using Microsoft.Extensions.Diagnostics.HealthChecks;
using Microsoft.Extensions.Options;
using ZB.MOM.WW.LocalDb.Replication;

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

/// <summary>
///     Pure decision function for the LocalDb replication probe, factored out of
///     <see cref="LocalDbReplicationHealthCheck"/> so the whole matrix is table-testable without
///     standing up a replication engine.
/// </summary>
public static class LocalDbReplicationDecision
{
    /// <summary>
    ///     Maps the resolved replication facts to a health status.
    /// </summary>
    /// <param name="peerConfigured">
    ///     True when this node participates in replication at all — it has a peer address to dial, or
    ///     a sync listener bound. False means default-OFF, and default-OFF must never degrade a node.
    /// </param>
    /// <param name="connected">Whether a sync session is currently running.</param>
    /// <param name="oplogBacklog">
    ///     The unacked oplog backlog, or <see langword="null"/> when it could not be read. Null is
    ///     deliberately not treated as zero: a failed poll is "unknown", not "caught up".
    /// </param>
    /// <param name="degradedThreshold">
    ///     Backlog at or above which a connected pair is judged to be falling behind.
    /// </param>
    /// <returns>The status plus a human-readable reason.</returns>
    public static (HealthStatus Status, string Description) Evaluate(
        bool peerConfigured, bool connected, long? oplogBacklog, long degradedThreshold)
    {
        if (!peerConfigured)
            return (HealthStatus.Healthy, "LocalDb replication is not configured on this node (default-OFF).");

        if (!connected)
            return (HealthStatus.Degraded, "LocalDb replication is configured but no sync session is connected.");

        if (oplogBacklog is null)
            return (HealthStatus.Degraded, "LocalDb replication is connected but its oplog backlog is unknown (the poll failed).");

        if (oplogBacklog.Value >= degradedThreshold)
            return (HealthStatus.Degraded,
                $"LocalDb replication is connected but its oplog backlog ({oplogBacklog.Value}) is at or above the degraded threshold ({degradedThreshold}).");

        return (HealthStatus.Healthy, $"LocalDb replication is connected; oplog backlog {oplogBacklog.Value}.");
    }
}

/// <summary>
///     Reports the health of this node's LocalDb replication link. Registered unconditionally with
///     the shared health pipeline; when LocalDb is not present at all (admin-only graphs) it reports
///     <see cref="HealthStatus.Healthy"/> rather than degrading — matching the
///     <c>ActiveNodeHealthCheck</c> precedent of "not applicable ⇒ Healthy".
/// </summary>
/// <remarks>
///     A node running LocalDb with replication switched off (no peer, no listener) is also Healthy:
///     that is the default posture for most of the fleet, and it must not look degraded. Only a node
///     that is <i>meant</i> to be replicating and is not — or is connected but cannot confirm it is
///     draining — is surfaced as Degraded. It never reports Unhealthy: a replication problem does not
///     stop the node serving its address space, so it must not fail a readiness gate.
/// </remarks>
public sealed class LocalDbReplicationHealthCheck : IHealthCheck
{
    /// <summary>
    ///     Backlog at or above which a connected pair is reported Degraded. Well below the library's
    ///     <c>MaxOplogRows</c> default (1,000,000, where a snapshot resync is forced), so the probe
    ///     flags a pair that is falling behind long before the engine itself intervenes.
    /// </summary>
    public const long DefaultBacklogDegradedThreshold = 100_000;

    private readonly ISyncStatus? _syncStatus;
    private readonly ReplicationOptions _options;
    private readonly int _syncListenPort;
    private readonly long _degradedThreshold;

    /// <summary>Creates the health check.</summary>
    /// <param name="syncStatus">
    ///     The replication status singleton, or <see langword="null"/> when the replication engine is
    ///     not registered (admin-only nodes) — in which case the check is a Healthy no-op.
    /// </param>
    /// <param name="options">The bound replication options (peer address, etc.).</param>
    /// <param name="syncListenPort">
    ///     The configured sync listener port; a value greater than zero counts as "replication
    ///     configured" even when this node is the passive side with no peer address.
    /// </param>
    /// <param name="degradedThreshold">Backlog degraded threshold; defaults to <see cref="DefaultBacklogDegradedThreshold"/>.</param>
    public LocalDbReplicationHealthCheck(
        ISyncStatus? syncStatus,
        IOptions<ReplicationOptions> options,
        int syncListenPort,
        long degradedThreshold = DefaultBacklogDegradedThreshold)
    {
        ArgumentNullException.ThrowIfNull(options);

        _syncStatus = syncStatus;
        _options = options.Value;
        _syncListenPort = syncListenPort;
        _degradedThreshold = degradedThreshold;
    }

    /// <inheritdoc />
    public Task<HealthCheckResult> CheckHealthAsync(
        HealthCheckContext context, CancellationToken cancellationToken = default)
    {
        // No replication engine registered at all → not applicable → Healthy. Admin-only nodes and
        // any graph that did not call AddOtOpcUaLocalDb land here.
        if (_syncStatus is null)
            return Task.FromResult(HealthCheckResult.Healthy(
                "LocalDb replication is not present on this node."));

        var peerConfigured =
            !string.IsNullOrWhiteSpace(_options.PeerAddress) || _syncListenPort > 0;

        var (status, description) = LocalDbReplicationDecision.Evaluate(
            peerConfigured, _syncStatus.Connected, _syncStatus.OplogBacklog, _degradedThreshold);

        var result = status switch
        {
            HealthStatus.Healthy => HealthCheckResult.Healthy(description),
            _ => HealthCheckResult.Degraded(description),
        };

        return Task.FromResult(result);
    }
}