package connectsrv

import (
	"context"
	"time"

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

// meterName scopes the gateway edge's OpenTelemetry instruments.
const meterName = "scrabble/gateway/edge"

// activeUserWindows are the rolling windows the active_users gauge reports.
var activeUserWindows = []struct {
	label string
	dur   time.Duration
}{
	{label: "24h", dur: 24 * time.Hour},
	{label: "7d", dur: 7 * 24 * time.Hour},
}

// serverMetrics holds the edge's operational instruments. It defaults to no-ops;
// NewServer installs the real meter when one is supplied in Deps.
type serverMetrics struct {
	edge   metric.Float64Histogram
	active *activeUsers
}

// newServerMetrics builds the instruments on meter (nil selects a no-op meter),
// falling back to a no-op histogram on the (rare) construction error. The
// active_users gauge is registered as an observable callback over the in-memory
// tracker.
func newServerMetrics(meter metric.Meter) *serverMetrics {
	if meter == nil {
		meter = noop.NewMeterProvider().Meter(meterName)
	}
	h, err := meter.Float64Histogram("edge_request_duration",
		metric.WithUnit("s"),
		metric.WithDescription("Seconds to serve one Connect Execute call, by message type and result."))
	if err != nil {
		h, _ = noop.NewMeterProvider().Meter(meterName).Float64Histogram("edge_request_duration")
	}
	m := &serverMetrics{edge: h, active: newActiveUsers()}

	gauge, err := meter.Int64ObservableGauge("active_users",
		metric.WithDescription("Distinct accounts that performed an authenticated action within the window (in-memory, single gateway instance)."))
	if err == nil {
		windows := make([]time.Duration, len(activeUserWindows))
		for i, w := range activeUserWindows {
			windows[i] = w.dur
		}
		_, _ = meter.RegisterCallback(func(_ context.Context, o metric.Observer) error {
			counts := m.active.counts(windows)
			for i, w := range activeUserWindows {
				o.ObserveInt64(gauge, int64(counts[i]), metric.WithAttributes(attribute.String("window", w.label)))
			}
			return nil
		}, gauge)
	}
	return m
}

// recordEdge records the duration of one Execute call labelled by message type and
// outcome (ok, domain, unauthenticated, rate_limited, unknown_type or internal).
func (m *serverMetrics) recordEdge(ctx context.Context, msgType, result string, start time.Time) {
	m.edge.Record(ctx, time.Since(start).Seconds(), metric.WithAttributes(
		attribute.String("message_type", msgType),
		attribute.String("result", result),
	))
}

// recordActive marks account uid active now, feeding the active_users gauge.
func (m *serverMetrics) recordActive(uid string) {
	m.active.seen(uid)
}