package messaging

import (
	"encoding/json"
	"log"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"

	"nearle/models"

	mqtt "github.com/eclipse/paho.mqtt.golang"
)

// Live console events.
//
// The console polls every 30 seconds. That is a safe floor, and it means a bill
// rung at 10:00:01 shows up on the owner's screen at 10:00:30. This closes that
// gap: the tills already publish every sale to the broker, so a second consumer
// listens to the same stream and pushes a nudge to whichever consoles are
// watching that outlet.
//
// Two things it deliberately is NOT:
//
//   - **It does not carry data.** An event says "stock at outlet 1185 changed,
//     these product ids are involved" and nothing more. The console then re-reads
//     through the normal API, which is the only thing that knows the authoritative
//     number after locks, dedup and rejections. Pushing figures from here would
//     mean a screen showing a total the database never agreed to.
//   - **It does not replace the poll.** The console keeps its 30-second refetch.
//     A dropped connection, a full buffer or a replica restart then costs latency
//     rather than correctness, which is the trade an operations screen wants.
//
// ── Why a second consumer and not a hook in the ingest path ──────────────────
//
// `StartPosMqttConsumer` runs on ONE replica (see `posConsumerElected`) because
// two writers ingesting the same bills would fight over the same rows. Hanging
// this off that consumer would mean only consoles that happened to land on the
// elected replica ever received anything, which is a bug that looks like flaky
// network. This client only reads and touches no database, so every replica can
// safely run one and serve its own connected consoles.
//
// Enabled by MQTT_URL, the same switch the ingest consumer uses. Unset, the hub
// still runs and simply never emits — the console degrades to polling, which is
// exactly what it does today.

const (
	// How many events a single console connection may fall behind before it
	// starts losing them. Small on purpose: these are nudges, and a client that
	// cannot keep up with a handful is better served by its next poll than by a
	// backlog of stale ones.
	liveBuffer = 16

	// Belt and braces around a burst. A terminal replaying a day of offline
	// bills would otherwise emit one event per batch as fast as the broker can
	// deliver them; the console cannot use more than a couple a second.
	liveMinInterval = 400 * time.Millisecond
)

// LiveEvent is one nudge. Kept small — it crosses the wire on every sale.
type LiveEvent struct {
	// "sale", "customer" or "terminal".
	Type string `json:"type"`
	// tenantlocations.locationid. The topic's store segment carries it as a
	// string (see models.PosOrderBatch).
	Locationid int `json:"locationid"`
	// Which products moved, when the event knows. Empty means "something did".
	Productids []int `json:"productids,omitempty"`
	// Which till, for the presence board.
	Terminalid string `json:"terminalid,omitempty"`
	At         string `json:"at"`
}

type liveSubscriber struct {
	ch   chan LiveEvent
	last time.Time
}

// LiveHub fans events out to the consoles currently watching an outlet.
type LiveHub struct {
	mu          sync.RWMutex
	subscribers map[int]map[*liveSubscriber]struct{}
	client      mqtt.Client
	dropped     uint64
}

// Hub is the process-wide hub. Nil until StartLiveHub runs; every method
// tolerates a nil receiver so callers never have to check.
var Hub *LiveHub

// StartLiveHub creates the hub and, if a broker is configured, connects a
// read-only consumer to it.
//
// Never returns nil: a hub with no broker is a working hub with no events, and
// the SSE endpoint must still accept connections so the console's reconnect
// logic has something to talk to.
func StartLiveHub() *LiveHub {
	hub := &LiveHub{subscribers: make(map[int]map[*liveSubscriber]struct{})}
	Hub = hub

	url := strings.TrimSpace(os.Getenv("MQTT_URL"))
	if url == "" {
		log.Println("live: MQTT_URL not set, console event stream will stay quiet")
		return hub
	}

	// A distinct client id per replica. Sharing one with the ingest consumer
	// would make the broker disconnect whichever connected first.
	clientID := "nearle-console-live-" + hostSuffix()

	opts := mqtt.NewClientOptions().
		AddBroker(url).
		SetClientID(clientID).
		SetAutoReconnect(true).
		SetConnectRetry(true).
		SetConnectRetryInterval(5 * time.Second).
		SetCleanSession(true). // No queued backlog on reconnect; stale nudges are noise.
		SetOrderMatters(false)

	// MQTT_USER is the name the ingest consumer and the env files use. This
	// read MQTT_USERNAME for a while, so the live stream connected to the
	// production broker with no credentials at all; MQTT_USERNAME is still
	// honoured for any deployment that set it.
	user := strings.TrimSpace(os.Getenv("MQTT_USER"))
	if user == "" {
		user = strings.TrimSpace(os.Getenv("MQTT_USERNAME"))
	}
	if user != "" {
		opts.SetUsername(user)
		opts.SetPassword(os.Getenv("MQTT_PASSWORD"))
	}

	opts.OnConnect = func(client mqtt.Client) {
		for topic, handler := range map[string]mqtt.MessageHandler{
			topicOrders:    hub.onOrders,
			topicCustomers: hub.onCustomers,
			topicHealth:    hub.onHealth,
		} {
			// QoS 0. A missed nudge costs at most one poll interval, and QoS 1
			// would have the broker retaining state for a listener that does
			// not need it.
			if token := client.Subscribe(topic, 0, handler); token.Wait() && token.Error() != nil {
				log.Printf("live: could not subscribe to %s: %v", topic, token.Error())
				continue
			}
			log.Printf("live: watching %s", topic)
		}
	}

	client := mqtt.NewClient(opts)
	hub.client = client

	// Connect in the background: a broker that is slow to answer must not hold
	// up the HTTP server coming online.
	go func() {
		if token := client.Connect(); token.Wait() && token.Error() != nil {
			log.Printf("live: broker unreachable, console falls back to polling: %v", token.Error())
		}
	}()

	return hub
}

/* ── Subscription ─────────────────────────────────────────────────────────── */

// Subscribe registers a console connection watching one outlet.
//
// Returns the channel to read and the function that releases it. The caller
// MUST call release — an SSE handler that returns without it leaks a channel
// the hub goes on writing to for the life of the process.
func (h *LiveHub) Subscribe(locationid int) (<-chan LiveEvent, func()) {
	if h == nil {
		// A closed channel reads immediately and forever, which would spin the
		// handler. An open one that never delivers is the honest no-op.
		return make(chan LiveEvent), func() {}
	}

	sub := &liveSubscriber{ch: make(chan LiveEvent, liveBuffer)}

	h.mu.Lock()
	if h.subscribers[locationid] == nil {
		h.subscribers[locationid] = make(map[*liveSubscriber]struct{})
	}
	h.subscribers[locationid][sub] = struct{}{}
	h.mu.Unlock()

	var once sync.Once
	release := func() {
		once.Do(func() {
			h.mu.Lock()
			if set := h.subscribers[locationid]; set != nil {
				delete(set, sub)
				if len(set) == 0 {
					delete(h.subscribers, locationid)
				}
			}
			h.mu.Unlock()
			close(sub.ch)
		})
	}
	return sub.ch, release
}

// Broadcast delivers an event to everyone watching that outlet.
//
// Never blocks. A subscriber whose buffer is full loses the event and is
// counted rather than waited for: one wedged console must not stall the fan-out
// to every other console, and the poll will collect what was missed.
func (h *LiveHub) Broadcast(event LiveEvent) {
	if h == nil || event.Locationid == 0 {
		return
	}
	if event.At == "" {
		event.At = time.Now().UTC().Format(time.RFC3339)
	}

	// A write lock, not a read lock, even though this only walks the map: the
	// loop below writes `sub.last` and `h.dropped`. Under RLock those are
	// concurrent writes from every broker goroutine at once — a data race, and
	// one `go test -race` would fail on. Fan-out is a handful of non-blocking
	// sends, so holding the write lock costs nothing worth reclaiming.
	h.mu.Lock()
	defer h.mu.Unlock()

	now := time.Now()
	for sub := range h.subscribers[event.Locationid] {
		// Coalesce a burst per subscriber, not globally: a busy outlet must not
		// throttle a quiet one.
		if now.Sub(sub.last) < liveMinInterval {
			continue
		}
		select {
		case sub.ch <- event:
			sub.last = now
		default:
			h.dropped++
		}
	}
}

/* ── Broker handlers ──────────────────────────────────────────────────────── */

func (h *LiveHub) onOrders(_ mqtt.Client, msg mqtt.Message) {
	var batch models.PosOrderBatch
	if err := json.Unmarshal(msg.Payload(), &batch); err != nil {
		return // The ingest consumer logs the bad payload; one complaint is enough.
	}

	store, terminal := topicIdentity(msg.Topic())
	if batch.Storeid == "" {
		batch.Storeid = store
	}
	if batch.Terminalid == "" {
		batch.Terminalid = terminal
	}

	// Deduplicated: a bill with four lines of the same SKU is one product.
	seen := make(map[int]struct{})
	productids := make([]int, 0, 8)
	for _, order := range batch.Orders {
		for _, item := range order.Items {
			id, err := strconv.Atoi(strings.TrimSpace(item.Productid))
			if err != nil || id == 0 {
				continue
			}
			if _, done := seen[id]; done {
				continue
			}
			seen[id] = struct{}{}
			productids = append(productids, id)
		}
	}

	h.Broadcast(LiveEvent{
		Type:       "sale",
		Locationid: locationFromStore(batch.Storeid),
		Productids: productids,
		Terminalid: batch.Terminalid,
	})
}

func (h *LiveHub) onCustomers(_ mqtt.Client, msg mqtt.Message) {
	var batch models.PosCustomerBatch
	if err := json.Unmarshal(msg.Payload(), &batch); err != nil {
		return
	}
	store, terminal := topicIdentity(msg.Topic())
	if batch.Storeid == "" {
		batch.Storeid = store
	}
	h.Broadcast(LiveEvent{
		Type:       "customer",
		Locationid: locationFromStore(batch.Storeid),
		Terminalid: firstNonEmpty(batch.Terminalid, terminal),
	})
}

// onHealth drives the presence board.
//
// Read from the topic rather than the body, the same rule bills follow: a till
// that could name its own store could appear in another tenant's console.
func (h *LiveHub) onHealth(_ mqtt.Client, msg mqtt.Message) {
	store, terminal := topicIdentity(msg.Topic())
	h.Broadcast(LiveEvent{
		Type:       "terminal",
		Locationid: locationFromStore(store),
		Terminalid: terminal,
	})
}

/* ── Helpers ──────────────────────────────────────────────────────────────── */

// locationFromStore parses the topic's store segment.
//
// `models.PosOrderBatch` documents it: "Storeid carries the numeric
// tenantlocations.locationid as a string." Anything unparseable yields 0, which
// Broadcast drops — an event with no outlet has nowhere to go.
func locationFromStore(store string) int {
	id, err := strconv.Atoi(strings.TrimSpace(store))
	if err != nil {
		return 0
	}
	return id
}

func firstNonEmpty(values ...string) string {
	for _, value := range values {
		if strings.TrimSpace(value) != "" {
			return value
		}
	}
	return ""
}

// hostSuffix keeps each replica's client id distinct without needing config.
func hostSuffix() string {
	if host := strings.TrimSpace(os.Getenv("HOSTNAME")); host != "" {
		return host
	}
	return strconv.FormatInt(time.Now().UnixNano(), 36)
}

// Stats reports what the hub is doing, for the health endpoint.
func (h *LiveHub) Stats() (outlets, connections int, dropped uint64) {
	if h == nil {
		return 0, 0, 0
	}
	h.mu.RLock()
	defer h.mu.RUnlock()
	for _, set := range h.subscribers {
		connections += len(set)
	}
	return len(h.subscribers), connections, h.dropped
}