using Shouldly;
using Xunit;

namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;

/// <summary>
///     Transport self-heal (review 07 P3), single in-gate connect attempt + failed-client disposal (L3)
///     and the response-PDU length check (H9).
/// </summary>
[Trait("Category", "Unit")]
public sealed class ModbusTransportSelfHealTests
{
    private static readonly byte[] ReadHr0 = [0x03, 0x00, 0x00, 0x00, 0x01];

    /// <summary>
    ///     P3: two lost replies in a row leave the socket torn down. The NEXT call must reconnect on its own
    ///     instead of failing fast with "Transport not connected" until the 90 s comms-loss watchdog.
    /// </summary>
    [Fact]
    public async Task After_two_consecutive_timeouts_the_next_call_reconnects()
    {
        await using var server = new ScriptedModbusTcpServer
        {
            Script = i => i < 2 ? ScriptedModbusTcpServer.Action.Stall : ScriptedModbusTcpServer.Action.Answer,
        };
        await using var transport = new ModbusTcpTransport("127.0.0.1", server.Port, TimeSpan.FromMilliseconds(500));
        await transport.ConnectAsync(TestContext.Current.CancellationToken);

        // First call: stall → teardown → reconnect → resend → stall again → propagates.
        await Should.ThrowAsync<ModbusTransportDesyncException>(
            () => transport.SendAsync(1, ReadHr0, TestContext.Current.CancellationToken));

        // Second call: the socket is gone; the transport must dial again and succeed.
        var resp = await transport.SendAsync(1, ReadHr0, TestContext.Current.CancellationToken);
        resp[0].ShouldBe((byte)0x03);
        server.AcceptedConnections.ShouldBe(3);
    }

    /// <summary>
    ///     P3 throttle: after a FAILED connect, calls inside the backoff window fail fast with
    ///     <see cref="ModbusTransportUnavailableException"/> and make no connect attempt; once the window has
    ///     passed the next call dials again. Positive control: the device is back before the throttled call,
    ///     so an un-throttled transport would have reconnected.
    /// </summary>
    [Fact]
    public async Task Failed_connect_is_throttled_then_retried_after_the_window()
    {
        await using var server = new ScriptedModbusTcpServer();
        await using var transport = new ModbusTcpTransport("127.0.0.1", server.Port, TimeSpan.FromSeconds(2));
        var now = 1_000_000L;
        transport.Lifecycle.NowMs = () => now;
        await transport.ConnectAsync(TestContext.Current.CancellationToken);
        (await transport.SendAsync(1, ReadHr0, TestContext.Current.CancellationToken))[0].ShouldBe((byte)0x03);

        server.Stop();
        // Dead socket → EOF → teardown → ONE reconnect attempt → refused.
        var first = await Should.ThrowAsync<Exception>(
            () => transport.SendAsync(1, ReadHr0, TestContext.Current.CancellationToken));
        ModbusSocketLifecycle.IsSocketLevelFailure(first).ShouldBeTrue();

        server.Start();
        var acceptedBefore = server.AcceptedConnections;
        await Should.ThrowAsync<ModbusTransportUnavailableException>(
            () => transport.SendAsync(1, ReadHr0, TestContext.Current.CancellationToken));
        server.AcceptedConnections.ShouldBe(acceptedBefore, "a throttled call must not dial");

        now += 60_000; // past any window (MaxDelay default 30 s)
        (await transport.SendAsync(1, ReadHr0, TestContext.Current.CancellationToken))[0].ShouldBe((byte)0x03);
        server.AcceptedConnections.ShouldBe(acceptedBefore + 1);
    }

    /// <summary>
    ///     L3: the reconnect never sleeps inside the transport gate. With a 30 s InitialDelay the old
    ///     <c>ConnectWithBackoffAsync</c> held the gate for 30 s before even dialling; now one attempt is made
    ///     and the call returns at once.
    /// </summary>
    [Fact]
    public async Task Reconnect_after_drop_does_not_sleep_inside_the_gate()
    {
        await using var server = new ScriptedModbusTcpServer();
        await using var transport = new ModbusTcpTransport("127.0.0.1", server.Port, TimeSpan.FromSeconds(2),
            reconnect: new ModbusReconnectOptions { InitialDelay = TimeSpan.FromSeconds(30) });
        await transport.ConnectAsync(TestContext.Current.CancellationToken);
        server.Stop();

        var call = transport.SendAsync(1, ReadHr0, TestContext.Current.CancellationToken);
        var ex = await Should.ThrowAsync<Exception>(() => call.WaitAsync(TimeSpan.FromSeconds(10), TestContext.Current.CancellationToken));
        ex.ShouldNotBeOfType<TimeoutException>("the send must not wait out the 30 s backoff inside the gate");
    }

    /// <summary>H9: an MBAP frame carrying no PDU is a truncated-frame desync, not an IndexOutOfRangeException.</summary>
    [Fact]
    public async Task Empty_response_pdu_is_a_truncated_frame_desync()
    {
        await using var server = new ScriptedModbusTcpServer
        {
            RawReply = (_, header, _) => ScriptedModbusTcpServer.Adu(header, []),
        };
        await using var transport = new ModbusTcpTransport("127.0.0.1", server.Port, TimeSpan.FromSeconds(2), autoReconnect: false);
        await transport.ConnectAsync(TestContext.Current.CancellationToken);

        var ex = await Should.ThrowAsync<ModbusTransportDesyncException>(
            () => transport.SendAsync(1, ReadHr0, TestContext.Current.CancellationToken));
        ex.Reason.ShouldBe(ModbusTransportDesyncException.DesyncReason.TruncatedFrame);
    }

    /// <summary>H9: an exception frame missing its exception code is a truncated-frame desync.</summary>
    [Fact]
    public async Task Exception_frame_without_code_is_a_truncated_frame_desync()
    {
        await using var server = new ScriptedModbusTcpServer
        {
            RawReply = (_, header, _) => ScriptedModbusTcpServer.Adu(header, [0x83]),
        };
        await using var transport = new ModbusTcpTransport("127.0.0.1", server.Port, TimeSpan.FromSeconds(2), autoReconnect: false);
        await transport.ConnectAsync(TestContext.Current.CancellationToken);

        var ex = await Should.ThrowAsync<ModbusTransportDesyncException>(
            () => transport.SendAsync(1, ReadHr0, TestContext.Current.CancellationToken));
        ex.Reason.ShouldBe(ModbusTransportDesyncException.DesyncReason.TruncatedFrame);
    }

    /// <summary>Positive control for the two H9 tests: a well-formed exception frame still surfaces as a
    /// <see cref="ModbusException"/> carrying its code.</summary>
    [Fact]
    public async Task Well_formed_exception_frame_still_surfaces_as_ModbusException()
    {
        await using var server = new ScriptedModbusTcpServer
        {
            RawReply = (_, header, _) => ScriptedModbusTcpServer.Adu(header, [0x83, 0x02]),
        };
        await using var transport = new ModbusTcpTransport("127.0.0.1", server.Port, TimeSpan.FromSeconds(2), autoReconnect: false);
        await transport.ConnectAsync(TestContext.Current.CancellationToken);

        var ex = await Should.ThrowAsync<ModbusException>(
            () => transport.SendAsync(1, ReadHr0, TestContext.Current.CancellationToken));
        ex.ExceptionCode.ShouldBe((byte)0x02);
    }
}