using Akka.Actor;
using Microsoft.Extensions.Diagnostics.HealthChecks;

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

/// <summary>
/// /health/active check: Healthy only on the oldest Up member — the node that
/// hosts the cluster singletons. Replaces the shared package's leader-based
/// ActiveNodeHealthCheck (review 01 [High]): leadership is address-ordered and
/// diverges from singleton placement after a restart, and during a partition
/// BOTH nodes compute themselves leader, so Traefik served both. Oldest-member
/// semantics keep exactly one active node through a partition (the non-oldest
/// side still sees the old oldest member until SBR resolves membership).
/// </summary>
public sealed class OldestNodeActiveHealthCheck : IHealthCheck
{
    private readonly ActorSystem _system;

    /// <summary>Initializes a new <see cref="OldestNodeActiveHealthCheck"/> bound to the running actor system.</summary>
    /// <param name="system">The live cluster actor system.</param>
    public OldestNodeActiveHealthCheck(ActorSystem system) => _system = system;

    /// <summary>Reports Healthy only when this node is the oldest Up cluster member (the singleton host); otherwise reports Unhealthy.</summary>
    /// <param name="context">The health check context (unused; the result depends only on cluster membership).</param>
    /// <param name="ct">Cancellation token.</param>
    /// <returns>A task that resolves to the health check result.</returns>
    public Task<HealthCheckResult> CheckHealthAsync(HealthCheckContext context, CancellationToken ct = default)
    {
        var cluster = Akka.Cluster.Cluster.Get(_system);
        return Task.FromResult(ClusterActivityEvaluator.SelfIsOldest(cluster)
            ? HealthCheckResult.Healthy("oldest Up member (singleton host)")
            : HealthCheckResult.Unhealthy("not the oldest Up member (standby)"));
    }
}