// Package events translates inbound `pushv1.PushEvent` frames received
// from backend into actions on the gateway-side push hub. It replaces
// the Stage <6.2 Redis Stream subscribers (`session_events`,
// `client_events`) with a single dispatcher driven by the gRPC
// SubscribePush stream.
package events

import (
	"context"
	"strings"

	pushv1 "galaxy/backend/proto/push/v1"
	"galaxy/gateway/internal/push"
	"galaxy/gateway/internal/telemetry"

	"go.opentelemetry.io/otel/attribute"
	"go.uber.org/zap"
)

// SessionInvalidator closes every active push subscription bound to a
// (device_session_id) or every session of a user when the backend emits
// a SessionInvalidation frame. *push.Hub satisfies this contract.
type SessionInvalidator interface {
	RevokeDeviceSession(deviceSessionID string)
	RevokeAllForUser(userID string)
}

// EventPublisher fans out a translated client event to active push
// subscriptions. *push.Hub satisfies this contract.
type EventPublisher interface {
	Publish(event push.Event)
}

// Dispatcher converts inbound `pushv1.PushEvent` frames into either a
// hub Publish or a hub revocation. Malformed frames are dropped and
// counted via telemetry; observability mirrors the previous
// RecordInternalEventDrop semantics.
type Dispatcher struct {
	publisher   EventPublisher
	invalidator SessionInvalidator
	logger      *zap.Logger
	metrics     *telemetry.Runtime
}

// NewDispatcher constructs a Dispatcher. publisher and invalidator are
// required; logger and metrics may be nil.
func NewDispatcher(publisher EventPublisher, invalidator SessionInvalidator, logger *zap.Logger, metrics *telemetry.Runtime) *Dispatcher {
	if logger == nil {
		logger = zap.NewNop()
	}
	return &Dispatcher{
		publisher:   publisher,
		invalidator: invalidator,
		logger:      logger.Named("push_dispatcher"),
		metrics:     metrics,
	}
}

// Handle implements backendclient.EventHandler. It is safe for
// concurrent use; the caller serialises ev within its goroutine.
func (d *Dispatcher) Handle(ctx context.Context, ev *pushv1.PushEvent) {
	if d == nil || ev == nil {
		return
	}

	switch kind := ev.GetKind().(type) {
	case *pushv1.PushEvent_ClientEvent:
		d.handleClientEvent(ctx, kind.ClientEvent, ev.GetCursor())
	case *pushv1.PushEvent_SessionInvalidation:
		d.handleSessionInvalidation(kind.SessionInvalidation)
	default:
		d.logger.Warn("dropped malformed push event",
			zap.String("cursor", ev.GetCursor()),
			zap.String("reason", "unknown_kind"),
		)
		d.recordDrop(ctx, "unknown_kind")
	}
}

func (d *Dispatcher) handleClientEvent(ctx context.Context, ce *pushv1.ClientEvent, cursor string) {
	if ce == nil || d.publisher == nil {
		return
	}

	userID := strings.TrimSpace(ce.GetUserId())
	kind := strings.TrimSpace(ce.GetKind())
	eventID := strings.TrimSpace(ce.GetEventId())
	if userID == "" || kind == "" || eventID == "" {
		d.logger.Warn("dropped malformed client event",
			zap.String("cursor", cursor),
			zap.String("user_id", userID),
			zap.String("kind", kind),
			zap.String("event_id", eventID),
		)
		d.recordDrop(ctx, "malformed_client_event")
		return
	}

	d.publisher.Publish(push.Event{
		UserID:          userID,
		DeviceSessionID: strings.TrimSpace(ce.GetDeviceSessionId()),
		EventType:       kind,
		EventID:         eventID,
		PayloadBytes:    cloneBytes(ce.GetPayload()),
		RequestID:       ce.GetRequestId(),
		TraceID:         ce.GetTraceId(),
	})
}

func (d *Dispatcher) handleSessionInvalidation(si *pushv1.SessionInvalidation) {
	if si == nil || d.invalidator == nil {
		return
	}

	userID := strings.TrimSpace(si.GetUserId())
	deviceSessionID := strings.TrimSpace(si.GetDeviceSessionId())

	switch {
	case deviceSessionID != "":
		d.invalidator.RevokeDeviceSession(deviceSessionID)
	case userID != "":
		d.invalidator.RevokeAllForUser(userID)
	default:
		d.logger.Warn("dropped malformed session_invalidation: user_id and device_session_id both empty")
	}
}

func (d *Dispatcher) recordDrop(ctx context.Context, reason string) {
	if d.metrics == nil {
		return
	}
	d.metrics.RecordInternalEventDrop(ctx,
		attribute.String("component", "push_dispatcher"),
		attribute.String("reason", reason),
	)
}

func cloneBytes(in []byte) []byte {
	if len(in) == 0 {
		return nil
	}
	out := make([]byte, len(in))
	copy(out, in)
	return out
}