// Package metrics defines the Prometheus collectors describing
// webhooker's delivery pipeline: how many events arrive, how many
// deliveries are attempted, how they end, how long they take, how
// deep the queues are, and how many circuit breakers are open.
//
// The inbound HTTP metrics come from the go-http-metrics recorder in
// internal/middleware and land on prometheus.DefaultRegisterer. These
// collectors register there too, so both surfaces are gathered by the
// one promhttp handler mounted on the authenticated /metrics route.
package metrics

import (
	"sync"
	"time"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"
	"sneak.berlin/go/webhooker/internal/database"
)

// namespace prefixes every collector defined here.
const namespace = "webhooker"

// targetTypeLabel is the only label any delivery metric carries, and
// cardinality is the whole reason for that.
//
// A target type is one of four compile-time constants, so the label
// domain is bounded by construction. Target ids, event ids and
// entrypoint ids are not: they are UUIDs minted per operator action
// or per inbound request, a series is never reclaimed once it exists,
// and labelling by any of them makes /metrics a memory leak that
// grows with traffic. normalizeTargetType enforces the bound at every
// call site — a type the registry does not know collapses into
// unknownTargetType rather than minting a series of its own.
const targetTypeLabel = "target_type"

// unknownTargetType is the bucket for a target type outside the known
// set, so an unrecognised value cannot mint a new series.
const unknownTargetType = "unknown"

// Delivery duration buckets, exponential from 5ms so the last bucket
// (about 98s) sits above the 30s outbound HTTP client timeout.
const (
	durationBucketStart  = 0.005
	durationBucketFactor = 3
	durationBucketCount  = 10
)

// knownTargetTypes is the fixed label domain: the target types the
// delivery engine implements.
//
//nolint:gochecknoglobals // the label domain, built once per process
var knownTargetTypes = []database.TargetType{
	database.TargetTypeHTTP,
	database.TargetTypeDatabase,
	database.TargetTypeLog,
	database.TargetTypeSlack,
}

// defaultSet is the process-wide metric set, registered on the same
// registry the HTTP middleware and the /metrics handler already use.
// It is built on first use rather than in an init so that a test
// binary that never touches metrics never registers them.
//
//nolint:gochecknoglobals // one process-wide registration, by design
var defaultSet = sync.OnceValue(func() *Set {
	return New(prometheus.DefaultRegisterer)
})

// Default returns the process-wide metric set.
func Default() *Set {
	return defaultSet()
}

// Set is one registered group of webhooker's delivery collectors.
// Production uses the single Default set; tests build their own
// against a private registry so assertions are not disturbed by
// deliveries other tests are making concurrently.
type Set struct {
	eventsReceived      prometheus.Counter
	deliveryAttempts    *prometheus.CounterVec
	deliveriesSucceeded *prometheus.CounterVec
	deliveriesFailed    *prometheus.CounterVec
	deliveryRetries     *prometheus.CounterVec
	deliveryDuration    *prometheus.HistogramVec
	deliveriesPending   *prometheus.GaugeVec
	deliveriesRetrying  *prometheus.GaugeVec
	circuitBreakersOpen *prometheus.GaugeVec
}

// New registers a full set of delivery collectors on reg and returns
// it. It panics if reg already holds them, which is the intended
// behaviour for a duplicate registration.
func New(reg prometheus.Registerer) *Set {
	factory := promauto.With(reg)

	s := &Set{
		eventsReceived: factory.NewCounter(
			prometheus.CounterOpts{
				Namespace: namespace,
				Name:      "events_received_total",
				Help: "Webhook events received and " +
					"stored, so the receive and deliver " +
					"sides can be compared.",
			},
		),
		deliveryDuration: factory.NewHistogramVec(
			prometheus.HistogramOpts{
				Namespace: namespace,
				Name:      "delivery_duration_seconds",
				Help: "Wall time of a single delivery " +
					"attempt, by target type.",
				Buckets: prometheus.ExponentialBuckets(
					durationBucketStart,
					durationBucketFactor,
					durationBucketCount,
				),
			},
			[]string{targetTypeLabel},
		),
	}

	s.registerCounters(factory)
	s.registerGauges(factory)
	s.initSeries()

	return s
}

// EventReceived counts one inbound webhook event stored.
func (s *Set) EventReceived() {
	s.eventsReceived.Inc()
}

// DeliveryAttempted counts one delivery attempt dispatched to a
// target.
func (s *Set) DeliveryAttempted(t database.TargetType) {
	s.deliveryAttempts.
		WithLabelValues(normalizeTargetType(t)).
		Inc()
}

// ObserveDeliveryDuration records how long one delivery attempt took.
func (s *Set) ObserveDeliveryDuration(
	t database.TargetType, d time.Duration,
) {
	s.deliveryDuration.
		WithLabelValues(normalizeTargetType(t)).
		Observe(d.Seconds())
}

// DeliveryStatusChanged counts a delivery's transition into a new
// status. The mapping from status to counter lives here, next to the
// collectors, so the engine has a single call for every transition it
// persists. A move back to pending is not an outcome and counts
// nothing.
func (s *Set) DeliveryStatusChanged(
	t database.TargetType, status database.DeliveryStatus,
) {
	label := normalizeTargetType(t)

	switch status {
	case database.DeliveryStatusDelivered:
		s.deliveriesSucceeded.WithLabelValues(label).Inc()
	case database.DeliveryStatusFailed:
		s.deliveriesFailed.WithLabelValues(label).Inc()
	case database.DeliveryStatusRetrying:
		s.deliveryRetries.WithLabelValues(label).Inc()
	case database.DeliveryStatusPending:
	}
}

// SetQueueDepths publishes the pending and retrying queue depths from
// one sample. Every known target type is written on every call, so a
// type whose queue has drained reads zero instead of holding its last
// value forever.
func (s *Set) SetQueueDepths(
	pending, retrying map[database.TargetType]int,
) {
	for _, t := range knownTargetTypes {
		label := string(t)

		s.deliveriesPending.WithLabelValues(label).
			Set(float64(pending[t]))
		s.deliveriesRetrying.WithLabelValues(label).
			Set(float64(retrying[t]))
	}
}

// SetCircuitBreakersOpen publishes how many of a target type's
// circuit breakers are currently open.
func (s *Set) SetCircuitBreakersOpen(
	t database.TargetType, open int,
) {
	s.circuitBreakersOpen.
		WithLabelValues(normalizeTargetType(t)).
		Set(float64(open))
}

func (s *Set) registerCounters(factory promauto.Factory) {
	s.deliveryAttempts = factory.NewCounterVec(
		prometheus.CounterOpts{
			Namespace: namespace,
			Name:      "delivery_attempts_total",
			Help: "Delivery attempts dispatched to a " +
				"target, by target type.",
		},
		[]string{targetTypeLabel},
	)

	s.deliveriesSucceeded = factory.NewCounterVec(
		prometheus.CounterOpts{
			Namespace: namespace,
			Name:      "deliveries_succeeded_total",
			Help: "Deliveries that reached the delivered " +
				"state, by target type.",
		},
		[]string{targetTypeLabel},
	)

	s.deliveriesFailed = factory.NewCounterVec(
		prometheus.CounterOpts{
			Namespace: namespace,
			Name:      "deliveries_failed_total",
			Help: "Deliveries that failed terminally and " +
				"will not be retried, by target type.",
		},
		[]string{targetTypeLabel},
	)

	s.deliveryRetries = factory.NewCounterVec(
		prometheus.CounterOpts{
			Namespace: namespace,
			Name:      "delivery_retries_total",
			Help: "Deliveries put back into the retrying " +
				"state, by target type.",
		},
		[]string{targetTypeLabel},
	)
}

func (s *Set) registerGauges(factory promauto.Factory) {
	s.deliveriesPending = factory.NewGaugeVec(
		prometheus.GaugeOpts{
			Namespace: namespace,
			Name:      "deliveries_pending",
			Help: "Deliveries currently in the pending " +
				"state, by target type.",
		},
		[]string{targetTypeLabel},
	)

	s.deliveriesRetrying = factory.NewGaugeVec(
		prometheus.GaugeOpts{
			Namespace: namespace,
			Name:      "deliveries_retrying",
			Help: "Deliveries currently in the retrying " +
				"state, by target type.",
		},
		[]string{targetTypeLabel},
	)

	s.circuitBreakersOpen = factory.NewGaugeVec(
		prometheus.GaugeOpts{
			Namespace: namespace,
			Name:      "circuit_breakers_open",
			Help: "Delivery circuit breakers currently " +
				"open, by target type.",
		},
		[]string{targetTypeLabel},
	)
}

// initSeries materialises every known-target-type series at zero, so
// a dashboard and an alert rule see a target type that has not
// delivered yet rather than a missing series.
func (s *Set) initSeries() {
	for _, t := range knownTargetTypes {
		label := string(t)

		s.deliveryAttempts.WithLabelValues(label)
		s.deliveriesSucceeded.WithLabelValues(label)
		s.deliveriesFailed.WithLabelValues(label)
		s.deliveryRetries.WithLabelValues(label)
		s.deliveriesPending.WithLabelValues(label)
		s.deliveriesRetrying.WithLabelValues(label)
		s.circuitBreakersOpen.WithLabelValues(label)
	}
}

// normalizeTargetType maps a target type onto the bounded label
// domain, collapsing anything outside it to unknownTargetType.
func normalizeTargetType(t database.TargetType) string {
	for _, known := range knownTargetTypes {
		if t == known {
			return string(known)
		}
	}

	return unknownTargetType
}