package dns

import (
	"bytes"
	"context"
	"encoding/binary"
	"net/url"
	"sync"
	"sync/atomic"
	"time"

	"github.com/xtls/xray-core/common"
	"github.com/xtls/xray-core/common/buf"
	"github.com/xtls/xray-core/common/log"
	"github.com/xtls/xray-core/common/net"
	"github.com/xtls/xray-core/common/net/cnc"
	"github.com/xtls/xray-core/common/protocol/dns"
	"github.com/xtls/xray-core/common/session"
	"github.com/xtls/xray-core/common/signal/pubsub"
	"github.com/xtls/xray-core/common/task"
	dns_feature "github.com/xtls/xray-core/features/dns"
	"github.com/xtls/xray-core/features/routing"
	"github.com/xtls/xray-core/transport/internet"
	"golang.org/x/net/dns/dnsmessage"
)

// TCPNameServer implemented DNS over TCP (RFC7766).
type TCPNameServer struct {
	sync.RWMutex
	name          string
	destination   *net.Destination
	ips           map[string]*record
	pub           *pubsub.Service
	cleanup       *task.Periodic
	reqID         uint32
	dial          func(context.Context) (net.Conn, error)
	queryStrategy QueryStrategy
}

// NewTCPNameServer creates DNS over TCP server object for remote resolving.
func NewTCPNameServer(
	url *url.URL,
	dispatcher routing.Dispatcher,
	queryStrategy QueryStrategy,
) (*TCPNameServer, error) {
	s, err := baseTCPNameServer(url, "TCP", queryStrategy)
	if err != nil {
		return nil, err
	}

	s.dial = func(ctx context.Context) (net.Conn, error) {
		link, err := dispatcher.Dispatch(toDnsContext(ctx, s.destination.String()), *s.destination)
		if err != nil {
			return nil, err
		}

		return cnc.NewConnection(
			cnc.ConnectionInputMulti(link.Writer),
			cnc.ConnectionOutputMulti(link.Reader),
		), nil
	}

	return s, nil
}

// NewTCPLocalNameServer creates DNS over TCP client object for local resolving
func NewTCPLocalNameServer(url *url.URL, queryStrategy QueryStrategy) (*TCPNameServer, error) {
	s, err := baseTCPNameServer(url, "TCPL", queryStrategy)
	if err != nil {
		return nil, err
	}

	s.dial = func(ctx context.Context) (net.Conn, error) {
		return internet.DialSystem(ctx, *s.destination, nil)
	}

	return s, nil
}

func baseTCPNameServer(url *url.URL, prefix string, queryStrategy QueryStrategy) (*TCPNameServer, error) {
	port := net.Port(53)
	if url.Port() != "" {
		var err error
		if port, err = net.PortFromString(url.Port()); err != nil {
			return nil, err
		}
	}
	dest := net.TCPDestination(net.ParseAddress(url.Hostname()), port)

	s := &TCPNameServer{
		destination:   &dest,
		ips:           make(map[string]*record),
		pub:           pubsub.NewService(),
		name:          prefix + "//" + dest.NetAddr(),
		queryStrategy: queryStrategy,
	}
	s.cleanup = &task.Periodic{
		Interval: time.Minute,
		Execute:  s.Cleanup,
	}

	return s, nil
}

// Name implements Server.
func (s *TCPNameServer) Name() string {
	return s.name
}

// Cleanup clears expired items from cache
func (s *TCPNameServer) Cleanup() error {
	now := time.Now()
	s.Lock()
	defer s.Unlock()

	if len(s.ips) == 0 {
		return newError("nothing to do. stopping...")
	}

	for domain, record := range s.ips {
		if record.A != nil && record.A.Expire.Before(now) {
			record.A = nil
		}
		if record.AAAA != nil && record.AAAA.Expire.Before(now) {
			record.AAAA = nil
		}

		if record.A == nil && record.AAAA == nil {
			newError(s.name, " cleanup ", domain).AtDebug().WriteToLog()
			delete(s.ips, domain)
		} else {
			s.ips[domain] = record
		}
	}

	if len(s.ips) == 0 {
		s.ips = make(map[string]*record)
	}

	return nil
}

func (s *TCPNameServer) updateIP(req *dnsRequest, ipRec *IPRecord) {
	elapsed := time.Since(req.start)

	s.Lock()
	rec, found := s.ips[req.domain]
	if !found {
		rec = &record{}
	}
	updated := false

	switch req.reqType {
	case dnsmessage.TypeA:
		if isNewer(rec.A, ipRec) {
			rec.A = ipRec
			updated = true
		}
	case dnsmessage.TypeAAAA:
		addr := make([]net.Address, 0)
		for _, ip := range ipRec.IP {
			if len(ip.IP()) == net.IPv6len {
				addr = append(addr, ip)
			}
		}
		ipRec.IP = addr
		if isNewer(rec.AAAA, ipRec) {
			rec.AAAA = ipRec
			updated = true
		}
	}
	newError(s.name, " got answer: ", req.domain, " ", req.reqType, " -> ", ipRec.IP, " ", elapsed).AtInfo().WriteToLog()

	if updated {
		s.ips[req.domain] = rec
	}
	switch req.reqType {
	case dnsmessage.TypeA:
		s.pub.Publish(req.domain+"4", nil)
	case dnsmessage.TypeAAAA:
		s.pub.Publish(req.domain+"6", nil)
	}
	s.Unlock()
	common.Must(s.cleanup.Start())
}

func (s *TCPNameServer) newReqID() uint16 {
	return uint16(atomic.AddUint32(&s.reqID, 1))
}

func (s *TCPNameServer) sendQuery(ctx context.Context, domain string, clientIP net.IP, option dns_feature.IPOption) {
	newError(s.name, " querying DNS for: ", domain).AtDebug().WriteToLog(session.ExportIDToError(ctx))

	reqs := buildReqMsgs(domain, option, s.newReqID, genEDNS0Options(clientIP))

	var deadline time.Time
	if d, ok := ctx.Deadline(); ok {
		deadline = d
	} else {
		deadline = time.Now().Add(time.Second * 5)
	}

	for _, req := range reqs {
		go func(r *dnsRequest) {
			dnsCtx := ctx

			if inbound := session.InboundFromContext(ctx); inbound != nil {
				dnsCtx = session.ContextWithInbound(dnsCtx, inbound)
			}

			dnsCtx = session.ContextWithContent(dnsCtx, &session.Content{
				Protocol:       "dns",
				SkipDNSResolve: true,
			})

			var cancel context.CancelFunc
			dnsCtx, cancel = context.WithDeadline(dnsCtx, deadline)
			defer cancel()

			b, err := dns.PackMessage(r.msg)
			if err != nil {
				newError("failed to pack dns query").Base(err).AtError().WriteToLog()
				return
			}

			conn, err := s.dial(dnsCtx)
			if err != nil {
				newError("failed to dial namesever").Base(err).AtError().WriteToLog()
				return
			}
			defer conn.Close()
			dnsReqBuf := buf.New()
			binary.Write(dnsReqBuf, binary.BigEndian, uint16(b.Len()))
			dnsReqBuf.Write(b.Bytes())
			b.Release()

			_, err = conn.Write(dnsReqBuf.Bytes())
			if err != nil {
				newError("failed to send query").Base(err).AtError().WriteToLog()
				return
			}
			dnsReqBuf.Release()

			respBuf := buf.New()
			defer respBuf.Release()
			n, err := respBuf.ReadFullFrom(conn, 2)
			if err != nil && n == 0 {
				newError("failed to read response length").Base(err).AtError().WriteToLog()
				return
			}
			var length int16
			err = binary.Read(bytes.NewReader(respBuf.Bytes()), binary.BigEndian, &length)
			if err != nil {
				newError("failed to parse response length").Base(err).AtError().WriteToLog()
				return
			}
			respBuf.Clear()
			n, err = respBuf.ReadFullFrom(conn, int32(length))
			if err != nil && n == 0 {
				newError("failed to read response length").Base(err).AtError().WriteToLog()
				return
			}

			rec, err := parseResponse(respBuf.Bytes())
			if err != nil {
				newError("failed to parse DNS over TCP response").Base(err).AtError().WriteToLog()
				return
			}

			s.updateIP(r, rec)
		}(req)
	}
}

func (s *TCPNameServer) findIPsForDomain(domain string, option dns_feature.IPOption) ([]net.IP, error) {
	s.RLock()
	record, found := s.ips[domain]
	s.RUnlock()

	if !found {
		return nil, errRecordNotFound
	}

	var err4 error
	var err6 error
	var ips []net.Address
	var ip6 []net.Address

	if option.IPv4Enable {
		ips, err4 = record.A.getIPs()
	}

	if option.IPv6Enable {
		ip6, err6 = record.AAAA.getIPs()
		ips = append(ips, ip6...)
	}

	if len(ips) > 0 {
		return toNetIP(ips)
	}

	if err4 != nil {
		return nil, err4
	}

	if err6 != nil {
		return nil, err6
	}

	return nil, dns_feature.ErrEmptyResponse
}

// QueryIP implements Server.
func (s *TCPNameServer) QueryIP(ctx context.Context, domain string, clientIP net.IP, option dns_feature.IPOption, disableCache bool) ([]net.IP, error) {
	fqdn := Fqdn(domain)
	option = ResolveIpOptionOverride(s.queryStrategy, option)
	if !option.IPv4Enable && !option.IPv6Enable {
		return nil, dns_feature.ErrEmptyResponse
	}

	if disableCache {
		newError("DNS cache is disabled. Querying IP for ", domain, " at ", s.name).AtDebug().WriteToLog()
	} else {
		ips, err := s.findIPsForDomain(fqdn, option)
		if err != errRecordNotFound {
			newError(s.name, " cache HIT ", domain, " -> ", ips).Base(err).AtDebug().WriteToLog()
			log.Record(&log.DNSLog{Server: s.name, Domain: domain, Result: ips, Status: log.DNSCacheHit, Elapsed: 0, Error: err})
			return ips, err
		}
	}

	// ipv4 and ipv6 belong to different subscription groups
	var sub4, sub6 *pubsub.Subscriber
	if option.IPv4Enable {
		sub4 = s.pub.Subscribe(fqdn + "4")
		defer sub4.Close()
	}
	if option.IPv6Enable {
		sub6 = s.pub.Subscribe(fqdn + "6")
		defer sub6.Close()
	}
	done := make(chan interface{})
	go func() {
		if sub4 != nil {
			select {
			case <-sub4.Wait():
			case <-ctx.Done():
			}
		}
		if sub6 != nil {
			select {
			case <-sub6.Wait():
			case <-ctx.Done():
			}
		}
		close(done)
	}()
	s.sendQuery(ctx, fqdn, clientIP, option)
	start := time.Now()

	for {
		ips, err := s.findIPsForDomain(fqdn, option)
		if err != errRecordNotFound {
			log.Record(&log.DNSLog{Server: s.name, Domain: domain, Result: ips, Status: log.DNSQueried, Elapsed: time.Since(start), Error: err})
			return ips, err
		}

		select {
		case <-ctx.Done():
			return nil, ctx.Err()
		case <-done:
		}
	}
}