package broker

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"log/slog"
	"net"
	"sync"
	"sync/atomic"
	"time"

	"git.asio.asia/nixevol/NixMsg/internal/app/port"
	"git.asio.asia/nixevol/NixMsg/internal/metrics"
	mqtt "github.com/mochi-mqtt/server/v2"
	"github.com/mochi-mqtt/server/v2/packets"
)

const (
	maxClients           = 2000
	maxPacketSize        = 786432
	uplinkQueueSize      = 256
	largeFrameBytes      = 64 * 1024
	largeFrameSlots      = 64
	packetOverheadBudget = 128 // 主题与 MQTT 包头预留
	keepaliveMin         = 10
	keepaliveMax         = 600
)

// ErrPayloadTooLarge 下行超过客户端 Maximum Packet Size（减包头预留）或 max_receive_bytes。
var ErrPayloadTooLarge = errors.New("broker: payload exceeds client limit")

// ErrNoConnection 目标端没有当前连接。
var ErrNoConnection = errors.New("broker: no active connection")

// ErrLargeFrameTimeout 全局大帧名额在有界等待内拿不到。
var ErrLargeFrameTimeout = errors.New("broker: large frame quota timeout")

// ErrBackpressure 该连接下行队列已满（帧数或字节数）。
var ErrBackpressure = errors.New("broker: downlink backpressure")

// ErrNotSubscribed 当前连接尚未订阅下行主题。
var ErrNotSubscribed = errors.New("broker: down topic not subscribed")

// ErrSessionWriteConflict 密码登录写令牌时发现库已被并发更新。
var ErrSessionWriteConflict = errors.New("broker: session token write conflict")

const (
	largeAcquireWait = 5 * time.Second
	downQueueMax     = 256
	downQueueBytes   = 16 << 20
)

// AuthResult 是登录校验结论（N3 实现真实逻辑；N2 默认拒绝）。
type AuthResult struct {
	OK           bool
	SessionToken string // 密码登录成功时由 N3 填写
}

// Authenticator 由 N3 实现；内部故障必须返回 error，不得当成密码错误。
type Authenticator interface {
	Authenticate(ctx context.Context, endpointID string, password []byte, remoteIP string) (AuthResult, error)
}

// RejectAuthenticator 默认拒绝所有客户端（CONNACK 用户名密码错误）。
type RejectAuthenticator struct{}

func (RejectAuthenticator) Authenticate(context.Context, string, []byte, string) (AuthResult, error) {
	return AuthResult{OK: false}, nil
}

// AllowAuthenticator 测试用：允许任意编号。
type AllowAuthenticator struct{}

func (AllowAuthenticator) Authenticate(context.Context, string, []byte, string) (AuthResult, error) {
	return AuthResult{OK: true}, nil
}

// PublishDroppedFunc 下行未写入发送队列时回调（对接消息线 OnPublishDropped）。
type PublishDroppedFunc func(ctx context.Context, endpointID string, connID port.ConnID, payload []byte)

// Options 装配 broker。
type Options struct {
	Authenticator Authenticator
	Uplink        port.UplinkHandler
	Logger        *slog.Logger
	// OnPublishDropped 可选；nil 时仅打 debug 日志。
	OnPublishDropped PublishDroppedFunc
	// Metrics 可选；会话建立/断开时更新 nixmsg_connections。
	Metrics *metrics.Registry
}

// Broker 内置 mochi，不自带监听端口。
type Broker struct {
	server  *mqtt.Server
	auth    Authenticator
	uplink  port.UplinkHandler
	log     *slog.Logger
	onDrop  PublishDroppedFunc
	metrics *metrics.Registry

	hook *nixHook

	connsMu  sync.RWMutex
	current  map[string]*connState
	byClient map[*mqtt.Client]*connState
	byConnID map[port.ConnID]*connState
	closedCh chan struct{}

	queuesMu sync.Mutex
	queues   map[string]*uplinkQueue

	largeSem chan struct{}
	closed   atomic.Bool

	lifeMu    sync.Mutex
	lifeLocks map[string]*sync.Mutex
}

type connState struct {
	connID         port.ConnID
	endpointID     string
	transport      port.Transport
	remoteIP       string
	client         *mqtt.Client
	maxPacketSize  uint32
	maxRecvBytes   int
	authOK         bool
	sessionToken   string
	handshook      bool
	subscribedDown bool
	largePIDs      map[uint16]struct{}
	largePending   int
	metricsCounted bool
	established    bool
	createdAt      time.Time
	closing        bool
	superseded     bool
	downCh         chan downItem
	downStop       chan struct{}
	downDone       chan struct{}
	downBytes      atomic.Int64
	sentPub        atomic.Int64
	mu             sync.Mutex

	handshakeTimer *time.Timer
}

// New 创建并 Serve mochi（无监听器）。
func New(opts Options) (*Broker, error) {
	auth := opts.Authenticator
	if auth == nil {
		auth = RejectAuthenticator{}
	}
	uplink := opts.Uplink
	if uplink == nil {
		uplink = port.StubUplinkHandler{}
	}
	log := opts.Logger
	if log == nil {
		log = slog.Default()
	}
	log = slog.New(newRedactHandler(log.Handler()))

	caps := mqtt.NewDefaultServerCapabilities()
	caps.MaximumClients = maxClients
	caps.MaximumQos = 1
	caps.MaximumPacketSize = maxPacketSize
	caps.MaximumSessionExpiryInterval = 0
	caps.ReceiveMaximum = 1024
	caps.MaximumInflight = 1024
	caps.MaximumClientWritesPending = 1024
	caps.RetainAvailable = 0
	caps.WildcardSubAvailable = 0
	caps.SharedSubAvailable = 0
	caps.TopicAliasMaximum = 0
	caps.Compatibilities.ObscureNotAuthorized = true

	srv := mqtt.New(&mqtt.Options{
		InlineClient: true,
		Capabilities: caps,
		Logger:       log,
	})

	b := &Broker{
		server:    srv,
		auth:      auth,
		uplink:    uplink,
		log:       log,
		onDrop:    opts.OnPublishDropped,
		metrics:   opts.Metrics,
		current:   make(map[string]*connState),
		byClient:  make(map[*mqtt.Client]*connState),
		byConnID:  make(map[port.ConnID]*connState),
		closedCh:  make(chan struct{}),
		queues:    make(map[string]*uplinkQueue),
		largeSem:  make(chan struct{}, largeFrameSlots),
		lifeLocks: make(map[string]*sync.Mutex),
	}
	b.hook = &nixHook{b: b}
	if err := srv.AddHook(b.hook, nil); err != nil {
		return nil, err
	}
	if err := srv.Serve(); err != nil {
		return nil, err
	}
	go b.sweepLoop()
	return b, nil
}

// Server 返回底层 mochi（测试用）。
func (b *Broker) Server() *mqtt.Server { return b.server }

// Close 关闭 broker。
func (b *Broker) Close() error {
	if b.closed.Swap(true) {
		return nil
	}
	select {
	case <-b.closedCh:
	default:
		close(b.closedCh)
	}
	b.queuesMu.Lock()
	for _, q := range b.queues {
		q.close()
	}
	b.queues = make(map[string]*uplinkQueue)
	b.queuesMu.Unlock()
	return b.server.Close()
}

// Shutdown 向所有连接发 MQTT 5 0x8B 后关闭。完整 HTTP 停机顺序见 L-03。
func (b *Broker) Shutdown(ctx context.Context) error {
	if b.closed.Load() {
		return nil
	}
	b.connsMu.RLock()
	clients := make([]*mqtt.Client, 0, len(b.byClient))
	for cl := range b.byClient {
		if cl != nil {
			clients = append(clients, cl)
		}
	}
	b.connsMu.RUnlock()
	for _, cl := range clients {
		_ = b.server.DisconnectClient(cl, packets.ErrServerShuttingDown)
	}
	if ctx != nil {
		select {
		case <-ctx.Done():
		default:
		}
	}
	return b.Close()
}

// AttachTCP 把裸 TCP/TLS 连接交给 mochi；阻塞到连接结束。
func (b *Broker) AttachTCP(conn net.Conn) error {
	return b.server.EstablishConnection("tcp", conn)
}

// AttachWS 把 WebSocket NetConn 交给 mochi；阻塞到连接结束。
func (b *Broker) AttachWS(conn net.Conn) error {
	return b.server.EstablishConnection("ws", conn)
}

// PublishDown 实现 port.Downlink。
func (b *Broker) PublishDown(ctx context.Context, endpointID string, connID port.ConnID, payload []byte, opts port.PublishOpts) error {
	qos := opts.QoS
	if qos > 1 {
		qos = 1
	}
	return b.enqueueDownlink(endpointID, connID, payload, qos, "")
}

// PublishThenDisconnect 把一帧写入该连接下行队列，写出后再断开（无固定 sleep）。
func (b *Broker) PublishThenDisconnect(_ context.Context, endpointID string, connID port.ConnID, payload []byte, qos byte, reason port.DisconnectReason) error {
	if qos > 1 {
		qos = 1
	}
	if reason == "" {
		reason = port.DisconnectNormal
	}
	return b.enqueueDownlink(endpointID, connID, payload, qos, reason)
}

func (b *Broker) enqueueDownlink(endpointID string, connID port.ConnID, payload []byte, qos byte, disconnect port.DisconnectReason) error {
	if b.closed.Load() {
		return errors.New("broker: closed")
	}
	st := b.lookupCurrent(endpointID, connID)
	if st == nil {
		return ErrNoConnection
	}

	st.mu.Lock()
	maxRecv := st.maxRecvBytes
	closing := st.closing
	superseded := st.superseded
	st.mu.Unlock()
	if closing || superseded {
		return ErrNoConnection
	}
	if !b.hasDownSub(st) {
		return ErrNotSubscribed
	}

	limit := EffectivePayloadLimit(st.maxPacketSize, maxRecv)
	if limit > 0 && len(payload) > limit {
		return ErrPayloadTooLarge
	}

	return st.enqueueDown(downItem{
		payload:    append([]byte(nil), payload...),
		qos:        qos,
		disconnect: disconnect,
	})
}

func (b *Broker) acquireLarge(ctx context.Context) error {
	timer := time.NewTimer(largeAcquireWait)
	defer timer.Stop()
	select {
	case b.largeSem <- struct{}{}:
		return nil
	case <-ctx.Done():
		return ctx.Err()
	case <-timer.C:
		return ErrLargeFrameTimeout
	}
}

func (b *Broker) releaseLargeSlot() {
	select {
	case <-b.largeSem:
	default:
	}
}

func (b *Broker) finishLargePublish(st *connState) {
	b.reconcileLargeInflight(st)
	st.mu.Lock()
	n := st.largePending
	st.largePending = 0
	st.mu.Unlock()
	for i := 0; i < n; i++ {
		b.releaseLargeSlot()
	}
}

func (b *Broker) releaseLargePID(st *connState, id uint16) {
	st.mu.Lock()
	_, ok := st.largePIDs[id]
	if ok {
		delete(st.largePIDs, id)
	}
	st.mu.Unlock()
	if ok {
		b.releaseLargeSlot()
	}
}

func (b *Broker) reconcileLargeInflight(st *connState) {
	if st == nil {
		return
	}
	st.mu.Lock()
	ids := make([]uint16, 0, len(st.largePIDs))
	for id := range st.largePIDs {
		ids = append(ids, id)
	}
	st.mu.Unlock()
	for _, id := range ids {
		if st.client != nil {
			if _, ok := st.client.State.Inflight.Get(id); ok {
				continue
			}
		}
		b.releaseLargePID(st, id)
	}
}

func (b *Broker) releaseAllLarge(st *connState) {
	st.mu.Lock()
	n := len(st.largePIDs) + st.largePending
	st.largePIDs = nil
	st.largePending = 0
	st.mu.Unlock()
	for i := 0; i < n; i++ {
		b.releaseLargeSlot()
	}
}

// Disconnect 实现 port.ConnControl。
func (b *Broker) Disconnect(_ context.Context, endpointID string, connID port.ConnID, reason port.DisconnectReason) error {
	st := b.lookupConn(endpointID, connID)
	if st == nil {
		return ErrNoConnection
	}
	code := packets.CodeDisconnect
	switch reason {
	case port.DisconnectTakenOver:
		code = packets.ErrSessionTakenOver
	case port.DisconnectKicked, port.DisconnectFatal:
		code = packets.ErrAdministrativeAction
	}
	err := b.server.DisconnectClient(st.client, code)
	// mochi 对错误类原因码会把 Code 当作 error 返回，表示已按该原因断开，不算失败。
	if _, ok := err.(packets.Code); ok {
		return nil
	}
	return err
}

func (b *Broker) lookupConn(endpointID string, connID port.ConnID) *connState {
	b.connsMu.RLock()
	defer b.connsMu.RUnlock()
	if connID != "" {
		st := b.byConnID[connID]
		if st != nil && st.endpointID == endpointID {
			return st
		}
		return nil
	}
	return b.current[endpointID]
}

// lookupCurrent 只返回该端当前连接；connID 非空时必须仍是当前连接。
func (b *Broker) lookupCurrent(endpointID string, connID port.ConnID) *connState {
	b.connsMu.RLock()
	defer b.connsMu.RUnlock()
	cur := b.current[endpointID]
	if cur == nil {
		return nil
	}
	if connID != "" && cur.connID != connID {
		return nil
	}
	return cur
}

func (b *Broker) endpointLife(endpointID string) *sync.Mutex {
	b.lifeMu.Lock()
	defer b.lifeMu.Unlock()
	m := b.lifeLocks[endpointID]
	if m == nil {
		m = &sync.Mutex{}
		b.lifeLocks[endpointID] = m
	}
	return m
}

func downTopic(endpointID string) string {
	return "nix/c/" + endpointID + "/down"
}

func upTopic(endpointID string) string {
	return "nix/c/" + endpointID + "/up"
}

// EffectivePayloadLimit 下行载荷上限：客户端 Maximum Packet Size 减包头预留，再与 max_receive_bytes 取更严者。
func EffectivePayloadLimit(maxPacketSize uint32, maxRecvBytes int) int {
	return effectivePayloadLimit(maxPacketSize, maxRecvBytes)
}

func effectivePayloadLimit(maxPacketSize uint32, maxRecvBytes int) int {
	limit := 0
	if maxPacketSize > 0 {
		if maxPacketSize > packetOverheadBudget {
			limit = int(maxPacketSize) - packetOverheadBudget
		}
	}
	if maxRecvBytes > 0 {
		if limit == 0 || maxRecvBytes < limit {
			limit = maxRecvBytes
		}
	}
	return limit
}

func randomConnID() port.ConnID {
	var b [16]byte
	_, _ = rand.Read(b[:])
	return port.ConnID(hex.EncodeToString(b[:]))
}

func transportOf(cl *mqtt.Client) port.Transport {
	if cl != nil && cl.Net.Listener == "ws" {
		return port.TransportWS
	}
	return port.TransportTCP
}

func remoteIPOf(cl *mqtt.Client) string {
	if cl == nil {
		return ""
	}
	addr := cl.Net.Remote
	if addr == "" && cl.Net.Conn != nil && cl.Net.Conn.RemoteAddr() != nil {
		addr = cl.Net.Conn.RemoteAddr().String()
	}
	host, _, err := net.SplitHostPort(addr)
	if err != nil {
		return addr
	}
	return host
}

// SetMaxReceiveBytes 供 N3 握手后设置；0 表示不限。
func (b *Broker) SetMaxReceiveBytes(endpointID string, connID port.ConnID, n int) {
	st := b.lookupConn(endpointID, connID)
	if st == nil {
		return
	}
	st.mu.Lock()
	st.maxRecvBytes = n
	st.mu.Unlock()
}

// ConnInfoOf 返回连接信息（测试/N3）。
func (b *Broker) ConnInfoOf(endpointID string) (port.ConnInfo, bool) {
	b.connsMu.RLock()
	st := b.current[endpointID]
	b.connsMu.RUnlock()
	if st == nil {
		return port.ConnInfo{}, false
	}
	return port.ConnInfo{
		ConnID:        st.connID,
		EndpointID:    st.endpointID,
		Transport:     st.transport,
		RemoteIP:      st.remoteIP,
		SessionToken:  st.sessionToken,
		MaxPacketSize: st.maxPacketSize,
	}, true
}

// IsHandshook 当前连接是否已完成握手。
func (b *Broker) IsHandshook(endpointID string) bool {
	b.connsMu.RLock()
	st := b.current[endpointID]
	b.connsMu.RUnlock()
	if st == nil {
		return false
	}
	st.mu.Lock()
	defer st.mu.Unlock()
	return st.handshook
}

// CurrentConnID 返回端的当前连接代号。
func (b *Broker) CurrentConnID(endpointID string) (port.ConnID, bool) {
	b.connsMu.RLock()
	st := b.current[endpointID]
	b.connsMu.RUnlock()
	if st == nil {
		return "", false
	}
	return st.connID, true
}

func (b *Broker) connStateOf(endpointID string, connID port.ConnID) *connState {
	b.connsMu.RLock()
	defer b.connsMu.RUnlock()
	st := b.byConnID[connID]
	if st == nil || st.endpointID != endpointID {
		return nil
	}
	return st
}

func (b *Broker) sweepLoop() {
	tick := time.NewTicker(time.Minute)
	defer tick.Stop()
	for {
		select {
		case <-tick.C:
			b.sweepUnestablished(time.Minute)
		case <-b.closedCh:
			return
		}
	}
}

func (b *Broker) sweepUnestablished(minAge time.Duration) {
	now := time.Now()
	b.connsMu.Lock()
	defer b.connsMu.Unlock()
	for cl, st := range b.byClient {
		if st.established {
			continue
		}
		if cl != nil && !cl.Closed() {
			continue
		}
		if minAge > 0 && now.Sub(st.createdAt) < minAge {
			continue
		}
		delete(b.byClient, cl)
		delete(b.byConnID, st.connID)
		if b.current[st.endpointID] == st {
			delete(b.current, st.endpointID)
		}
	}
}

func (b *Broker) hasDownSub(st *connState) bool {
	if st == nil {
		return false
	}
	st.mu.Lock()
	defer st.mu.Unlock()
	if st.subscribedDown {
		return true
	}
	// 回退：直接看 mochi 订阅表
	if st.client != nil && st.client.State.Subscriptions != nil {
		_, ok := st.client.State.Subscriptions.Get(downTopic(st.endpointID))
		return ok
	}
	return false
}

func (b *Broker) startHandshakeDeadline(endpointID string, connID port.ConnID, d time.Duration) {
	st := b.connStateOf(endpointID, connID)
	if st == nil {
		return
	}
	st.mu.Lock()
	if st.handshook {
		st.mu.Unlock()
		return
	}
	if st.handshakeTimer != nil {
		st.handshakeTimer.Stop()
	}
	st.handshakeTimer = time.AfterFunc(d, func() {
		cur := b.connStateOf(endpointID, connID)
		if cur == nil {
			return
		}
		cur.mu.Lock()
		done := cur.handshook
		cur.mu.Unlock()
		if done {
			return
		}
		_ = b.Disconnect(context.Background(), endpointID, connID, port.DisconnectIdle)
	})
	st.mu.Unlock()
}

func (b *Broker) cancelHandshakeDeadline(endpointID string, connID port.ConnID) {
	st := b.connStateOf(endpointID, connID)
	if st == nil {
		return
	}
	st.mu.Lock()
	if st.handshakeTimer != nil {
		st.handshakeTimer.Stop()
		st.handshakeTimer = nil
	}
	st.mu.Unlock()
}

func (b *Broker) enqueueUplink(endpointID string, conn port.ConnInfo, payload []byte) {
	b.queuesMu.Lock()
	q, ok := b.queues[endpointID]
	if !ok {
		q = newUplinkQueue(b, endpointID)
		b.queues[endpointID] = q
	}
	b.queuesMu.Unlock()
	q.push(uplinkItem{conn: conn, payload: payload})
}

var (
	_ port.Downlink    = (*Broker)(nil)
	_ port.ConnControl = (*Broker)(nil)
)