using Akka.Cluster;

namespace ZB.MOM.WW.ScadaBridge.Host.Health;

/// <summary>
/// THE single definition of "active node" (review 01 [High]): 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
/// cluster.State.Leader.
/// </summary>
public static class ClusterActivityEvaluator
{
    /// <summary>True when self is Up and no other Up member (in the role scope) is older.</summary>
    public static bool SelfIsOldest(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));
    }

    /// <summary>The oldest Up member in the role scope, or null while none is Up. Used for Primary/Standby labelling.</summary>
    public static Member? OldestUpMember(Cluster cluster, string? role = null)
    {
        Member? oldest = null;
        foreach (var m in cluster.State.Members.Where(m => m.Status == MemberStatus.Up))
        {
            if (role != null && !m.HasRole(role)) continue;
            if (oldest == null || m.IsOlderThan(oldest)) oldest = m;
        }
        return oldest;
    }
}