package otelsetup

import (
	"context"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/metric"
)

// subscriptionMeterName is the instrumentation scope for the cross-service
// subscription push metrics emitted by SubscriptionMetrics.
const subscriptionMeterName = "gitea.unbound.se/shiny/otelsetup/subscriptions"

// SubscriptionMetrics records cross-service subscription push outcomes (the
// read-your-writes subscriptions described in ADR-0012) as OpenTelemetry
// counters.
//
// Its method set (Pushed/PushSkipped/Dropped/ChannelFull) satisfies the
// Observer interface of gitea.unbound.se/shiny/subscriptions *structurally* —
// otelsetup does not import that library. A service constructs it and passes it
// to the registry:
//
//	metrics, err := otelsetup.NewSubscriptionMetrics("availableCompanies")
//	if err != nil { return err }
//	reg := subscriptions.New[T](subscriptions.WithObserver(metrics))
//
// The WithObserver call type-checks the structural match, so a drift in the
// Observer interface fails the service build. Add a
// `var _ subscriptions.Observer = (*otelsetup.SubscriptionMetrics)(nil)` next to
// it for an explicit guard.
//
// Outcomes are recorded against a low-cardinality (subscription, outcome) pair.
// The subscriber key (company id / user email) the Observer methods receive is
// deliberately NOT used as a metric attribute — that would be unbounded
// cardinality; the key stays in the subscriptions library's logs for
// correlation.
type SubscriptionMetrics struct {
	notifications metric.Int64Counter
	subscription  attribute.KeyValue
}

// NewSubscriptionMetrics builds a SubscriptionMetrics that records to the global
// OpenTelemetry MeterProvider. subscription is the low-cardinality name of the
// subscription field (e.g. "availableCompanies", "entryBasesChanged"), recorded
// as an attribute so one counter covers every subscription.
//
// SetupOTelSDK must have run first so the global MeterProvider is configured;
// when metrics are disabled the global provider is a no-op and recording is
// effectively free.
func NewSubscriptionMetrics(subscription string) (*SubscriptionMetrics, error) {
	c, err := otel.Meter(subscriptionMeterName).Int64Counter(
		"subscription.notifications",
		metric.WithDescription("Cross-service subscription push outcomes, by outcome (pushed/skipped/dropped/channel_full)."),
	)
	if err != nil {
		return nil, err
	}
	return &SubscriptionMetrics{
		notifications: c,
		subscription:  attribute.String("subscription", subscription),
	}, nil
}

func (s *SubscriptionMetrics) record(outcome string) {
	s.notifications.Add(context.Background(), 1, metric.WithAttributes(
		s.subscription,
		attribute.String("outcome", outcome),
	))
}

// Pushed records a change that was gated and delivered to the key's subscribers
// — the denominator for a skip/drop rate.
func (s *SubscriptionMetrics) Pushed(string) { s.record("pushed") }

// PushSkipped records a push skipped because the read view never reflected the
// change within the retry budget.
func (s *SubscriptionMetrics) PushSkipped(string) { s.record("skipped") }

// Dropped records a notification dropped because the worker queue was full.
func (s *SubscriptionMetrics) Dropped(string) { s.record("dropped") }

// ChannelFull records a notification dropped because a subscriber's buffer was
// full.
func (s *SubscriptionMetrics) ChannelFull(string) { s.record("channel_full") }