package events

import (
	"encoding/json"
	"fmt"
	"log"
	"time"

	"github.com/nats-io/nats.go"
)

// EventBus is the platform event bus interface.
// Publish sends a typed payload to the given subject.
// Subscribe registers a handler for a subject pattern (supports NATS wildcards).
// The returned func() unsubscribes when called.
type EventBus interface {
	Publish(subject string, payload any) error
	Subscribe(subject string, handler func(subject string, data []byte)) (func(), error)
	Close()
}

// NATSBus is the NATS-backed EventBus. Events are published to core NATS subjects.
// Phase 2A uses core NATS (ephemeral). Phase 2B will add JetStream for CI durability.
type NATSBus struct {
	nc *nats.Conn
}

func NewNATSBus(url string) (*NATSBus, error) {
	nc, err := nats.Connect(url,
		nats.MaxReconnects(-1),
		nats.ReconnectWait(2*time.Second),
		nats.DisconnectErrHandler(func(_ *nats.Conn, err error) {
			if err != nil {
				log.Printf("nats: disconnected: %v", err)
			}
		}),
		nats.ReconnectHandler(func(_ *nats.Conn) {
			log.Printf("nats: reconnected")
		}),
	)
	if err != nil {
		return nil, fmt.Errorf("nats connect %s: %w", url, err)
	}
	log.Printf("nats: connected to %s", url)
	return &NATSBus{nc: nc}, nil
}

func (b *NATSBus) Publish(subject string, payload any) error {
	data, err := json.Marshal(payload)
	if err != nil {
		return fmt.Errorf("marshal: %w", err)
	}
	return b.nc.Publish(subject, data)
}

func (b *NATSBus) Subscribe(subject string, handler func(subject string, data []byte)) (func(), error) {
	sub, err := b.nc.Subscribe(subject, func(msg *nats.Msg) {
		handler(msg.Subject, msg.Data)
	})
	if err != nil {
		return nil, err
	}
	return func() { sub.Unsubscribe() }, nil //nolint:errcheck
}

func (b *NATSBus) Close() {
	if err := b.nc.Drain(); err != nil {
		log.Printf("nats: drain: %v", err)
	}
}

// NoOpBus is a no-op EventBus used when NATS_URL is not configured.
// Events are silently dropped. The app works normally; just no real-time push.
type NoOpBus struct{}

func (NoOpBus) Publish(_ string, _ any) error                              { return nil }
func (NoOpBus) Subscribe(_ string, _ func(string, []byte)) (func(), error) { return func() {}, nil }
func (NoOpBus) Close()                                                      {}

// New returns a NATSBus if url is non-empty, otherwise a NoOpBus.
func New(url string) (EventBus, error) {
	if url == "" {
		log.Printf("events: NATS_URL not set — using no-op bus (real-time push disabled)")
		return NoOpBus{}, nil
	}
	return NewNATSBus(url)
}