package tlstunnel

import (
	"context"
	"crypto/tls"
	"fmt"
	"io"
	"log"
	"net"
	"strings"
	"sync/atomic"

	"git.sr.ht/~emersion/go-scfg"
	"github.com/caddyserver/certmagic"
	"github.com/pires/go-proxyproto"
	"github.com/pires/go-proxyproto/tlvparse"
)

type Server struct {
	Listeners map[string]*Listener // indexed by listening address
	Frontends []*Frontend

	ManagedNames   []string
	UnmanagedCerts []tls.Certificate

	ACMEManager *certmagic.ACMEManager
	ACMEConfig  *certmagic.Config

	cancelACME context.CancelFunc
}

func NewServer() *Server {
	cfg := certmagic.NewDefault()

	mgr := certmagic.NewACMEManager(cfg, certmagic.DefaultACME)
	mgr.Agreed = true
	// We're a TLS server, we don't speak HTTP
	mgr.DisableHTTPChallenge = true
	cfg.Issuer = mgr
	cfg.Revoker = mgr

	return &Server{
		Listeners:   make(map[string]*Listener),
		ACMEManager: mgr,
		ACMEConfig:  cfg,
	}
}

func (srv *Server) Load(cfg scfg.Block) error {
	return parseConfig(srv, cfg)
}

func (srv *Server) RegisterListener(addr string) *Listener {
	// TODO: normalize addr with net.LookupPort
	ln, ok := srv.Listeners[addr]
	if !ok {
		ln = newListener(srv, addr)
		srv.Listeners[addr] = ln
	}
	return ln
}

func (srv *Server) startACME() error {
	var ctx context.Context
	ctx, srv.cancelACME = context.WithCancel(context.Background())

	for _, cert := range srv.UnmanagedCerts {
		if err := srv.ACMEConfig.CacheUnmanagedTLSCertificate(cert, nil); err != nil {
			return err
		}
	}

	if err := srv.ACMEConfig.ManageAsync(ctx, srv.ManagedNames); err != nil {
		return fmt.Errorf("failed to manage TLS certificates: %v", err)
	}

	return nil
}

func (srv *Server) Start() error {
	if err := srv.startACME(); err != nil {
		return err
	}

	for _, ln := range srv.Listeners {
		if err := ln.Start(); err != nil {
			return err
		}
	}
	return nil
}

func (srv *Server) Stop() {
	srv.cancelACME()
	// TODO: clean cached unmanaged certs
	for _, ln := range srv.Listeners {
		ln.Stop()
	}
}

// Replace starts the server but takes over existing listeners from an old
// Server instance. The old instance keeps running unchanged if Replace
// returns an error.
func (srv *Server) Replace(old *Server) error {
	// Try to start new listeners
	for addr, ln := range srv.Listeners {
		if _, ok := old.Listeners[addr]; ok {
			continue
		}
		if err := ln.Start(); err != nil {
			for _, ln2 := range srv.Listeners {
				ln2.Stop()
			}
			return err
		}
	}

	// Restart ACME
	old.cancelACME()
	if err := srv.startACME(); err != nil {
		for _, ln2 := range srv.Listeners {
			ln2.Stop()
		}
		return err
	}
	// TODO: clean cached unmanaged certs

	// Take over existing listeners and terminate old ones
	for addr, oldLn := range old.Listeners {
		if ln, ok := srv.Listeners[addr]; ok {
			srv.Listeners[addr] = oldLn.UpdateFrom(ln)
		} else {
			oldLn.Stop()
		}
	}

	return nil
}

type listenerHandles struct {
	Server    *Server
	Frontends map[string]*Frontend // indexed by server name
}

type Listener struct {
	Address string
	netLn   net.Listener
	atomic  atomic.Value
}

func newListener(srv *Server, addr string) *Listener {
	ln := &Listener{
		Address: addr,
	}
	ln.atomic.Store(&listenerHandles{
		Server:    srv,
		Frontends: make(map[string]*Frontend),
	})
	return ln
}

func (ln *Listener) RegisterFrontend(name string, fe *Frontend) error {
	fes := ln.atomic.Load().(*listenerHandles).Frontends
	if _, ok := fes[name]; ok {
		return fmt.Errorf("listener %q: duplicate frontends for server name %q", ln.Address, name)
	}
	fes[name] = fe
	return nil
}

func (ln *Listener) Start() error {
	var err error
	ln.netLn, err = net.Listen("tcp", ln.Address)
	if err != nil {
		return err
	}
	log.Printf("listening on %q", ln.Address)

	go func() {
		if err := ln.serve(); err != nil {
			log.Fatalf("listener %q: %v", ln.Address, err)
		}
	}()

	return nil
}

func (ln *Listener) Stop() {
	ln.netLn.Close()
}

func (ln *Listener) UpdateFrom(new *Listener) *Listener {
	ln.atomic.Store(new.atomic.Load())
	return ln
}

func (ln *Listener) serve() error {
	for {
		conn, err := ln.netLn.Accept()
		if err != nil && strings.Contains(err.Error(), "use of closed network connection") {
			// Listening socket has been closed by Stop()
			return nil
		} else if err != nil {
			return fmt.Errorf("failed to accept connection: %v", err)
		}

		go func() {
			if err := ln.handle(conn); err != nil {
				log.Printf("listener %q: %v", ln.Address, err)
			}
		}()
	}
}

func (ln *Listener) handle(conn net.Conn) error {
	defer conn.Close()
	srv := ln.atomic.Load().(*listenerHandles).Server

	// TODO: setup timeouts
	tlsConfig := srv.ACMEConfig.TLSConfig()
	getConfigForClient := tlsConfig.GetConfigForClient
	tlsConfig.GetConfigForClient = func(hello *tls.ClientHelloInfo) (*tls.Config, error) {
		// Call previous GetConfigForClient function, if any
		var tlsConfig *tls.Config
		if getConfigForClient != nil {
			var err error
			tlsConfig, err = getConfigForClient(hello)
			if err != nil {
				return nil, err
			}
		} else {
			tlsConfig = srv.ACMEConfig.TLSConfig()
		}

		fe, err := ln.matchFrontend(hello.ServerName)
		if err != nil {
			return nil, err
		}

		tlsConfig.NextProtos = fe.Protocols
		return tlsConfig, nil
	}
	tlsConn := tls.Server(conn, tlsConfig)
	if err := tlsConn.Handshake(); err != nil {
		return err
	}

	tlsState := tlsConn.ConnectionState()
	fe, err := ln.matchFrontend(tlsState.ServerName)
	if err != nil {
		return err
	}

	return fe.handle(tlsConn, &tlsState)
}

func (ln *Listener) matchFrontend(serverName string) (*Frontend, error) {
	fes := ln.atomic.Load().(*listenerHandles).Frontends

	fe, ok := fes[serverName]
	if !ok {
		// Match wildcard certificates, allowing only a single, non-partial
		// wildcard, in the left-most label
		i := strings.IndexByte(serverName, '.')
		// Don't allow wildcards with only a TLD (e.g. *.com)
		if i >= 0 && strings.IndexByte(serverName[i+1:], '.') >= 0 {
			fe, ok = fes["*"+serverName[i:]]
		}
	}
	if !ok {
		fe, ok = fes[""]
	}
	if !ok {
		return nil, fmt.Errorf("can't find frontend for server name %q", serverName)
	}

	return fe, nil
}

type Frontend struct {
	Backend   Backend
	Protocols []string
}

func (fe *Frontend) handle(downstream net.Conn, tlsState *tls.ConnectionState) error {
	defer downstream.Close()

	be := &fe.Backend
	upstream, err := net.Dial(be.Network, be.Address)
	if err != nil {
		return fmt.Errorf("failed to dial backend: %v", err)
	}
	if be.TLSConfig != nil {
		upstream = tls.Client(upstream, be.TLSConfig)
	}
	defer upstream.Close()

	if be.Proxy {
		h := proxyproto.HeaderProxyFromAddrs(2, downstream.RemoteAddr(), downstream.LocalAddr())

		var tlvs []proxyproto.TLV
		if tlsState.ServerName != "" {
			tlvs = append(tlvs, authorityTLV(tlsState.ServerName))
		}
		if tlsState.NegotiatedProtocol != "" {
			tlvs = append(tlvs, alpnTLV(tlsState.NegotiatedProtocol))
		}
		if tlv, err := sslTLV(tlsState); err != nil {
			return fmt.Errorf("failed to set PROXY protocol header SSL TLV: %v", err)
		} else {
			tlvs = append(tlvs, tlv)
		}
		if err := h.SetTLVs(tlvs); err != nil {
			return fmt.Errorf("failed to set PROXY protocol header TLVs: %v", err)
		}

		if _, err := h.WriteTo(upstream); err != nil {
			return fmt.Errorf("failed to write PROXY protocol header: %v", err)
		}
	}

	return duplexCopy(upstream, downstream)
}

type Backend struct {
	Network   string
	Address   string
	Proxy     bool
	TLSConfig *tls.Config // nil if no TLS
}

func duplexCopy(a, b io.ReadWriter) error {
	done := make(chan error, 2)
	go func() {
		_, err := io.Copy(a, b)
		done <- err
	}()
	go func() {
		_, err := io.Copy(b, a)
		done <- err
	}()
	return <-done
}

func authorityTLV(name string) proxyproto.TLV {
	return proxyproto.TLV{
		Type:  proxyproto.PP2_TYPE_AUTHORITY,
		Value: []byte(name),
	}
}

func alpnTLV(proto string) proxyproto.TLV {
	return proxyproto.TLV{
		Type:  proxyproto.PP2_TYPE_ALPN,
		Value: []byte(proto),
	}
}

func sslTLV(state *tls.ConnectionState) (proxyproto.TLV, error) {
	pp2ssl := tlvparse.PP2SSL{
		Client: tlvparse.PP2_BITFIELD_CLIENT_SSL, // all of our connections are TLS
		Verify: 1,                                // we haven't checked the client cert
	}

	var version string
	switch state.Version {
	case tls.VersionTLS10:
		version = "TLSv1.0"
	case tls.VersionTLS11:
		version = "TLSv1.1"
	case tls.VersionTLS12:
		version = "TLSv1.2"
	case tls.VersionTLS13:
		version = "TLSv1.3"
	}
	if version != "" {
		versionTLV := proxyproto.TLV{
			Type:  proxyproto.PP2_SUBTYPE_SSL_VERSION,
			Value: []byte(version),
		}
		pp2ssl.TLV = append(pp2ssl.TLV, versionTLV)
	}

	// TODO: add PP2_SUBTYPE_SSL_CIPHER, PP2_SUBTYPE_SSL_SIG_ALG, PP2_SUBTYPE_SSL_KEY_ALG
	// TODO: check client-provided cert, if any

	return pp2ssl.Marshal()
}