    // -----------------------------------------------------------------------
    // MqttPacketWriter response helper tests
    // -----------------------------------------------------------------------

    [Fact]
    public void WriteConnAck_encodes_correctly()
    {
        var data = MqttPacketWriter.WriteConnAck(0x01, 0x00);
        var packet = MqttPacketReader.Read(data);
        packet.Type.ShouldBe(MqttControlPacketType.ConnAck);
        packet.RemainingLength.ShouldBe(2);
        packet.Payload.Span[0].ShouldBe((byte)0x01); // session present
        packet.Payload.Span[1].ShouldBe((byte)0x00); // accepted
    }

    [Fact]
    public void WritePubAck_round_trips_packet_id()
    {
        var data = MqttPacketWriter.WritePubAck(42);
        var packet = MqttPacketReader.Read(data);
        packet.Type.ShouldBe(MqttControlPacketType.PubAck);
        var id = (ushort)((packet.Payload.Span[0] << 8) | packet.Payload.Span[1]);
        id.ShouldBe((ushort)42);
    }

    [Fact]
    public void WriteSubAck_encodes_granted_qos()
    {
        byte[] grantedQoS = [0, 1, 2, 0x80]; // 0x80 = failure
        var data = MqttPacketWriter.WriteSubAck(99, grantedQoS);
        var packet = MqttPacketReader.Read(data);
        packet.Type.ShouldBe(MqttControlPacketType.SubAck);
        // Packet ID
        var id = (ushort)((packet.Payload.Span[0] << 8) | packet.Payload.Span[1]);
        id.ShouldBe((ushort)99);
        // QoS values
        packet.Payload.Span[2].ShouldBe((byte)0);
        packet.Payload.Span[3].ShouldBe((byte)1);
        packet.Payload.Span[4].ShouldBe((byte)2);
        packet.Payload.Span[5].ShouldBe((byte)0x80);
    }

    [Fact]
    public void WriteUnsubAck_round_trips_packet_id()
    {
        var data = MqttPacketWriter.WriteUnsubAck(7);
        var packet = MqttPacketReader.Read(data);
        packet.Type.ShouldBe(MqttControlPacketType.UnsubAck);
        var id = (ushort)((packet.Payload.Span[0] << 8) | packet.Payload.Span[1]);
        id.ShouldBe((ushort)7);
    }

    [Fact]
    public void WritePingResp_is_correct()
    {
        var data = MqttPacketWriter.WritePingResp();
        var packet = MqttPacketReader.Read(data);
        packet.Type.ShouldBe(MqttControlPacketType.PingResp);
        packet.RemainingLength.ShouldBe(0);
    }

    [Fact]
    public void WritePubRec_round_trips_packet_id()
    {
        var data = MqttPacketWriter.WritePubRec(100);
        var packet = MqttPacketReader.Read(data);
        packet.Type.ShouldBe(MqttControlPacketType.PubRec);
        var id = (ushort)((packet.Payload.Span[0] << 8) | packet.Payload.Span[1]);
        id.ShouldBe((ushort)100);
    }

    [Fact]
    public void WritePubRel_has_correct_flags()
    {
        var data = MqttPacketWriter.WritePubRel(50);
        var packet = MqttPacketReader.Read(data);
        packet.Type.ShouldBe(MqttControlPacketType.PubRel);
        packet.Flags.ShouldBe((byte)0x02); // PUBREL must have flags 0x02
    }

    [Fact]
    public void WritePubComp_round_trips_packet_id()
    {
        var data = MqttPacketWriter.WritePubComp(200);
        var packet = MqttPacketReader.Read(data);
        packet.Type.ShouldBe(MqttControlPacketType.PubComp);
        var id = (ushort)((packet.Payload.Span[0] << 8) | packet.Payload.Span[1]);
        id.ShouldBe((ushort)200);
    }

    [Fact]
    public void WritePublish_qos0_no_packet_id()
    {
        var data = MqttPacketWriter.WritePublish("test/topic", "hello"u8, qos: 0);
        var packet = MqttPacketReader.Read(data);
        packet.Type.ShouldBe(MqttControlPacketType.Publish);
        var pub = MqttBinaryDecoder.ParsePublish(packet.Payload.Span, packet.Flags);
        pub.Topic.ShouldBe("test/topic");
        pub.QoS.ShouldBe((byte)0);
        pub.PacketId.ShouldBe((ushort)0);
        Encoding.UTF8.GetString(pub.Payload.Span).ShouldBe("hello");
    }

    [Fact]
    public void WritePublish_qos1_with_flags()
    {
        var data = MqttPacketWriter.WritePublish("a/b", "data"u8, qos: 1, retain: true, dup: true, packetId: 5);
        var packet = MqttPacketReader.Read(data);
        var pub = MqttBinaryDecoder.ParsePublish(packet.Payload.Span, packet.Flags);
        pub.QoS.ShouldBe((byte)1);
        pub.Retain.ShouldBeTrue();
        pub.Dup.ShouldBeTrue();
        pub.PacketId.ShouldBe((ushort)5);
    }

    // -----------------------------------------------------------------------
    // Enum completeness
    // -----------------------------------------------------------------------

    [Theory]
    [InlineData(MqttControlPacketType.PubRec, 5)]
    [InlineData(MqttControlPacketType.PubRel, 6)]
    [InlineData(MqttControlPacketType.PubComp, 7)]
    [InlineData(MqttControlPacketType.Unsubscribe, 10)]
    [InlineData(MqttControlPacketType.UnsubAck, 11)]
    public void Enum_has_all_mqtt_packet_types(MqttControlPacketType type, byte expectedValue)
    {
        ((byte)type).ShouldBe(expectedValue);
    }

    // -----------------------------------------------------------------------
    // Binary connection integration tests (MQTT 3.1.1 compliance)
    // -----------------------------------------------------------------------

    [Fact]
    public async Task Binary_connect_and_ping_pong()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();

        // Send CONNECT
        await SendMqttPacketAsync(stream, BuildConnectPacket("ping-client"));

        // Read CONNACK
        var connAck = await ReadMqttPacketAsync(stream);
        connAck.Type.ShouldBe(MqttControlPacketType.ConnAck);
        connAck.Payload.Span[1].ShouldBe(MqttProtocolConstants.ConnAckAccepted);

        // Send PINGREQ
        await SendMqttPacketAsync(stream, MqttPacketWriter.Write(MqttControlPacketType.PingReq, []));

        // Read PINGRESP
        var pingResp = await ReadMqttPacketAsync(stream);
        pingResp.Type.ShouldBe(MqttControlPacketType.PingResp);
    }

    [Fact]
    public async Task Binary_first_packet_must_be_connect()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();

        // Send PINGREQ as first packet (not CONNECT) — should be disconnected
        await SendMqttPacketAsync(stream, MqttPacketWriter.Write(MqttControlPacketType.PingReq, []));

        // Connection should be closed
        var response = await ReadWithTimeoutAsync(stream, 500);
        response.ShouldBeNull();
    }

    [Fact]
    public async Task Binary_reject_bad_protocol_level()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();

        // CONNECT with protocol level 5 (not 4)
        var connectPayload = BuildConnectPayload("bad-level", protocolLevel: 5);
        await SendMqttPacketAsync(stream, MqttPacketWriter.Write(MqttControlPacketType.Connect, connectPayload));

        var connAck = await ReadMqttPacketAsync(stream);
        connAck.Type.ShouldBe(MqttControlPacketType.ConnAck);
        connAck.Payload.Span[1].ShouldBe(MqttProtocolConstants.ConnAckUnacceptableProtocolVersion);
    }

    [Fact]
    public async Task Binary_empty_clientid_clean_session_generates_id()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();

        // CONNECT with empty client ID + clean session
        var connectPayload = BuildConnectPayload("", cleanSession: true);
        await SendMqttPacketAsync(stream, MqttPacketWriter.Write(MqttControlPacketType.Connect, connectPayload));

        var connAck = await ReadMqttPacketAsync(stream);
        connAck.Payload.Span[1].ShouldBe(MqttProtocolConstants.ConnAckAccepted);
    }

    [Fact]
    public async Task Binary_empty_clientid_persistent_session_rejected()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();

        // CONNECT with empty client ID + persistent session
        var connectPayload = BuildConnectPayload("", cleanSession: false);
        await SendMqttPacketAsync(stream, MqttPacketWriter.Write(MqttControlPacketType.Connect, connectPayload));

        var connAck = await ReadMqttPacketAsync(stream);
        connAck.Payload.Span[1].ShouldBe(MqttProtocolConstants.ConnAckIdentifierRejected);
    }

    [Fact]
    public async Task Binary_auth_failure_returns_not_authorized()
    {
        await using var listener = new MqttListener("127.0.0.1", 0,
            requiredUsername: "admin", requiredPassword: "pass");
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();

        // CONNECT with wrong credentials
        var connectPayload = BuildConnectPayload("auth-fail", username: "wrong", password: "creds");
        await SendMqttPacketAsync(stream, MqttPacketWriter.Write(MqttControlPacketType.Connect, connectPayload));

        var connAck = await ReadMqttPacketAsync(stream);
        connAck.Payload.Span[1].ShouldBe(MqttProtocolConstants.ConnAckNotAuthorized);
    }

    [Fact]
    public async Task Binary_auth_success_with_credentials()
    {
        await using var listener = new MqttListener("127.0.0.1", 0,
            requiredUsername: "admin", requiredPassword: "secret");
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();

        var connectPayload = BuildConnectPayload("auth-ok", username: "admin", password: "secret");
        await SendMqttPacketAsync(stream, MqttPacketWriter.Write(MqttControlPacketType.Connect, connectPayload));

        var connAck = await ReadMqttPacketAsync(stream);
        connAck.Payload.Span[1].ShouldBe(MqttProtocolConstants.ConnAckAccepted);
    }

    [Fact]
    public async Task Binary_subscribe_and_publish_qos0()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        // Subscriber
        using var subTcp = new TcpClient();
        await subTcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var subStream = subTcp.GetStream();
        await ConnectAsync(subStream, "sub-client");

        // Subscribe to "test/topic"
        await SendMqttPacketAsync(subStream, BuildSubscribePacket(1, "test/topic", 0));
        var subAck = await ReadMqttPacketAsync(subStream);
        subAck.Type.ShouldBe(MqttControlPacketType.SubAck);

        // Publisher
        using var pubTcp = new TcpClient();
        await pubTcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var pubStream = pubTcp.GetStream();
        await ConnectAsync(pubStream, "pub-client");

        // Publish to "test/topic"
        await SendMqttPacketAsync(pubStream,
            MqttPacketWriter.WritePublish("test/topic", "hello binary"u8));

        // Subscriber should receive PUBLISH
        var received = await ReadMqttPacketAsync(subStream);
        received.Type.ShouldBe(MqttControlPacketType.Publish);
        var pub = MqttBinaryDecoder.ParsePublish(received.Payload.Span, received.Flags);
        pub.Topic.ShouldBe("test/topic");
        Encoding.UTF8.GetString(pub.Payload.Span).ShouldBe("hello binary");
    }

    [Fact]
    public async Task Binary_publish_qos1_gets_puback()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();
        await ConnectAsync(stream, "qos1-pub");

        // Publish QoS 1
        await SendMqttPacketAsync(stream,
            MqttPacketWriter.WritePublish("qos1/topic", "msg"u8, qos: 1, packetId: 42));

        var pubAck = await ReadMqttPacketAsync(stream);
        pubAck.Type.ShouldBe(MqttControlPacketType.PubAck);
        var id = (ushort)((pubAck.Payload.Span[0] << 8) | pubAck.Payload.Span[1]);
        id.ShouldBe((ushort)42);
    }

    [Fact]
    public async Task Binary_publish_qos2_full_flow()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();
        await ConnectAsync(stream, "qos2-pub");

        // Step 1: PUBLISH QoS 2
        await SendMqttPacketAsync(stream,
            MqttPacketWriter.WritePublish("qos2/topic", "msg"u8, qos: 2, packetId: 10));

        // Step 2: Receive PUBREC
        var pubRec = await ReadMqttPacketAsync(stream);
        pubRec.Type.ShouldBe(MqttControlPacketType.PubRec);

        // Step 3: Send PUBREL
        await SendMqttPacketAsync(stream, MqttPacketWriter.WritePubRel(10));

        // Step 4: Receive PUBCOMP
        var pubComp = await ReadMqttPacketAsync(stream);
        pubComp.Type.ShouldBe(MqttControlPacketType.PubComp);
        var id = (ushort)((pubComp.Payload.Span[0] << 8) | pubComp.Payload.Span[1]);
        id.ShouldBe((ushort)10);
    }

    [Fact]
    public async Task Binary_unsubscribe_returns_unsuback()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();
        await ConnectAsync(stream, "unsub-client");

        // Subscribe
        await SendMqttPacketAsync(stream, BuildSubscribePacket(1, "test/unsub", 0));
        _ = await ReadMqttPacketAsync(stream); // SUBACK

        // Unsubscribe
        await SendMqttPacketAsync(stream, BuildUnsubscribePacket(2, "test/unsub"));
        var unsubAck = await ReadMqttPacketAsync(stream);
        unsubAck.Type.ShouldBe(MqttControlPacketType.UnsubAck);
        var id = (ushort)((unsubAck.Payload.Span[0] << 8) | unsubAck.Payload.Span[1]);
        id.ShouldBe((ushort)2);
    }

    [Fact]
    public async Task Binary_unsubscribe_stops_message_delivery()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        // Subscriber
        using var subTcp = new TcpClient();
        await subTcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var subStream = subTcp.GetStream();
        await ConnectAsync(subStream, "unsub-recv");

        await SendMqttPacketAsync(subStream, BuildSubscribePacket(1, "nosub/topic", 0));
        _ = await ReadMqttPacketAsync(subStream); // SUBACK

        // Unsubscribe
        await SendMqttPacketAsync(subStream, BuildUnsubscribePacket(2, "nosub/topic"));
        _ = await ReadMqttPacketAsync(subStream); // UNSUBACK

        // Publisher
        using var pubTcp = new TcpClient();
        await pubTcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var pubStream = pubTcp.GetStream();
        await ConnectAsync(pubStream, "unsub-pub");

        await SendMqttPacketAsync(pubStream,
            MqttPacketWriter.WritePublish("nosub/topic", "invisible"u8));

        // Subscriber should NOT receive anything
        var result = await ReadWithTimeoutAsync(subStream, 200);
        result.ShouldBeNull();
    }

    [Fact]
    public async Task Binary_disconnect_clears_will_message()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        // Subscriber for will topic
        using var subTcp = new TcpClient();
        await subTcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var subStream = subTcp.GetStream();
        await ConnectAsync(subStream, "will-sub");
        await SendMqttPacketAsync(subStream, BuildSubscribePacket(1, "will/topic", 0));
        _ = await ReadMqttPacketAsync(subStream); // SUBACK

        // Client with will
        using var willTcp = new TcpClient();
        await willTcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var willStream = willTcp.GetStream();
        var connectPayload = BuildConnectPayload("will-client",
            willTopic: "will/topic", willMessage: "oops");
        await SendMqttPacketAsync(willStream, MqttPacketWriter.Write(MqttControlPacketType.Connect, connectPayload));
        _ = await ReadMqttPacketAsync(willStream); // CONNACK

        // Clean DISCONNECT — should clear will
        await SendMqttPacketAsync(willStream,
            MqttPacketWriter.Write(MqttControlPacketType.Disconnect, []));

        // Wait a bit and check that will was NOT published
        var result = await ReadWithTimeoutAsync(subStream, 300);
        result.ShouldBeNull();
    }

    [Fact]
    public async Task Binary_duplicate_clientid_takeover()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        // First connection
        using var tcp1 = new TcpClient();
        await tcp1.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream1 = tcp1.GetStream();
        await ConnectAsync(stream1, "dup-client");

        // Second connection with same client-id (takeover)
        using var tcp2 = new TcpClient();
        await tcp2.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream2 = tcp2.GetStream();
        await ConnectAsync(stream2, "dup-client");

        // First connection should be closed
        var result = await ReadWithTimeoutAsync(stream1, 500);
        result.ShouldBeNull();

        // Second connection should still work (PINGREQ/PINGRESP)
        await SendMqttPacketAsync(stream2, MqttPacketWriter.Write(MqttControlPacketType.PingReq, []));
        var pingResp = await ReadMqttPacketAsync(stream2);
        pingResp.Type.ShouldBe(MqttControlPacketType.PingResp);
    }

    [Fact]
    public async Task Binary_subscribe_flags_validation()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();
        await ConnectAsync(stream, "bad-sub-flags");

        // Send SUBSCRIBE with wrong flags (0x00 instead of 0x02)
        var subPayload = BuildSubscribePayload(1, "test/topic", 0);
        var badPacket = MqttPacketWriter.Write(MqttControlPacketType.Subscribe, subPayload, flags: 0x00);
        await SendMqttPacketAsync(stream, badPacket);

        // Connection should be closed
        var result = await ReadWithTimeoutAsync(stream, 500);
        result.ShouldBeNull();
    }

    [Fact]
    public async Task Binary_retained_message_tombstone()
    {
        await using var listener = new MqttListener("127.0.0.1", 0);
        await listener.StartAsync(CancellationToken.None);

        using var tcp = new TcpClient();
        await tcp.ConnectAsync(IPAddress.Loopback, listener.Port);
        var stream = tcp.GetStream();
        await ConnectAsync(stream, "retain-client");

        // Publish retained message
        await SendMqttPacketAsync(stream,
            MqttPacketWriter.WritePublish("retain/topic", "kept"u8, retain: true));

        // Wait for the server to process the retained publish
        for (var i = 0; i < 20; i++)
        {
            if (listener.GetRetainedMessage("retain/topic") != null)
                break;
            await Task.Delay(25);
        }

        // Verify retained
        listener.GetRetainedMessage("retain/topic").ShouldBe("kept");

        // Publish empty retained (tombstone)
        await SendMqttPacketAsync(stream,
            MqttPacketWriter.WritePublish("retain/topic", ReadOnlySpan<byte>.Empty, retain: true));

        // Wait for the server to process the packet
        for (var i = 0; i < 20; i++)
        {
            if (listener.GetRetainedMessage("retain/topic") == null)
                break;
            await Task.Delay(25);
        }

        // Verify tombstoned
        listener.GetRetainedMessage("retain/topic").ShouldBeNull();
    }

    // -----------------------------------------------------------------------
    // Helpers
    // -----------------------------------------------------------------------

    private static async Task ConnectAsync(NetworkStream stream, string clientId)
    {
        await SendMqttPacketAsync(stream, BuildConnectPacket(clientId));
        var connAck = await ReadMqttPacketAsync(stream);
        connAck.Type.ShouldBe(MqttControlPacketType.ConnAck);
        connAck.Payload.Span[1].ShouldBe(MqttProtocolConstants.ConnAckAccepted);
    }

    private static byte[] BuildConnectPacket(string clientId, string? username = null, string? password = null,
        bool cleanSession = true, byte protocolLevel = 4, string? willTopic = null, string? willMessage = null)
    {
        var payload = BuildConnectPayload(clientId, username, password, cleanSession, protocolLevel, willTopic, willMessage);
        return MqttPacketWriter.Write(MqttControlPacketType.Connect, payload);
    }

    private static byte[] BuildConnectPayload(string clientId, string? username = null, string? password = null,
        bool cleanSession = true, byte protocolLevel = 4, string? willTopic = null, string? willMessage = null)
    {
        var buf = new List<byte>();

        // Protocol name "MQTT"
        buf.AddRange(MqttPacketWriter.WriteString("MQTT"));

        // Protocol level
        buf.Add(protocolLevel);

        // Connect flags
        byte flags = 0;
        if (cleanSession) flags |= 0x02;
        if (username != null) flags |= 0x80;
        if (password != null) flags |= 0x40;
        if (willTopic != null)
        {
            flags |= 0x04; // will flag
            // will QoS = 0, will retain = 0
        }
        buf.Add(flags);

        // Keep-alive (60 seconds)
        buf.Add(0x00);
        buf.Add(0x3C);

        // Client ID
        buf.AddRange(MqttPacketWriter.WriteString(clientId));

        // Will topic + message
        if (willTopic != null)
        {
            buf.AddRange(MqttPacketWriter.WriteString(willTopic));
            buf.AddRange(MqttPacketWriter.WriteBytes(
                Encoding.UTF8.GetBytes(willMessage ?? "")));
        }

        // Username
        if (username != null)
            buf.AddRange(MqttPacketWriter.WriteString(username));

        // Password
        if (password != null)
            buf.AddRange(MqttPacketWriter.WriteString(password));

        return [.. buf];
    }

    private static byte[] BuildSubscribePacket(ushort packetId, string topic, byte qos)
    {
        var payload = BuildSubscribePayload(packetId, topic, qos);
        return MqttPacketWriter.Write(MqttControlPacketType.Subscribe, payload, flags: 0x02);
    }

    private static byte[] BuildSubscribePayload(ushort packetId, string topic, byte qos)
    {
        var buf = new List<byte>();
        buf.Add((byte)(packetId >> 8));
        buf.Add((byte)(packetId & 0xFF));
        buf.AddRange(MqttPacketWriter.WriteString(topic));
        buf.Add(qos);
        return [.. buf];
    }

    private static byte[] BuildUnsubscribePacket(ushort packetId, string topic)
    {
        var buf = new List<byte>();
        buf.Add((byte)(packetId >> 8));
        buf.Add((byte)(packetId & 0xFF));
        buf.AddRange(MqttPacketWriter.WriteString(topic));
        return MqttPacketWriter.Write(MqttControlPacketType.Unsubscribe, [.. buf], flags: 0x02);
    }

    private static async Task SendMqttPacketAsync(NetworkStream stream, byte[] packet)
    {
        await stream.WriteAsync(packet);
        await stream.FlushAsync();
    }

    private static async Task<MqttControlPacket> ReadMqttPacketAsync(NetworkStream stream, int timeoutMs = 2000)
    {
        using var cts = new CancellationTokenSource(timeoutMs);
        var buf = new byte[4096];
        var offset = 0;

        while (true)
        {
            var read = await stream.ReadAsync(buf.AsMemory(offset), cts.Token);
            if (read == 0)
                throw new IOException("Connection closed while reading MQTT packet");
            offset += read;

            var seq = new ReadOnlySequence<byte>(buf.AsMemory(0, offset));
            if (MqttPacketReader.TryRead(seq, out var packet, out _))
                return packet!;
        }
    }

    private static async Task<MqttControlPacket?> ReadWithTimeoutAsync(NetworkStream stream, int timeoutMs)
    {
        try
        {
            return await ReadMqttPacketAsync(stream, timeoutMs);
        }
        catch
        {
            return null;
        }
    }

    /// <summary>
    /// Helper for creating segmented ReadOnlySequence for split-packet tests.
    /// </summary>
    private sealed class MemorySegment<T> : ReadOnlySequenceSegment<T>
    {
        public MemorySegment(ReadOnlyMemory<T> memory)
        {
            Memory = memory;
        }

        public MemorySegment<T> Append(ReadOnlyMemory<T> memory)
        {
            var segment = new MemorySegment<T>(memory)
            {
                RunningIndex = RunningIndex + Memory.Length,
            };
            Next = segment;
            return segment;
        }
    }
}