using System.Collections.Immutable;
using Akka.Actor;
using Akka.Cluster.Tools.Client;
using Akka.Cluster.Tools.PublishSubscribe;
using Akka.Event;
using Microsoft.EntityFrameworkCore;
using ZB.MOM.WW.OtOpcUa.Cluster;
using ZB.MOM.WW.OtOpcUa.Commons.Messages.Deploy;
using ZB.MOM.WW.OtOpcUa.Commons.Messages.Mesh;
using ZB.MOM.WW.OtOpcUa.Configuration;

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

/// <summary>
///     Central-side end of the central↔node command boundary (per-cluster mesh Phase 2). Routes
///     every central→node command out — over DistributedPubSub or ClusterClient, per
///     <see cref="MeshTransportOptions.Mode"/> — and forwards inbound <see cref="ApplyAck"/>s to the
///     deploy coordinator singleton.
/// </summary>
/// <remarks>
///     <para>
///         <b>Registered per node, NOT as a cluster singleton.</b> A driver node's ClusterClient
///         rotates across contact points and must find a live comm actor at whichever central node
///         answers. A singleton would be reachable only through the one node hosting it, and the
///         rotation would fail silently against the other.
///     </para>
///     <para>
///         <b>Exactly one ClusterClient, fleet-wide.</b> See <see cref="RebuildClient"/> — this is
///         the single most important thing to understand before changing this actor.
///     </para>
/// </remarks>
public sealed class CentralCommunicationActor : ReceiveActor, IWithTimers
{
    private const string RefreshTimerKey = "mesh-contact-refresh";

    private readonly IDbContextFactory<OtOpcUaConfigDbContext> _dbFactory;
    private readonly MeshTransportOptions _options;
    private readonly IMeshClusterClientFactory _clientFactory;
    private readonly Func<IActorRef?> _coordinator;
    private readonly ILoggingAdapter _log = Context.GetLogger();
    private readonly bool _useClusterClient;

    private IActorRef? _client;
    private ImmutableHashSet<string> _currentContacts = ImmutableHashSet<string>.Empty;
    private CancellationTokenSource _lifecycle = new();

    /// <summary>Gets the timer scheduler driving the periodic contact refresh.</summary>
    public ITimerScheduler Timers { get; set; } = null!;

    /// <summary>Creates the props for this actor.</summary>
    /// <param name="dbFactory">Factory for the config database holding <c>ClusterNode</c> rows.</param>
    /// <param name="options">The bound mesh-transport options.</param>
    /// <param name="clientFactory">Creates the ClusterClient; substituted in tests.</param>
    /// <param name="coordinator">
    ///     Resolves the deploy-coordinator singleton proxy. A <see cref="Func{TResult}"/> rather than
    ///     an <see cref="IActorRef"/> so this actor never depends on registration order at wiring
    ///     time — it resolves on first use and caches.
    /// </param>
    /// <returns>The props.</returns>
    public static Props Props(
        IDbContextFactory<OtOpcUaConfigDbContext> dbFactory,
        MeshTransportOptions options,
        IMeshClusterClientFactory clientFactory,
        Func<IActorRef?> coordinator) =>
        Akka.Actor.Props.Create(() =>
            new CentralCommunicationActor(dbFactory, options, clientFactory, coordinator));

    /// <summary>Initializes a new instance of the <see cref="CentralCommunicationActor"/> class.</summary>
    /// <param name="dbFactory">Factory for the config database.</param>
    /// <param name="options">The bound mesh-transport options.</param>
    /// <param name="clientFactory">Creates the ClusterClient.</param>
    /// <param name="coordinator">Lazily resolves the deploy-coordinator singleton proxy.</param>
    public CentralCommunicationActor(
        IDbContextFactory<OtOpcUaConfigDbContext> dbFactory,
        MeshTransportOptions options,
        IMeshClusterClientFactory clientFactory,
        Func<IActorRef?> coordinator)
    {
        _dbFactory = dbFactory;
        _options = options;
        _clientFactory = clientFactory;
        _coordinator = coordinator;
        _useClusterClient = string.Equals(
            options.Mode, MeshTransportOptions.ModeClusterClient, StringComparison.OrdinalIgnoreCase);

        Receive<MeshCommand>(HandleMeshCommand);
        Receive<ApplyAck>(HandleApplyAck);
        Receive<RefreshContacts>(_ => LoadContactsFromDb());
        Receive<ContactsLoaded>(HandleContactsLoaded);

        // A faulted LoadContactsFromDb task is piped here as a Status.Failure. Without this handler
        // the failure is an unhandled message (debug level only) and the refresh fails silently —
        // an operator cannot distinguish "no nodes configured" from "the database is down", and the
        // contact set silently freezes at whatever it last held.
        Receive<Status.Failure>(f => _log.Warning(
            f.Cause,
            "Failed to load ClusterNode contact points; the mesh ClusterClient contact set was NOT "
            + "refreshed and may be stale or empty. Commands may be dropped until the next refresh"));

        Receive<SubscribeAck>(_ => { /* DPS subscribe confirmation */ });
    }

    /// <inheritdoc />
    protected override void PreStart()
    {
        if (!_useClusterClient)
        {
            _log.Info(
                "Mesh transport is {Mode}: commands are published on DistributedPubSub and no "
                + "ClusterClient is created", _options.Mode);
            return;
        }

        Timers.StartPeriodicTimer(
            RefreshTimerKey,
            new RefreshContacts(),
            TimeSpan.Zero,
            TimeSpan.FromSeconds(_options.ContactRefreshSeconds));
    }

    /// <inheritdoc />
    protected override void PostStop()
    {
        _lifecycle.Cancel();
        _lifecycle.Dispose();
    }

    private void HandleMeshCommand(MeshCommand cmd)
    {
        if (!_useClusterClient)
        {
            DistributedPubSub.Get(Context.System).Mediator.Tell(new Publish(cmd.Topic, cmd.Message));
            return;
        }

        if (_client is null)
        {
            // App-level drop, matching the sister project's decision and this repo's buffer-size = 0.
            // The command is NOT queued and will NOT be retried: a deploy fails at the coordinator's
            // apply deadline naming the silent nodes, which is a better operator signal than a
            // command arriving minutes late against a config the DB already recorded as failed.
            _log.Warning(
                "No mesh ClusterClient — dropping {MessageType}. The last contact refresh produced "
                + "no usable contact point from the enabled, non-maintenance ClusterNode rows; the "
                + "command is not buffered and will not be retried",
                cmd.Message.GetType().Name);
            return;
        }

        // SendToAll, NEVER Send. Today's DPS publish reaches EVERY DriverHostActor — there is no
        // ClusterId or node filter on the node side at all; scoping happens later, inside the
        // artifact (DeploymentArtifact.ResolveClusterScope). ClusterClient.Send delivers to exactly
        // ONE registered actor, which would deploy to a single node of the fleet while every other
        // node silently kept its old configuration — and the deployment would still seal green if
        // the ack set happened to be satisfied.
        _client.Tell(new ClusterClient.SendToAll(MeshPaths.NodeCommunication, cmd.Message));
    }

    private void HandleApplyAck(ApplyAck ack)
    {
        var coordinator = _coordinator();
        if (coordinator is null)
        {
            _log.Warning(
                "Received ApplyAck for {DeploymentId} from {NodeId} but the deploy coordinator is "
                + "not resolvable on this node; the ack is lost and the deployment will time out "
                + "naming a node that applied successfully",
                ack.DeploymentId, ack.NodeId);
            return;
        }

        // Forward, not Tell: preserves the original sender across the hop so the coordinator sees
        // the node rather than this relay.
        coordinator.Forward(ack);
    }

    private void LoadContactsFromDb()
    {
        var self = Self;
        CancellationToken ct;
        try
        {
            ct = _lifecycle.Token;
        }
        catch (ObjectDisposedException)
        {
            return; // Stopping.
        }

        var systemName = Context.System.Name;
        var dbFactory = _dbFactory;

        // Off the actor thread, result piped back as a message. A synchronous DB read here would
        // block every command dispatch behind a slow or unreachable SQL Server.
        Task.Run(async () =>
        {
            await using var db = await dbFactory.CreateDbContextAsync(ct).ConfigureAwait(false);
            var rows = await db.ClusterNodes
                .AsNoTracking()
                .Where(n => n.Enabled && !n.MaintenanceMode)
                .Select(n => new { n.NodeId, n.Host, n.AkkaPort })
                .ToListAsync(ct)
                .ConfigureAwait(false);

            var contacts = new List<string>(rows.Count);
            var malformed = new List<string>();
            foreach (var row in rows)
            {
                var address = $"akka.tcp://{systemName}@{row.Host}:{row.AkkaPort}/system/receptionist";

                // Parse up front, per row, inside the loop's own guard. A single malformed row must
                // not abort the whole refresh and leave the contact set half-built — the sister
                // project shipped that bug and its regression test deliberately orders the bad row
                // first.
                if (ActorPath.TryParse(address, out _)) contacts.Add(address);
                else malformed.Add($"{row.NodeId} -> {address}");
            }

            return new ContactsLoaded(contacts, malformed);
        }, ct).PipeTo(self);
    }

    private void HandleContactsLoaded(ContactsLoaded msg)
    {
        foreach (var bad in msg.Malformed)
        {
            _log.Warning(
                "ClusterNode {Row} does not yield a usable receptionist address; skipping it in this "
                + "refresh (other nodes are unaffected). Check the row's Host and AkkaPort", bad);
        }

        var contacts = msg.Contacts.ToImmutableHashSet();

        if (contacts.IsEmpty)
        {
            _log.Warning(
                "No usable ClusterNode contact points; the mesh ClusterClient was not created and "
                + "every central→node command will be dropped until a refresh finds one");
            return;
        }

        if (_client is not null && _currentContacts.SetEquals(contacts)) return;

        RebuildClient(contacts);
    }

    /// <summary>Stops any existing client and creates a replacement for the new contact set.</summary>
    /// <remarks>
    ///     <para>
    ///         <b>TODO(Phase 6): one client per application <c>Cluster</c>.</b> Today this actor
    ///         creates exactly <b>one</b> client whose contacts are the whole fleet, and that is not
    ///         a compromise — it is the only correct shape while the fleet is a single Akka mesh.
    ///     </para>
    ///     <para>
    ///         A <c>ClusterClientReceptionist</c> serves its entire cluster, so
    ///         <c>SendToAll</c> reaches every registered node-comm actor in the mesh <i>regardless of
    ///         which node's address was used as the contact point</i>. One client per Cluster would
    ///         therefore fan each command out once per cluster — N× duplicate
    ///         <c>DispatchDeployment</c> and N× duplicate <c>ApplyAck</c>. Phase 6 splits the meshes,
    ///         at which point per-cluster clients become both correct and necessary.
    ///     </para>
    ///     <para>
    ///         Corollary worth stating because it is easy to assume otherwise: <b>the contact set
    ///         does not scope delivery.</b> Excluding a maintenance-mode node from the contacts does
    ///         not stop it receiving commands — it still receives them, exactly as it does under
    ///         today's DPS broadcast. The filter is about which receptionists are worth <i>dialling</i>
    ///         (a node in maintenance may be switched off), not about who gets the message.
    ///     </para>
    /// </remarks>
    /// <param name="contacts">The receptionist addresses to dial.</param>
    private void RebuildClient(ImmutableHashSet<string> contacts)
    {
        if (_client is not null)
        {
            _log.Info("Mesh contact set changed; replacing the ClusterClient");
            Context.Stop(_client);

            // Cleared now: if the create below throws, a stale ref would route commands into a
            // stopping actor and they would vanish without a warning.
            _client = null;
            _currentContacts = ImmutableHashSet<string>.Empty;
        }

        try
        {
            var paths = contacts.Select(ActorPath.Parse).ToImmutableHashSet();
            _client = _clientFactory.Create(Context.System, paths);
            _currentContacts = contacts;
            _log.Info("Mesh ClusterClient created with {Count} contact point(s)", contacts.Count);
        }
        catch (Exception ex)
        {
            _log.Error(ex,
                "Failed to create the mesh ClusterClient; central→node commands are dropped until "
                + "the next contact refresh succeeds");
        }
    }

    /// <summary>Internal tick asking for a contact-point refresh.</summary>
    public sealed record RefreshContacts;

    /// <summary>Result of a contact-point load, piped back from the DB read.</summary>
    /// <param name="Contacts">Receptionist addresses that parsed cleanly.</param>
    /// <param name="Malformed">Rows that did not, described for the log.</param>
    public sealed record ContactsLoaded(
        IReadOnlyList<string> Contacts,
        IReadOnlyList<string> Malformed);
}