package dns

import (
	"bytes"
	"context"
	"crypto/tls"
	"fmt"
	"io"
	"net/http"
	"sync"
	"time"

	"github.com/miekg/dns"
)

// DNSQuery DNS查询记录
type DNSQuery struct {
	Domain    string
	QueryType string
	Timestamp time.Time
	Count     int
}

type DNSServer struct {
	Port         int
	UpstreamDNS  string
	UseDoH       bool
	DoHURL       string
	queries      map[string]*DNSQuery
	queriesMutex sync.RWMutex
	server       *dns.Server
	dnsClient    *dns.Client
	httpClient   *http.Client
}

// NewDNSServer 创建DNS服务器
func NewDNSServer(port int, upstreamDNS string) *DNSServer {
	return &DNSServer{
		Port:        port,
		UpstreamDNS: upstreamDNS,
		UseDoH:      false,
		queries:     make(map[string]*DNSQuery),
		dnsClient: &dns.Client{
			Net:          "udp",
			Timeout:      3 * time.Second,
			DialTimeout:  2 * time.Second,
			ReadTimeout:  3 * time.Second,
			WriteTimeout: 2 * time.Second,
		},
	}
}

// NewDNSServerWithDoH 创建使用DoH的DNS服务器
func NewDNSServerWithDoH(port int, dohURL string) *DNSServer {
	return &DNSServer{
		Port:    port,
		UseDoH:  true,
		DoHURL:  dohURL,
		queries: make(map[string]*DNSQuery),
		httpClient: &http.Client{
			Timeout: 3 * time.Second,
			Transport: &http.Transport{
				ForceAttemptHTTP2:   true,
				MaxIdleConns:        100,
				MaxIdleConnsPerHost: 100,
				IdleConnTimeout:     90 * time.Second,
				TLSHandshakeTimeout: 2 * time.Second,
				DisableCompression:  false,
				TLSClientConfig: &tls.Config{
					InsecureSkipVerify: false,
				},
			},
		},
	}
}

// Start 启动DNS服务器
func (s *DNSServer) Start() error {
	s.server = &dns.Server{
		Addr:    fmt.Sprintf("0.0.0.0:%d", s.Port),
		Net:     "udp",
		Handler: dns.HandlerFunc(s.handleDNSRequest),
		UDPSize: 65535,
	}

	fmt.Printf("DNS Server started on 0.0.0.0:%d\n", s.Port)
	if s.UseDoH {
		fmt.Printf("Using DoH: %s\n", s.DoHURL)
	} else {
		fmt.Printf("Upstream DNS: %s\n", s.UpstreamDNS)
	}

	go func() {
		if err := s.server.ListenAndServe(); err != nil {
			fmt.Printf("DNS server error: %v\n", err)
		}
	}()

	return nil
}

// Stop 停止DNS服务器
func (s *DNSServer) Stop() {
	if s.server != nil {
		s.server.Shutdown()
	}
}

// handleDNSRequest 处理DNS请求
func (s *DNSServer) handleDNSRequest(w dns.ResponseWriter, req *dns.Msg) {
	// 记录查询
	if len(req.Question) > 0 {
		q := req.Question[0]
		s.recordQuery(q.Name, dns.TypeToString[q.Qtype])
	}

	// 转发请求
	var resp *dns.Msg
	var err error

	if s.UseDoH {
		resp, err = s.forwardDoH(req)
	} else {
		resp, err = s.forwardUDP(req)
	}

	if err != nil {
		// 返回 SERVFAIL
		resp = new(dns.Msg)
		resp.SetRcode(req, dns.RcodeServerFailure)
	}

	w.WriteMsg(resp)
}

// forwardUDP 通过UDP转发
func (s *DNSServer) forwardUDP(req *dns.Msg) (*dns.Msg, error) {
	resp, _, err := s.dnsClient.Exchange(req, s.UpstreamDNS)
	return resp, err
}

// forwardDoH 通过DoH转发
func (s *DNSServer) forwardDoH(req *dns.Msg) (*dns.Msg, error) {
	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
	defer cancel()

	// 将 DNS 消息打包
	packed, err := req.Pack()
	if err != nil {
		return nil, err
	}

	// 创建 HTTP POST 请求
	httpReq, err := http.NewRequestWithContext(ctx, "POST", s.DoHURL, bytes.NewReader(packed))
	if err != nil {
		return nil, err
	}

	// 设置 DoH 请求头
	httpReq.Header.Set("Content-Type", "application/dns-message")
	httpReq.Header.Set("Accept", "application/dns-message")

	// 发送请求
	resp, err := s.httpClient.Do(httpReq)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("DoH server returned status %d", resp.StatusCode)
	}

	// 读取响应
	body, err := io.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}

	// 解析 DNS 响应
	dnsResp := new(dns.Msg)
	if err := dnsResp.Unpack(body); err != nil {
		return nil, err
	}

	return dnsResp, nil
}

// recordQuery 记录DNS查询
func (s *DNSServer) recordQuery(domain, queryType string) {
	s.queriesMutex.Lock()
	defer s.queriesMutex.Unlock()

	key := domain
	if q, exists := s.queries[key]; exists {
		q.Count++
		q.Timestamp = time.Now()
	} else {
		s.queries[key] = &DNSQuery{
			Domain:    domain,
			QueryType: queryType,
			Timestamp: time.Now(),
			Count:     1,
		}
	}

	// 限制缓存大小，防止内存溢出
	if len(s.queries) > 500 {
		// 删除最旧的条目
		oldestKey := ""
		oldestTime := time.Now()
		for key, q := range s.queries {
			if q.Timestamp.Before(oldestTime) {
				oldestTime = q.Timestamp
				oldestKey = key
			}
		}
		if oldestKey != "" {
			delete(s.queries, oldestKey)
		}
	}
}

// GetRecentQueries 获取最近的DNS查询
func (s *DNSServer) GetRecentQueries(limit int) []DNSQuery {
	s.queriesMutex.RLock()
	defer s.queriesMutex.RUnlock()

	queries := make([]DNSQuery, 0, len(s.queries))
	for _, q := range s.queries {
		queries = append(queries, *q)
	}

	// 按时间排序（最新的在前）
	for i := 0; i < len(queries)-1; i++ {
		for j := i + 1; j < len(queries); j++ {
			if queries[j].Timestamp.After(queries[i].Timestamp) {
				queries[i], queries[j] = queries[j], queries[i]
			}
		}
	}

	if len(queries) > limit {
		queries = queries[:limit]
	}

	return queries
}