using Akka.Cluster;

namespace ZB.MOM.WW.ScadaBridge.Communication.ClusterState;

/// <summary>
/// THE single definition of "active node" (review 01 [High]; review 02 round 2 [Critical] N1):
/// a node is active when it is the OLDEST Up member (optionally within a role scope) — i.e.
/// the member the ClusterSingletonManager places singletons on. Cluster LEADERSHIP (lowest
/// address) is an Akka-internal concept that diverges from singleton placement permanently
/// once the original first node restarts and rejoins; every product-level active/standby
/// decision must use this evaluator, never <c>cluster.State.Leader</c>.
/// <para>
/// Lives in Communication (not Host) so <c>SiteCommunicationActor</c> can default to it —
/// Host cannot be referenced from there. The Host's
/// <c>ClusterActivityEvaluator.SelfIsOldest</c> delegates here, so the S&amp;F delivery gate
/// (<c>IClusterNodeProvider.SelfIsPrimary</c>) and the heartbeat IsActive stamp share one
/// implementation.
/// <para>
/// It also backed <c>SiteReplicationActor</c>'s resync authority checks until LocalDb
/// Phase 2 deleted that actor. Those checks existed because the bespoke resync applied a
/// destructive delete-all-then-insert-all, so running it in the wrong direction wiped a
/// live store-and-forward buffer. LocalDb's snapshot resync merges per row under
/// last-writer-wins and never deletes, so there is no destructive apply left to gate — the
/// evaluator survives for the delivery gate and the heartbeat, which still genuinely need
/// a single active node.
/// </para>
/// </para>
/// </summary>
public static class ActiveNodeEvaluator
{
    /// <summary>True when self is Up and no other Up member (in the role scope) is older.</summary>
    /// <param name="cluster">The Akka cluster to evaluate.</param>
    /// <param name="role">Optional role scope; when set, only members with this role are considered.</param>
    /// <returns><c>true</c> when self is Up and the oldest Up member in the role scope.</returns>
    public static bool SelfIsOldestUp(Cluster cluster, string? role = null)
    {
        var self = cluster.SelfMember;
        if (self.Status != MemberStatus.Up)
            return false;
        if (role != null && !self.HasRole(role))
            return false;

        return cluster.State.Members
            .Where(m => m.Status == MemberStatus.Up)
            .Where(m => role == null || m.HasRole(role))
            .All(m => m.UniqueAddress.Equals(self.UniqueAddress) || self.IsOlderThan(m));
    }
}