using Prometheus;

/// <summary>
/// Application-level Prometheus metrics for the digitization pipeline.
/// All metrics are static singletons — safe for concurrent use across
/// all services and background workers.
///
/// prometheus-net throws InvalidOperationException if a metric with the
/// same name but different label configuration is registered twice.
/// Static fields guarantee each metric is created exactly once.
/// </summary>
public static class DiagnosticsMetrics
{
    /// <summary>
    /// Gauge: count of digitization batches per status.
    /// Updated periodically by MetricsCollectorService.
    /// Labels: status (UPLOADED, IN_ENTRY, PENDING_VERIFICATION, etc.)
    ///
    /// This is a gauge (not a counter) because statuses change — a batch
    /// moves from UPLOADED to IN_ENTRY, decrementing one label and
    /// incrementing another. The gauge is set to the current count
    /// each collection cycle.
    /// </summary>
    public static readonly Gauge BatchesByStatus = Metrics.CreateGauge(
        "digitization_batches_by_status",
        "Number of digitization batches grouped by current status.",
        new GaugeConfiguration
        {
            LabelNames = new[] { "status" }
        });

    /// <summary>
    /// Histogram: how long a promotion operation takes in seconds.
    /// Recorded in PromotionService when a batch transitions to Promoted.
    /// Buckets tuned for typical promotion durations (50ms to 30s).
    ///
    /// The p50/p95/p99 can be derived from the bucket boundaries in
    /// Grafana using histogram_quantile().
    /// </summary>
    public static readonly Histogram PromotionDuration = Metrics.CreateHistogram(
        "digitization_promotion_duration_seconds",
        "Duration of batch promotion operations in seconds.",
        new HistogramConfiguration
        {
            Buckets = new[] { 0.05, 0.1, 0.25, 0.5, 1.0, 2.5, 5.0, 10.0, 30.0 }
        });

    /// <summary>
    /// Counter: total number of batch rejections.
    /// Incremented in VerificationService on every rejection.
    /// Labels: reason_category (verification_failed, clinical_rejected)
    ///
    /// Supervisors need to distinguish between verification-stage rejections
    /// (data entry errors) and clinical-stage rejections (clinical judgment
    /// issues). The label enables separate alerting thresholds.
    /// </summary>
    public static readonly Counter RejectionTotal = Metrics.CreateCounter(
        "digitization_rejection_total",
        "Total number of digitization batch rejections.",
        new CounterConfiguration
        {
            LabelNames = new[] { "reason_category" }
        });

    private static readonly string[] RejectionReasonCategories =
    {
        "verification_failed",
        "clinical_rejected"
    };

    static DiagnosticsMetrics()
    {
        // Expose all reason_category label combinations at 0 before any rejections occur.
        foreach (var category in RejectionReasonCategories)
            RejectionTotal.WithLabels(category).Inc(0);
    }

    /// <summary>
    /// Gauge: age in seconds of the oldest batch in PendingVerification status.
    /// Updated periodically by MetricsCollectorService.
    /// A high value indicates the verification queue is backed up.
    ///
    /// This is a gauge because it reflects a point-in-time measurement —
    /// the age of the oldest pending batch right now.
    /// </summary>
    public static readonly Gauge QueueAgeSeconds = Metrics.CreateGauge(
        "digitization_queue_age_seconds",
        "Age in seconds of the oldest batch in pending_verification status.");
}