// Package server implements the olcrtc tunnel server logic.
package server

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net"
	"strconv"
	"sync"
	"time"

	"github.com/google/uuid"
	"github.com/openlibrecommunity/olcrtc/internal/control"
	"github.com/openlibrecommunity/olcrtc/internal/crypto"
	"github.com/openlibrecommunity/olcrtc/internal/handshake"
	"github.com/openlibrecommunity/olcrtc/internal/logger"
	"github.com/openlibrecommunity/olcrtc/internal/muxconn"
	"github.com/openlibrecommunity/olcrtc/internal/names"
	"github.com/openlibrecommunity/olcrtc/internal/runtime"
	"github.com/openlibrecommunity/olcrtc/internal/transport"
	"github.com/xtaci/smux"
)

const connectCommand = "connect"

var (
	// ErrKeyRequired re-exports runtime.ErrKeyRequired for compatibility with
	// pre-runtime callers that errors.Is-checked it.
	ErrKeyRequired = runtime.ErrKeyRequired
	// ErrKeySize re-exports runtime.ErrKeySize for the same reason.
	ErrKeySize = runtime.ErrKeySize
	// ErrSocks5AuthFailed is returned when SOCKS5 authentication fails.
	ErrSocks5AuthFailed = errors.New("SOCKS5 auth failed")
	// ErrSocks5ConnectFailed is returned when SOCKS5 connection fails.
	ErrSocks5ConnectFailed = errors.New("SOCKS5 connect failed")
)

// SessionOpenFunc is called after a successful handshake, before the server
// accepts tunnel streams on that session.
type SessionOpenFunc func(sessionID, deviceID string, claims map[string]any)

// SessionCloseFunc is called when a session is torn down. Possible reasons:
// "reconnect" (carrier dropped and was reestablished), "closed" (graceful
// shutdown or ctx cancel).
type SessionCloseFunc func(sessionID, reason string)

// TrafficFunc is called once per tunnel stream, after the copy loops finish.
// bytesIn counts client→target bytes; bytesOut counts target→client bytes.
type TrafficFunc func(sessionID, addr string, bytesIn, bytesOut uint64)

// HealthFunc is called when the server control health snapshot changes.
type HealthFunc func(control.Status)

// Server handles incoming tunnel connections and proxies their traffic.
type Server struct {
	ln             transport.Transport
	cipher         *crypto.Cipher
	conn           *muxconn.Conn
	session        *smux.Session
	controlStop    context.CancelFunc
	sessMu         sync.RWMutex
	reinstallMu    sync.Mutex
	wg             sync.WaitGroup
	authHook       handshake.AuthFunc
	onOpen         SessionOpenFunc
	onClose        SessionCloseFunc
	onTraffic      TrafficFunc
	deviceID       string
	sessionID      string
	dnsServer      string
	resolver       *net.Resolver
	socksProxyAddr string
	socksProxyPort int
	liveness       control.Config
	health         *runtime.HealthTracker
}

// ConnectRequest is a message from the client to establish a new connection.
type ConnectRequest struct {
	Cmd  string `json:"cmd"`
	Addr string `json:"addr"`
	Port int    `json:"port"`
}

// Config holds runtime configuration for [Run].
type Config struct {
	Transport        string
	Carrier          string
	RoomURL          string
	ChannelID        string
	KeyHex           string
	DNSServer        string
	SOCKSProxyAddr   string
	SOCKSProxyPort   int
	TransportOptions transport.Options
	Engine           string
	URL              string
	Token            string
	Liveness         control.Config
	Traffic          transport.TrafficConfig

	// AuthHook is invoked after CLIENT_HELLO to authorize the client and
	// return a session ID. If nil, every client is admitted with a random UUID.
	AuthHook handshake.AuthFunc

	// OnSessionOpen fires after a successful handshake. Nil means no-op.
	OnSessionOpen SessionOpenFunc
	// OnSessionClose fires when the session is torn down (reconnect, closed). Nil means no-op.
	OnSessionClose SessionCloseFunc
	// OnTraffic fires once per tunnel stream after both copy loops finish. Nil means no-op.
	OnTraffic TrafficFunc
	// OnHealth fires when liveness/reconnect status changes. Nil means no-op.
	OnHealth HealthFunc
}

// Run starts the server with the given configuration.
func Run(ctx context.Context, cfg Config) error {
	runCtx, cancel := context.WithCancel(ctx)
	defer cancel()

	cipher, err := setupCipher(cfg.KeyHex)
	if err != nil {
		return fmt.Errorf("setupCipher failed: %w", err)
	}

	hook := cfg.AuthHook
	if hook == nil {
		hook = defaultAuthHook
	}
	onOpen := cfg.OnSessionOpen
	if onOpen == nil {
		onOpen = func(string, string, map[string]any) {}
	}
	onClose := cfg.OnSessionClose
	if onClose == nil {
		onClose = func(string, string) {}
	}
	onTraffic := cfg.OnTraffic
	if onTraffic == nil {
		onTraffic = func(string, string, uint64, uint64) {}
	}
	s := &Server{
		cipher:         cipher,
		authHook:       hook,
		onOpen:         onOpen,
		onClose:        onClose,
		onTraffic:      onTraffic,
		dnsServer:      cfg.DNSServer,
		socksProxyAddr: cfg.SOCKSProxyAddr,
		socksProxyPort: cfg.SOCKSProxyPort,
		liveness:       cfg.Liveness,
		health:         runtime.NewHealthTracker(cfg.OnHealth),
	}
	s.setupResolver()

	// Register shutdown BEFORE bringUpLink so a partial setup (e.g.
	// link.New succeeded but ln.Connect timed out) still tears the
	// link down and sends MUC presence-unavailable. Without this, an
	// early bringUpLink error returns straight to the caller and the
	// already-joined MUC presence stays behind as a ghost participant
	// for subsequent tests against the same room. shutdown is
	// idempotent and safe to call before s.serve runs.
	defer func() {
		s.shutdown()
		s.wg.Wait()
	}()

	if err := s.bringUpLink(runCtx, cfg, cancel); err != nil {
		return err
	}

	go func() {
		<-runCtx.Done()
		s.closeSession()
	}()

	s.serve(runCtx)

	return nil
}

func setupCipher(keyHex string) (*crypto.Cipher, error) {
	cipher, err := runtime.SetupCipher(keyHex)
	if err != nil {
		return nil, fmt.Errorf("server: %w", err)
	}
	return cipher, nil
}

func (s *Server) setupResolver() {
	s.resolver = &net.Resolver{
		PreferGo: true,
		Dial: func(ctx context.Context, network, _ string) (net.Conn, error) {
			d := net.Dialer{Timeout: 3 * time.Second}
			return d.DialContext(ctx, network, s.dnsServer)
		},
	}
}

func smuxConfig(maxWirePayload int) *smux.Config {
	return runtime.SmuxConfig(maxWirePayload)
}

func linkMaxPayload(tr transport.Transport) int {
	return runtime.MaxPayload(tr)
}

func (s *Server) bringUpLink(
	ctx context.Context,
	cfg Config,
	cancel context.CancelFunc,
) error {
	ln, err := transport.New(ctx, cfg.Transport, transport.Config{
		Carrier:   cfg.Carrier,
		RoomURL:   cfg.RoomURL,
		Engine:    cfg.Engine,
		URL:       cfg.URL,
		Token:     cfg.Token,
		ChannelID: cfg.ChannelID,
		DeviceID:  "",
		Name:      names.Generate(),
		OnData:    s.onData,
		DNSServer: s.dnsServer,
		ProxyAddr: s.socksProxyAddr,
		ProxyPort: s.socksProxyPort,
		Options:   cfg.TransportOptions,
		Traffic:   cfg.Traffic,
	})
	if err != nil {
		return fmt.Errorf("failed to create transport: %w", err)
	}
	s.ln = ln

	ln.SetEndedCallback(func(reason string) {
		logger.Infof("Server link reported conference end: %s", reason)
		cancel()
	})
	ln.SetShouldReconnect(func() bool { return ctx.Err() == nil })
	ln.SetReconnectCallback(func() {
		if ctx.Err() != nil {
			return
		}
		s.handleReconnect()
	})

	logger.Infof("Connecting transport=%s carrier=%s ...", cfg.Transport, cfg.Carrier)
	s.installSession()

	if err := ln.Connect(ctx); err != nil {
		return fmt.Errorf("failed to connect link: %w", err)
	}
	logger.Infof("Link connected")

	s.wg.Add(1)
	go func() {
		defer s.wg.Done()
		ln.WatchConnection(ctx)
	}()
	return nil
}

func (s *Server) installSession() {
	conn := muxconn.New(s.ln, s.cipher)
	sess, err := smux.Server(conn, smuxConfig(linkMaxPayload(s.ln)))
	if err != nil {
		logger.Warnf("smux server init failed: %v", err)
		return
	}
	s.sessMu.Lock()
	s.conn = conn
	s.session = sess
	s.sessMu.Unlock()
}

func (s *Server) handleReconnect() {
	s.recordReconnect()
	logger.Infof("server reconnect reason=carrier - tearing down smux session")
	s.sessMu.RLock()
	current := s.session
	s.sessMu.RUnlock()
	s.reinstallSession(current)
}

func (s *Server) reinstallSession(dead *smux.Session) {
	s.reinstallMu.Lock()
	defer s.reinstallMu.Unlock()

	// Pre-build the replacement so we can swap atomically below.
	newConn := muxconn.New(s.ln, s.cipher)
	newSess, err := smux.Server(newConn, smuxConfig(linkMaxPayload(s.ln)))
	if err != nil {
		logger.Warnf("smux server init failed: %v", err)
		_ = newConn.Close()
		return
	}

	s.sessMu.Lock()
	if s.session != dead {
		// Someone else already reinstalled — discard our build.
		s.sessMu.Unlock()
		_ = newSess.Close()
		_ = newConn.Close()
		return
	}
	oldSess := s.session
	oldConn := s.conn
	oldControlStop := s.controlStop
	oldSID := s.sessionID
	s.session = newSess
	s.conn = newConn
	s.controlStop = nil
	s.sessionID = ""
	s.deviceID = ""
	s.sessMu.Unlock()

	if oldControlStop != nil {
		oldControlStop()
	}
	if oldSess != nil {
		_ = oldSess.Close()
	}
	if oldConn != nil {
		_ = oldConn.Close()
	}
	if oldSID != "" {
		s.onClose(oldSID, "reconnect")
	}
}

func (s *Server) closeSession() {
	s.sessMu.Lock()
	sess := s.session
	conn := s.conn
	controlStop := s.controlStop
	s.session = nil
	s.conn = nil
	s.controlStop = nil
	oldSID := s.sessionID
	s.sessionID = ""
	s.deviceID = ""
	s.sessMu.Unlock()

	if controlStop != nil {
		controlStop()
	}
	if sess != nil {
		_ = sess.Close()
	}
	if conn != nil {
		_ = conn.Close()
	}
	if oldSID != "" {
		s.onClose(oldSID, "closed")
	}
}

func (s *Server) onData(data []byte) {
	s.sessMu.RLock()
	conn := s.conn
	s.sessMu.RUnlock()
	if conn != nil {
		conn.Push(data)
	}
}

// serve drives the smux Accept loop. The first accepted stream on a given
// smux session is the control stream — the handshake runs there. Subsequent
// streams are tunnel streams and proxy traffic.
func (s *Server) serve(ctx context.Context) {
	for {
		if contextDone(ctx) {
			return
		}

		s.sessMu.RLock()
		sess := s.session
		s.sessMu.RUnlock()
		if sess == nil {
			select {
			case <-ctx.Done():
				return
			case <-time.After(50 * time.Millisecond):
				continue
			}
		}

		if !s.handshakeReady() {
			if !s.acceptHandshake(ctx, sess) {
				continue
			}
		}

		stream, err := sess.AcceptStream()
		if err != nil {
			if contextDone(ctx) {
				return
			}
			logger.Debugf("AcceptStream returned %v - reinstalling session", err)
			s.reinstallSession(sess)
			continue
		}

		s.wg.Add(1)
		go func() {
			defer s.wg.Done()
			s.handleStream(ctx, stream)
		}()
	}
}

func contextDone(ctx context.Context) bool {
	select {
	case <-ctx.Done():
		return true
	default:
		return false
	}
}

// handshakeReady reports whether the current session has completed its
// handshake. The session is reset on reconnect, so this is recomputed.
func (s *Server) handshakeReady() bool {
	s.sessMu.RLock()
	defer s.sessMu.RUnlock()
	return s.sessionID != ""
}

func (s *Server) acceptHandshake(ctx context.Context, sess *smux.Session) bool {
	stream, err := sess.AcceptStream()
	if err != nil {
		select {
		case <-ctx.Done():
			return false
		default:
		}
		logger.Debugf("AcceptStream(control) returned %v - reinstalling session", err)
		s.resetLinkPeer()
		s.reinstallSession(sess)
		return false
	}
	_ = stream.SetDeadline(time.Now().Add(handshake.DefaultTimeout))
	hello, sid, err := handshake.Server(stream, s.authHook)
	_ = stream.SetDeadline(time.Time{})
	if err != nil {
		logger.Warnf("handshake failed: %v", err)
		_ = stream.Close()
		s.resetLinkPeer()
		s.reinstallSession(sess)
		return false
	}
	s.sessMu.Lock()
	s.deviceID = hello.DeviceID
	s.sessionID = sid
	s.sessMu.Unlock()
	s.recordSession(sid)
	s.onOpen(sid, hello.DeviceID, hello.Claims)
	logger.Infof("session %s opened (device=%s)", sid, hello.DeviceID)
	s.startControlLoop(ctx, sess, stream)
	return true
}

func (s *Server) resetLinkPeer() {
	s.sessMu.RLock()
	ln := s.ln
	s.sessMu.RUnlock()
	if resetter, ok := ln.(interface{ ResetPeer() }); ok {
		resetter.ResetPeer()
	}
}

func (s *Server) startControlLoop(ctx context.Context, sess *smux.Session, stream *smux.Stream) {
	controlCtx, stop := context.WithCancel(ctx)
	s.sessMu.Lock()
	s.controlStop = stop
	s.sessMu.Unlock()

	liveness := s.liveness
	onPong := liveness.OnPong
	onMissedPong := liveness.OnMissedPong
	onUnhealthy := liveness.OnUnhealthy
	liveness.OnPong = func(h control.Health) {
		s.sessMu.RLock()
		sid := s.sessionID
		s.sessMu.RUnlock()
		s.recordPong(h)
		logger.Debugf("control alive session=%s rtt=%v seq=%d", sid, h.RTT, h.Seq)
		if onPong != nil {
			onPong(h)
		}
	}
	liveness.OnMissedPong = func(missed int) {
		s.recordMissed(missed)
		logger.Warnf("control missed pong on server: missed_pongs=%d", missed)
		if onMissedPong != nil {
			onMissedPong(missed)
		}
	}
	liveness.OnUnhealthy = func(missed int) {
		s.recordUnhealthy(missed)
		logger.Warnf("control stream unhealthy on server: missed_pongs=%d", missed)
		if onUnhealthy != nil {
			onUnhealthy(missed)
		}
	}

	s.wg.Add(1)
	go func() {
		defer s.wg.Done()
		defer func() { _ = stream.Close() }()
		err := control.Run(controlCtx, stream, liveness)
		if controlCtx.Err() != nil || ctx.Err() != nil {
			return
		}
		if err != nil {
			logger.Warnf("server control stream ended: %v", err)
		}
		s.recordReconnect()
		logger.Infof("server reconnect reason=liveness - reinstalling smux session")
		s.reinstallSession(sess)
	}()
}

// Status returns the latest server-side control health snapshot.
func (s *Server) Status() control.Status {
	return s.health.Status()
}

func (s *Server) recordSession(sessionID string) { s.health.RecordSession(sessionID) }
func (s *Server) recordPong(h control.Health)    { s.health.RecordPong(h) }
func (s *Server) recordMissed(missed int)        { s.health.RecordMissed(missed) }
func (s *Server) recordUnhealthy(missed int)     { s.health.RecordUnhealthy(missed) }
func (s *Server) recordReconnect()               { s.health.RecordReconnect() }

func (s *Server) shutdown() {
	s.closeSession()
	if s.ln != nil {
		_ = s.ln.Close()
	}
}

func (s *Server) handleStream(_ context.Context, stream *smux.Stream) {
	defer func() { _ = stream.Close() }()

	// Read the connect JSON. The client writes the whole JSON in one
	// stream.Write so it usually arrives intact; tolerate fragmentation
	// by reading incrementally up to a sane cap.
	const maxConnReq = 4096
	header := make([]byte, 0, 256)
	tmp := make([]byte, 256)
	_ = stream.SetReadDeadline(time.Now().Add(15 * time.Second))
	for {
		n, err := stream.Read(tmp)
		if n > 0 {
			header = append(header, tmp[:n]...)
			if req, ok := parseConnectRequest(header); ok {
				_ = stream.SetReadDeadline(time.Time{})
				s.dispatch(stream, req)
				return
			}
		}
		if err != nil {
			return
		}
		if len(header) > maxConnReq {
			return
		}
	}
}

func parseConnectRequest(buf []byte) (ConnectRequest, bool) {
	var req ConnectRequest
	if err := json.Unmarshal(buf, &req); err != nil {
		return req, false
	}
	if req.Cmd != connectCommand {
		return req, false
	}
	return req, true
}

// defaultAuthHook admits every client and assigns a random session ID.
// Replace it via [Config.AuthHook] to plug in real authorization.
func defaultAuthHook(_ string, _ map[string]any) (string, error) {
	return uuid.NewString(), nil
}

func (s *Server) dispatch(stream *smux.Stream, req ConnectRequest) {
	addr := net.JoinHostPort(req.Addr, strconv.Itoa(req.Port))
	logger.Infof("sid=%d connect %s", stream.ID(), addr)

	s.sessMu.RLock()
	sid := s.sessionID
	s.sessMu.RUnlock()

	dialStart := time.Now()
	conn, err := s.dial(req)
	dialElapsed := time.Since(dialStart)

	if err != nil {
		logger.Infof("sid=%d dial %s failed (%v): %v", stream.ID(), addr, dialElapsed, err)
		return
	}
	defer func() { _ = conn.Close() }()

	logger.Infof("sid=%d connected %s in %v", stream.ID(), addr, dialElapsed)

	if _, err := stream.Write([]byte{0x00}); err != nil {
		return
	}

	var bytesOut uint64
	done := make(chan struct{})
	go func() {
		n, _ := io.Copy(stream, conn)
		if n > 0 {
			bytesOut = uint64(n)
		}
		_ = stream.Close()
		close(done)
	}()
	in, _ := io.Copy(conn, stream)
	_ = conn.Close()
	<-done
	bytesIn := uint64(0)
	if in > 0 {
		bytesIn = uint64(in)
	}
	if s.onTraffic != nil {
		s.onTraffic(sid, addr, bytesIn, bytesOut)
	}
}

func (s *Server) dial(req ConnectRequest) (net.Conn, error) {
	addr := net.JoinHostPort(req.Addr, strconv.Itoa(req.Port))
	if s.socksProxyAddr == "" {
		dialer := &net.Dialer{
			Timeout:   10 * time.Second,
			KeepAlive: 30 * time.Second,
			Resolver:  s.resolver,
		}
		conn, err := dialer.Dial("tcp4", addr)
		if err != nil {
			return nil, fmt.Errorf("dial failed: %w", err)
		}
		return conn, nil
	}

	proxyAddr := net.JoinHostPort(s.socksProxyAddr, strconv.Itoa(s.socksProxyPort))
	dialer := &net.Dialer{
		Timeout:   10 * time.Second,
		KeepAlive: 30 * time.Second,
	}
	conn, err := dialer.Dial("tcp4", proxyAddr)
	if err != nil {
		return nil, fmt.Errorf("failed to dial proxy: %w", err)
	}

	if err := s.socks5Connect(conn, req.Addr, req.Port); err != nil {
		_ = conn.Close()
		return nil, err
	}
	return conn, nil
}

func (s *Server) socks5Connect(conn net.Conn, targetAddr string, targetPort int) error {
	if _, err := conn.Write([]byte{5, 1, 0}); err != nil {
		return fmt.Errorf("failed to write socks5 auth: %w", err)
	}

	resp := make([]byte, 2)
	if _, err := io.ReadFull(conn, resp); err != nil {
		return fmt.Errorf("failed to read socks5 auth resp: %w", err)
	}
	if resp[0] != 5 || resp[1] != 0 {
		return ErrSocks5AuthFailed
	}

	addrLen := len(targetAddr)
	if addrLen > 255 {
		addrLen = 255
		targetAddr = targetAddr[:255]
	}

	req := make([]byte, 0, 7+addrLen)
	req = append(req, 5, 1, 0, 3, byte(addrLen))
	req = append(req, []byte(targetAddr)...)
	req = append(req, byte(targetPort>>8), byte(targetPort)) //nolint:gosec,lll // G115: bounded conversion verified by surrounding logic

	if _, err := conn.Write(req); err != nil {
		return fmt.Errorf("failed to write socks5 connect req: %w", err)
	}

	resp = make([]byte, 10)
	if _, err := io.ReadFull(conn, resp); err != nil {
		return fmt.Errorf("failed to read socks5 connect resp: %w", err)
	}
	if resp[0] != 5 || resp[1] != 0 {
		return fmt.Errorf("%w: %d", ErrSocks5ConnectFailed, resp[1])
	}

	return nil
}