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
	rateLimited metric.Int64Counter
	banned      metric.Int64Counter
	active      *activeUsers
	// Client-reported local move-preview adoption (see localEvalMetricsHandler).
	localColdStart metric.Int64Counter
	localDictLoad  metric.Int64Counter
	localPreview   metric.Int64Counter
	// Clients turned away by the unsupported-engine boot screen (see unsupportedEngineHandler).
	unsupportedEngine metric.Int64Counter
}

// 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")
	}
	c, err := meter.Int64Counter("gateway_rate_limited_total",
		metric.WithDescription("Rate-limiter rejections at the edge, by limiter class (user, public, email or admin) — aggregate only, no per-user attributes."))
	if err != nil {
		c, _ = noop.NewMeterProvider().Meter(meterName).Int64Counter("gateway_rate_limited_total")
	}
	b, err := meter.Int64Counter("gateway_abuse_banned_total",
		metric.WithDescription("Temporary IP bans applied at the edge, by reason (rejections, tripwire or honeytoken)."))
	if err != nil {
		b, _ = noop.NewMeterProvider().Meter(meterName).Int64Counter("gateway_abuse_banned_total")
	}
	m := &serverMetrics{
		edge: h, rateLimited: c, banned: b, active: newActiveUsers(),
		localColdStart: counterOf(meter, "local_eval_cold_start_total", "App cold starts reported by clients — the denominator for local-move-preview adoption."),
		localDictLoad:  counterOf(meter, "local_eval_dict_load_total", "Client dictionary loads for the local move preview, by result (fetched, cache_hit or miss)."),
		localPreview:   counterOf(meter, "local_eval_preview_total", "Client move previews, by path (local on-device, or network fallback)."),
		unsupportedEngine: counterOf(meter, "unsupported_engine_total",
			"Clients that hit the unsupported-engine boot screen (the app cannot run), by reason (no_bigint, no_proxy, boot_error, other) and Chromium major — a deduped beacon from the index.html boot guard; the full user agent is logged, not labelled."),
	}

	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)
}

// recordRateLimited counts one limiter rejection under class.
func (m *serverMetrics) recordRateLimited(ctx context.Context, class string) {
	m.rateLimited.Add(ctx, 1, metric.WithAttributes(attribute.String("class", class)))
}

// recordBan counts one temporary IP ban under reason (rejections, tripwire or
// honeytoken).
func (m *serverMetrics) recordBan(ctx context.Context, reason string) {
	m.banned.Add(ctx, 1, metric.WithAttributes(attribute.String("reason", reason)))
}

// recordUnsupportedEngine counts one client turned away by the unsupported-engine boot screen,
// labelled by reason and Chromium major. The caller passes both already reduced to bounded label
// sets (see normalizeUnsupported) so a spoofed beacon cannot explode the metric cardinality.
func (m *serverMetrics) recordUnsupportedEngine(ctx context.Context, reason, chromium string) {
	m.unsupportedEngine.Add(ctx, 1, metric.WithAttributes(
		attribute.String("reason", reason),
		attribute.String("chromium", chromium),
	))
}

// localEvalReport is the client-reported local move-preview telemetry batch — deltas since
// the client's previous report. It backs the adoption dashboard: app cold starts vs cached
// dictionaries vs on-device previews.
type localEvalReport struct {
	ColdStart      int `json:"cold_start"`
	DictFetched    int `json:"dict_fetched"`
	DictCacheHit   int `json:"dict_cache_hit"`
	DictMiss       int `json:"dict_miss"`
	PreviewLocal   int `json:"preview_local"`
	PreviewNetwork int `json:"preview_network"`
}

// recordLocalEval folds one client report into the edge's local-eval counters.
func (m *serverMetrics) recordLocalEval(ctx context.Context, r localEvalReport) {
	add := func(c metric.Int64Counter, n int, opts ...metric.AddOption) {
		if n > 0 {
			c.Add(ctx, int64(n), opts...)
		}
	}
	add(m.localColdStart, r.ColdStart)
	add(m.localDictLoad, r.DictFetched, metric.WithAttributes(attribute.String("result", "fetched")))
	add(m.localDictLoad, r.DictCacheHit, metric.WithAttributes(attribute.String("result", "cache_hit")))
	add(m.localDictLoad, r.DictMiss, metric.WithAttributes(attribute.String("result", "miss")))
	add(m.localPreview, r.PreviewLocal, metric.WithAttributes(attribute.String("path", "local")))
	add(m.localPreview, r.PreviewNetwork, metric.WithAttributes(attribute.String("path", "network")))
}

// counterOf builds an Int64Counter on meter, falling back to a no-op on the rare
// construction error so metrics never fail startup.
func counterOf(meter metric.Meter, name, desc string) metric.Int64Counter {
	c, err := meter.Int64Counter(name, metric.WithDescription(desc))
	if err != nil {
		c, _ = noop.NewMeterProvider().Meter(meterName).Int64Counter(name)
	}
	return c
}