package main

import (
	"bufio"
	"fmt"
	"net"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/miekg/dns"
	"errors"
	"crypto/tls"
)

type ResolvError struct {
	qname, net  string
	nameservers []string
}

func (e ResolvError) Error() string {
	errmsg := fmt.Sprintf("%s resolv failed on %s (%s)", e.qname, strings.Join(e.nameservers, "; "), e.net)
	return errmsg
}

type RResp struct {
	msg        *dns.Msg
	nameserver string
	rtt        time.Duration
}

type Resolver struct {
	servers       []string
	domain_server *suffixTreeNode
	config        *ResolvSettings

	tcpClient   *dns.Client
	udpClient   *dns.Client
	httpsClient *dns.Client
}

func NewResolver(c ResolvSettings) *Resolver {
	r := &Resolver{
		servers:       []string{},
		domain_server: newSuffixTreeRoot(),
		config:        &c,
	}

	if serverList := os.Getenv("SERVER_LIST_FILE"); serverList != "" {
		r.ReadServerListFile(serverList)
	} else if len(c.ServerListFile) > 0 {
		r.ReadServerListFile(c.ServerListFile)
	}

	if len(c.ResolvFile) > 0 {
		clientConfig, err := dns.ClientConfigFromFile(c.ResolvFile)

		if err != nil {
			logger.Error(":%s is not a valid resolv.conf file\n", c.ResolvFile)
			logger.Error("%s", err)
			panic(err)
		}

		for _, server := range clientConfig.Servers {
			r.servers = append(r.servers, net.JoinHostPort(server, clientConfig.Port))
		}
	}

	if len(c.DOHServer) > 0 {
		r.servers = append([]string{c.DOHServer}, r.servers...)
	}

	timeout := r.Timeout()

	r.udpClient = &dns.Client{
		Net:          "udp",
		ReadTimeout:  timeout,
		WriteTimeout: timeout,
	}

	r.tcpClient = &dns.Client{
		Net:          "tcp",
		ReadTimeout:  timeout,
		WriteTimeout: timeout,
	}

	r.httpsClient = &dns.Client{
		Net:          "https",
		ReadTimeout:  timeout,
		WriteTimeout: timeout,
	}

	return r
}

func (r *Resolver) parseServerListFile(buf *os.File) {
	scanner := bufio.NewScanner(buf)

	var line string
	var idx int

	for scanner.Scan() {
		line = strings.TrimSpace(scanner.Text())

		if !strings.HasPrefix(line, "server") {
			continue
		}

		idx = strings.Index(line, "=")

		if idx == -1 {
			continue
		}

		line = strings.TrimSpace(line[idx:])

		if strings.HasPrefix(line, "https://") {
			r.servers = append(r.servers, line)
			continue
		}

		tokens := strings.Split(line, "/")
		switch len(tokens) {
		case 3:
			domain := tokens[1]
			ip := tokens[2]

			if !isDomain(domain) || !isIP(ip) {
				continue
			}

			r.domain_server.sinsert(strings.Split(domain, "."), ip)
		case 1:
			srv_port := strings.Split(line, "#")

			if len(srv_port) > 2 {
				continue
			}

			ip := ""

			if ip = srv_port[0]; !isIP(ip) {
				continue
			}

			port := "53"

			if len(srv_port) == 2 {
				if _, err := strconv.Atoi(srv_port[1]); err != nil {
					continue
				}

				port = srv_port[1]
			}

			r.servers = append(r.servers, net.JoinHostPort(ip, port))
		}
	}

}

func (r *Resolver) ReadServerListFile(path string) {
	files := strings.Split(path, ";")
	for _, file := range files {
		buf, err := os.Open(file)
		if err != nil {
			panic("Can't open " + file)
		}
		defer buf.Close()
		r.parseServerListFile(buf)
	}
}

// Lookup will ask each nameserver in top-to-bottom fashion, starting a new request
// in every second, and return as early as possbile (have an answer).
// It returns an error if no request has succeeded.
func (r *Resolver) Lookup(net string, req *dns.Msg) (message *dns.Msg, err error) {
	if net == "udp" && settings.ResolvConfig.SetEDNS0 {
		req = req.SetEdns0(65535, true)
	}

	qname := req.Question[0].Name

	res := make(chan *RResp, 1)
	var wg sync.WaitGroup
	L := func(resolver *Resolver, nameserver string) {
		defer wg.Done()

		c, err := resolver.resolverFor(net, nameserver)

		if err != nil {
			logger.Warn("error:%s", err.Error())
			return
		}

		r, rtt, err := c.Exchange(req, nameserver)

		if err != nil {
			logger.Warn("%s socket error on %s", qname, nameserver)
			logger.Warn("error:%s", err.Error())
			return
		}
		// If SERVFAIL happen, should return immediately and try another upstream resolver.
		// However, other Error code like NXDOMAIN is an clear response stating
		// that it has been verified no such domain existas and ask other resolvers
		// would make no sense. See more about #20
		if r != nil && r.Rcode != dns.RcodeSuccess {
			logger.Warn("%s failed to get an valid answer on %s", qname, nameserver)
			if r.Rcode == dns.RcodeServerFailure {
				return
			}
		}
		re := &RResp{r, nameserver, rtt}
		select {
		case res <- re:
		default:
		}
	}

	ticker := time.NewTicker(time.Duration(settings.ResolvConfig.Interval) * time.Millisecond)
	defer ticker.Stop()
	// Start lookup on each nameserver top-down, in every second
	nameservers := r.Nameservers(qname)
	for _, nameserver := range nameservers {
		wg.Add(1)
		go L(r, nameserver)
		// but exit early, if we have an answer
		select {
		case re := <-res:
			logger.Debug("%s resolv on %s rtt: %v", UnFqdn(qname), re.nameserver, re.rtt)
			return re.msg, nil
		case <-ticker.C:
			continue
		}
	}
	// wait for all the namservers to finish
	wg.Wait()
	select {
	case re := <-res:
		logger.Debug("%s resolv on %s rtt: %v", UnFqdn(qname), re.nameserver, re.rtt)
		return re.msg, nil
	default:
		return nil, ResolvError{qname, net, nameservers}
	}
}

func (r *Resolver) resolverFor(net, nameserver string) (*dns.Client, error) {
	if strings.HasPrefix(nameserver, "https") {
		return r.httpsClient, nil
	} else if strings.HasSuffix(nameserver, ":853") {
		// TODO We need to set the server name so we can confirm the TLS connection. This may require a rewrite of storing nameservers.
		return &dns.Client{
			Net:          "tcp-tls",
			ReadTimeout:  r.Timeout(),
			WriteTimeout: r.Timeout(),
			TLSConfig: &tls.Config{
				ServerName: "",
			},
		}, nil
	} else if net == "udp" {
		return r.udpClient, nil
	} else if net == "tcp" {
		return r.tcpClient, nil
	}

	return nil, errors.New("no client for nameserver")
}

// Namservers return the array of nameservers, with port number appended.
// '#' in the name is treated as port separator, as with dnsmasq.

func (r *Resolver) Nameservers(qname string) []string {
	queryKeys := strings.Split(qname, ".")
	queryKeys = queryKeys[:len(queryKeys)-1] // ignore last '.'

	ns := []string{}
	if v, found := r.domain_server.search(queryKeys); found {
		logger.Debug("%s be found in domain server list, upstream: %v", qname, v)

		ns = append(ns, net.JoinHostPort(v, "53"))
		//Ensure query the specific upstream nameserver in async Lookup() function.
		return ns
	}

	for _, nameserver := range r.servers {
		ns = append(ns, nameserver)
	}

	return ns
}

func (r *Resolver) Timeout() time.Duration {
	return time.Duration(r.config.Timeout) * time.Second
}