using System.Text.Json;
using Shouldly;
using Xunit;

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

/// <summary>
///     Unit tests for upstream-redundancy failover (PR-14, issue #286). The driver
///     exposes two test seams — <see cref="OpcUaClientDriver.InjectServiceLevelDropForTest"/>
///     and <see cref="OpcUaClientDriver.RedundancyFailoverHookForTest"/> — that bypass
///     the SDK's session-create + TransferSubscriptions machinery so we can assert the
///     decision logic without standing up two real OPC UA sessions.
/// </summary>
[Trait("Category", "Unit")]
public sealed class OpcUaClientRedundancyTests
{
    [Fact]
    public void Redundancy_options_default_to_disabled()
    {
        var opts = new OpcUaClientDriverOptions();
        opts.Redundancy.ShouldNotBeNull();
        opts.Redundancy.Enabled.ShouldBeFalse(
            "default deployments do client-side failover via EndpointUrls; upstream redundancy is opt-in");
        opts.Redundancy.ServiceLevelThreshold.ShouldBe((ushort)200,
            "OPC UA spec convention: 200+ = healthy, lower = degraded");
        opts.Redundancy.ResolvedRecheckInterval.ShouldBe(TimeSpan.FromSeconds(5));
    }

    [Fact]
    public void DTO_json_round_trip_preserves_redundancy_settings()
    {
        var opts = new OpcUaClientDriverOptions
        {
            EndpointUrl = "opc.tcp://primary:4840",
            Redundancy = new RedundancyOptions(
                Enabled: true,
                ServiceLevelThreshold: 150,
                RecheckInterval: TimeSpan.FromSeconds(10)),
        };

        var json = JsonSerializer.Serialize(opts);
        var roundTripped = JsonSerializer.Deserialize<OpcUaClientDriverOptions>(json);

        roundTripped.ShouldNotBeNull();
        roundTripped!.Redundancy.Enabled.ShouldBeTrue();
        roundTripped.Redundancy.ServiceLevelThreshold.ShouldBe((ushort)150);
        roundTripped.Redundancy.ResolvedRecheckInterval.ShouldBe(TimeSpan.FromSeconds(10));
    }

    [Fact]
    public void Disabled_redundancy_does_not_failover_on_low_servicelevel()
    {
        // Even a value of 0 (unrecoverable per the spec) should be a no-op when the
        // feature is disabled — the driver shouldn't be reading ServerArray or watching
        // ServiceLevel at all in that mode.
        var opts = new OpcUaClientDriverOptions
        {
            EndpointUrl = "opc.tcp://primary:4840",
            Redundancy = new RedundancyOptions(Enabled: false),
        };
        using var drv = new OpcUaClientDriver(opts, "opcua-redundancy-disabled");
        var hookFired = false;
        drv.RedundancyFailoverHookForTest = (_, _) => { hookFired = true; return Task.FromResult(true); };

        drv.InjectServiceLevelDropForTest(0);

        hookFired.ShouldBeFalse(
            "Redundancy.Enabled=false means ServiceLevel drops must not trigger failover");
        drv.RedundancyFailoverInvocationsForTest.ShouldBe(0);
    }

    [Fact]
    public void ServiceLevel_above_threshold_does_not_trigger_failover()
    {
        var opts = new OpcUaClientDriverOptions
        {
            EndpointUrl = "opc.tcp://primary:4840",
            Redundancy = new RedundancyOptions(Enabled: true, ServiceLevelThreshold: 200),
        };
        using var drv = new OpcUaClientDriver(opts, "opcua-redundancy-healthy");
        SeedPeers(drv, "opc.tcp://secondary:4840");
        var hookFired = false;
        drv.RedundancyFailoverHookForTest = (_, _) => { hookFired = true; return Task.FromResult(true); };

        // Equal to threshold = healthy boundary; spec semantics treat 200 as healthy.
        drv.InjectServiceLevelDropForTest(200);
        // Just above threshold = healthy.
        drv.InjectServiceLevelDropForTest(220);

        hookFired.ShouldBeFalse(
            "ServiceLevel >= threshold must not trigger failover — healthy primary stays put");
    }

    [Fact]
    public void ServiceLevel_below_threshold_triggers_failover_with_secondary_uri()
    {
        var opts = new OpcUaClientDriverOptions
        {
            EndpointUrl = "opc.tcp://primary:4840",
            Redundancy = new RedundancyOptions(Enabled: true, ServiceLevelThreshold: 200),
        };
        using var drv = new OpcUaClientDriver(opts, "opcua-redundancy-failover");
        SeedPeers(drv, "opc.tcp://primary:4840", "opc.tcp://secondary:4840");
        SeedActive(drv, "opc.tcp://primary:4840");

        string? failoverTarget = null;
        drv.RedundancyFailoverHookForTest = (uri, _) =>
        {
            failoverTarget = uri;
            return Task.FromResult(true);
        };

        drv.InjectServiceLevelDropForTest(50);

        // Wait for the fire-and-forget Task to complete. The driver dispatches FailoverAsync
        // via discard — give it a beat to land.
        Wait(() => failoverTarget is not null);

        failoverTarget.ShouldBe("opc.tcp://secondary:4840",
            "the failover path picks the next URI in ServerArray that isn't the active one");
        var diags1 = drv.GetHealth().Diagnostics;
        diags1.ShouldNotBeNull();
        diags1!.ShouldContainKey("RedundancyFailoverCount");
        diags1["RedundancyFailoverCount"].ShouldBe(1);
    }

    [Fact]
    public void Empty_peer_list_does_not_trigger_failover()
    {
        // Upstream with RedundancySupport=None (or one that simply doesn't expose the
        // ServerUriArray node) leaves _redundancyPeers empty. ServiceLevel drops in that
        // mode are diagnostic-only — the driver has no peer to swap to.
        var opts = new OpcUaClientDriverOptions
        {
            EndpointUrl = "opc.tcp://primary:4840",
            Redundancy = new RedundancyOptions(Enabled: true),
        };
        using var drv = new OpcUaClientDriver(opts, "opcua-redundancy-no-peers");
        var hookFired = false;
        drv.RedundancyFailoverHookForTest = (_, _) => { hookFired = true; return Task.FromResult(true); };

        drv.InjectServiceLevelDropForTest(50);

        hookFired.ShouldBeFalse(
            "ServerArray empty means there's nowhere to fail over to — drop is informational only");
    }

    [Fact]
    public void Failover_with_only_active_uri_in_peer_list_does_not_swap_to_self()
    {
        // Edge case: the upstream advertises itself in ServerUriArray but no actual peers.
        // The driver must not try to fail over to the URI it's already on.
        var opts = new OpcUaClientDriverOptions
        {
            EndpointUrl = "opc.tcp://primary:4840",
            Redundancy = new RedundancyOptions(Enabled: true),
        };
        using var drv = new OpcUaClientDriver(opts, "opcua-redundancy-self-only");
        SeedPeers(drv, "opc.tcp://primary:4840");
        SeedActive(drv, "opc.tcp://primary:4840");
        var hookFired = false;
        drv.RedundancyFailoverHookForTest = (_, _) => { hookFired = true; return Task.FromResult(true); };

        drv.InjectServiceLevelDropForTest(50);

        hookFired.ShouldBeFalse(
            "the only peer in the list is the active URI itself — there's nothing to swap to");
    }

    [Fact]
    public void Failover_failure_increments_failures_counter_and_keeps_session()
    {
        var opts = new OpcUaClientDriverOptions
        {
            EndpointUrl = "opc.tcp://primary:4840",
            Redundancy = new RedundancyOptions(Enabled: true),
        };
        using var drv = new OpcUaClientDriver(opts, "opcua-redundancy-failure");
        SeedPeers(drv, "opc.tcp://primary:4840", "opc.tcp://secondary:4840");
        SeedActive(drv, "opc.tcp://primary:4840");

        drv.RedundancyFailoverHookForTest = (_, _) => Task.FromResult(false);

        drv.InjectServiceLevelDropForTest(50);

        Wait(() => drv.GetHealth().Diagnostics is { } d
            && d.TryGetValue("RedundancyFailoverFailures", out var f) && f >= 1);

        var diags = drv.GetHealth().Diagnostics;
        diags.ShouldNotBeNull();
        diags!.ShouldContainKey("RedundancyFailoverFailures");
        diags["RedundancyFailoverFailures"].ShouldBe(1);
        diags["RedundancyFailoverCount"].ShouldBe(0,
            "a failed swap must not bump the success counter");
    }

    [Fact]
    public void Repeated_drops_within_recheck_interval_only_failover_once()
    {
        var opts = new OpcUaClientDriverOptions
        {
            EndpointUrl = "opc.tcp://primary:4840",
            Redundancy = new RedundancyOptions(
                Enabled: true,
                ServiceLevelThreshold: 200,
                RecheckInterval: TimeSpan.FromMinutes(5)),
        };
        using var drv = new OpcUaClientDriver(opts, "opcua-redundancy-debounce");
        SeedPeers(drv, "opc.tcp://primary:4840", "opc.tcp://secondary:4840");
        SeedActive(drv, "opc.tcp://primary:4840");

        var calls = 0;
        drv.RedundancyFailoverHookForTest = (_, _) =>
        {
            Interlocked.Increment(ref calls);
            return Task.FromResult(true);
        };

        drv.InjectServiceLevelDropForTest(50);
        Wait(() => calls >= 1);
        drv.InjectServiceLevelDropForTest(50);
        drv.InjectServiceLevelDropForTest(40);

        // RecheckInterval = 5 minutes — the second + third drops should be suppressed
        // because the first failover landed inside the window.
        calls.ShouldBe(1,
            "RecheckInterval suppresses oscillation around the threshold so a flapping primary doesn't ping-pong");
    }

    [Fact]
    public void Diagnostics_exposes_redundancy_counters_in_snapshot()
    {
        // The `driver-diagnostics` RPC reads through GetHealth(); operators expect the
        // redundancy counters in the snapshot regardless of whether failover ever fired.
        var opts = new OpcUaClientDriverOptions
        {
            EndpointUrl = "opc.tcp://primary:4840",
            Redundancy = new RedundancyOptions(Enabled: true),
        };
        using var drv = new OpcUaClientDriver(opts, "opcua-redundancy-diag");

        var d = drv.GetHealth().Diagnostics;
        d.ShouldNotBeNull();
        d!.ShouldContainKey("RedundancyFailoverCount");
        d.ShouldContainKey("RedundancyFailoverFailures");
        d["RedundancyFailoverCount"].ShouldBe(0);
        d["RedundancyFailoverFailures"].ShouldBe(0);
    }

    // ---- helpers ----

    private static void SeedPeers(OpcUaClientDriver drv, params string[] peers)
    {
        // The driver normally populates _redundancyPeers from a session ReadValue call.
        // For unit testing we use reflection to seed the field directly — the alternative
        // (mocking ISession) brings most of the OPC UA SDK into the test surface.
        var field = typeof(OpcUaClientDriver).GetField(
            "_redundancyPeers",
            System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!;
        field.SetValue(drv, (IReadOnlyList<string>)peers);
    }

    private static void SeedActive(OpcUaClientDriver drv, string uri)
    {
        var diag = drv.DiagnosticsForTest;
        diag.SetActiveServerUri(uri);
    }

    private static void Wait(Func<bool> predicate, int timeoutMs = 2000)
    {
        var deadline = DateTime.UtcNow.AddMilliseconds(timeoutMs);
        while (DateTime.UtcNow < deadline)
        {
            if (predicate()) return;
            Thread.Sleep(10);
        }
    }
}