using Akka.Actor;
using Akka.Cluster;
using Akka.Cluster.Tools.PublishSubscribe;
using Akka.Event;
using ZB.MOM.WW.OtOpcUa.Commons.Messages.Redundancy;
using ZB.MOM.WW.OtOpcUa.Commons.Types;
using CommonsRedundancyRole = ZB.MOM.WW.OtOpcUa.Commons.Messages.Redundancy.RedundancyRole;

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

/// <summary>
/// Admin-role cluster singleton that aggregates per-node cluster events into a
/// <see cref="RedundancyStateChanged"/> snapshot and publishes it on the <c>redundancy-state</c>
/// DistributedPubSub topic. Subscribers (notably the OPC UA host's ServiceLevel calc) react to
/// topology changes without polling.
///
/// Recomputation is debounced by <see cref="DebounceWindow"/> — a burst of cluster events from
/// a rolling restart should produce one published snapshot, not one per event.
/// </summary>
public sealed class RedundancyStateActor : ReceiveActor, IWithTimers
{
    public const string Topic = "redundancy-state";
    public static readonly TimeSpan DebounceWindow = TimeSpan.FromMilliseconds(250);

    private readonly ILoggingAdapter _log = Context.GetLogger();
    private readonly Akka.Cluster.Cluster _cluster;
    private readonly Action<object>? _broadcastOverride;
    private bool _dirty;

    public ITimerScheduler Timers { get; set; } = null!;

    public static Props Props(Action<object>? broadcast = null) =>
        Akka.Actor.Props.Create(() => new RedundancyStateActor(broadcast));

    public RedundancyStateActor() : this(broadcast: null) { }

    public RedundancyStateActor(Action<object>? broadcast)
    {
        _cluster = Akka.Cluster.Cluster.Get(Context.System);
        _broadcastOverride = broadcast;

        Receive<ClusterEvent.IMemberEvent>(_ => MarkDirty());
        Receive<ClusterEvent.LeaderChanged>(_ => MarkDirty());
        Receive<ClusterEvent.RoleLeaderChanged>(_ => MarkDirty());
        Receive<ClusterEvent.CurrentClusterState>(_ => MarkDirty());
        Receive<ClusterEvent.ReachabilityEvent>(_ => MarkDirty());
        Receive<RecomputeNow>(_ => PublishIfDirty());
    }

    protected override void PreStart()
    {
        _cluster.Subscribe(
            Self,
            ClusterEvent.InitialStateAsEvents,
            typeof(ClusterEvent.IMemberEvent),
            typeof(ClusterEvent.LeaderChanged),
            typeof(ClusterEvent.RoleLeaderChanged),
            typeof(ClusterEvent.ReachabilityEvent));
    }

    protected override void PostStop() => _cluster.Unsubscribe(Self);

    private void MarkDirty()
    {
        _dirty = true;
        Timers.StartSingleTimer("debounce", RecomputeNow.Instance, DebounceWindow);
    }

    private void PublishIfDirty()
    {
        if (!_dirty) return;
        _dirty = false;

        var snapshot = BuildSnapshot();
        var msg = new RedundancyStateChanged(snapshot, CorrelationId.NewId());
        if (_broadcastOverride is not null) _broadcastOverride(msg);
        else DistributedPubSub.Get(Context.System).Mediator.Tell(new Publish(Topic, msg));
        _log.Debug("Published RedundancyStateChanged with {Count} nodes", snapshot.Count);
    }

    private IReadOnlyList<NodeRedundancyState> BuildSnapshot()
    {
        var driverLeader = _cluster.State.RoleLeader("driver");
        var clusterLeader = _cluster.State.Leader;
        var now = DateTime.UtcNow;

        var list = new List<NodeRedundancyState>(_cluster.State.Members.Count);
        foreach (var member in _cluster.State.Members)
        {
            var host = member.Address.Host;
            if (string.IsNullOrWhiteSpace(host)) continue;

            var role = member.Roles.Contains("driver")
                ? (driverLeader == member.Address ? CommonsRedundancyRole.Primary : CommonsRedundancyRole.Secondary)
                : CommonsRedundancyRole.Detached;

            list.Add(new NodeRedundancyState(
                NodeId.Parse(host),
                role,
                IsClusterLeader: clusterLeader == member.Address,
                IsRoleLeaderForDriver: driverLeader == member.Address,
                AsOfUtc: now));
        }
        return list;
    }

    public sealed class RecomputeNow
    {
        public static readonly RecomputeNow Instance = new();
        private RecomputeNow() { }
    }
}