using ZB.MOM.WW.OtOpcUa.Configuration.Enums;

namespace ZB.MOM.WW.OtOpcUa.Server.Redundancy;

/// <summary>
///     Orchestrates Phase 6.3 Stream C: feeds the <see cref="ServiceLevelCalculator"/> with the
///     current (topology, peer reachability, apply-in-progress, recovery dwell, self health)
///     inputs and emits the resulting <see cref="byte"/> + labelled <see cref="ServiceLevelBand"/>
///     to subscribers. The OPC UA <c>ServiceLevel</c> variable node consumes this via
///     <see cref="OnStateChanged"/> on every tick.
/// </summary>
/// <remarks>
///     Pure orchestration — no background timer, no OPC UA stack dep. The caller (a
///     HostedService in a future PR, or a test) drives <see cref="ComputeAndPublish"/> at
///     whatever cadence is appropriate. Each call reads the inputs + recomputes the ServiceLevel
///     byte; state is fired on the <see cref="OnStateChanged"/> event when the byte differs from
///     the last emitted value (edge-triggered). The <see cref="OnServerUriArrayChanged"/> event
///     fires whenever the topology's <c>ServerUriArray</c> content changes.
/// </remarks>
public sealed class RedundancyStatePublisher
{
    private readonly RedundancyCoordinator _coordinator;
    private readonly ApplyLeaseRegistry _leases;
    private readonly RecoveryStateManager _recovery;
    private readonly PeerReachabilityTracker _peers;
    private readonly Func<bool> _selfHealthy;
    private readonly Func<bool> _operatorMaintenance;
    private byte _lastByte = 255; // start at Authoritative — harmless before first tick
    private IReadOnlyList<string>? _lastServerUriArray;

    public RedundancyStatePublisher(
        RedundancyCoordinator coordinator,
        ApplyLeaseRegistry leases,
        RecoveryStateManager recovery,
        PeerReachabilityTracker peers,
        Func<bool>? selfHealthy = null,
        Func<bool>? operatorMaintenance = null)
    {
        ArgumentNullException.ThrowIfNull(coordinator);
        ArgumentNullException.ThrowIfNull(leases);
        ArgumentNullException.ThrowIfNull(recovery);
        ArgumentNullException.ThrowIfNull(peers);

        _coordinator = coordinator;
        _leases = leases;
        _recovery = recovery;
        _peers = peers;
        _selfHealthy = selfHealthy ?? (() => true);
        _operatorMaintenance = operatorMaintenance ?? (() => false);
    }

    /// <summary>
    ///     Fires with the current ServiceLevel byte + band on every call to
    ///     <see cref="ComputeAndPublish"/> when the byte differs from the previously-emitted one.
    /// </summary>
    public event Action<ServiceLevelSnapshot>? OnStateChanged;

    /// <summary>
    ///     Fires when the cluster's ServerUriArray (self + peers) content changes — e.g. an
    ///     operator adds or removes a peer. Consumer is the OPC UA <c>ServerUriArray</c>
    ///     variable node in Stream C.2.
    /// </summary>
    public event Action<IReadOnlyList<string>>? OnServerUriArrayChanged;

    /// <summary>Snapshot of the last-published ServiceLevel byte — diagnostics + tests.</summary>
    public byte LastByte => _lastByte;

    /// <summary>
    ///     Compute the current ServiceLevel + emit change events if anything moved. Caller
    ///     drives cadence — a 1 s tick in production is reasonable; tests drive it directly.
    /// </summary>
    public ServiceLevelSnapshot ComputeAndPublish()
    {
        var topology = _coordinator.Current;
        if (topology is null)
        {
            // Not yet initialized — surface NoData so clients don't treat us as authoritative.
            return Emit((byte)ServiceLevelBand.NoData, null);
        }

        // Aggregate peer reachability. For 2-node v2.0 clusters there is at most one peer;
        // treat "all peers healthy" as the boolean input to the calculator.
        var peerReachable = topology.Peers.All(p => _peers.Get(p.NodeId).BothHealthy);
        var peerUaHealthy = topology.Peers.All(p => _peers.Get(p.NodeId).UaHealthy);
        var peerHttpHealthy = topology.Peers.All(p => _peers.Get(p.NodeId).HttpHealthy);

        var role = MapRole(topology.SelfRole);

        var value = ServiceLevelCalculator.Compute(
            role: role,
            selfHealthy: _selfHealthy(),
            peerUaHealthy: peerUaHealthy,
            peerHttpHealthy: peerHttpHealthy,
            applyInProgress: _leases.IsApplyInProgress,
            recoveryDwellMet: _recovery.IsDwellMet(),
            topologyValid: _coordinator.IsTopologyValid,
            operatorMaintenance: _operatorMaintenance());

        MaybeFireServerUriArray(topology);
        return Emit(value, topology);
    }

    private static RedundancyRole MapRole(RedundancyRole role) => role switch
    {
        // Standalone is serving; treat as Primary for the matrix since the calculator
        // already special-cases Standalone inside its Compute.
        RedundancyRole.Primary => RedundancyRole.Primary,
        RedundancyRole.Secondary => RedundancyRole.Secondary,
        _ => RedundancyRole.Standalone,
    };

    private ServiceLevelSnapshot Emit(byte value, RedundancyTopology? topology)
    {
        var snap = new ServiceLevelSnapshot(
            Value: value,
            Band: ServiceLevelCalculator.Classify(value),
            Topology: topology);

        if (value != _lastByte)
        {
            _lastByte = value;
            OnStateChanged?.Invoke(snap);
        }
        return snap;
    }

    private void MaybeFireServerUriArray(RedundancyTopology topology)
    {
        var current = topology.ServerUriArray();
        if (_lastServerUriArray is null || !current.SequenceEqual(_lastServerUriArray, StringComparer.Ordinal))
        {
            _lastServerUriArray = current;
            OnServerUriArrayChanged?.Invoke(current);
        }
    }
}

/// <summary>Per-tick output of <see cref="RedundancyStatePublisher.ComputeAndPublish"/>.</summary>
public sealed record ServiceLevelSnapshot(
    byte Value,
    ServiceLevelBand Band,
    RedundancyTopology? Topology);