package dns

import (
	"fmt"
	"net"
	"strings"
	"sync"
	"time"

	"github.com/azoic/wormhole-client/pkg/logger"
)

// DNSProxy DNS代理服务器
type DNSProxy struct {
	upstreamDNS string
	localPort   int
	server      *net.UDPConn
	cache       *dnsCache
	running     bool
	mutex       sync.RWMutex
}

// dnsCache DNS缓存
type dnsCache struct {
	entries map[string]*cacheEntry
	mutex   sync.RWMutex
}

type cacheEntry struct {
	response []byte
	expiry   time.Time
}

// NewDNSProxy 创建DNS代理
func NewDNSProxy(upstreamDNS string, localPort int) *DNSProxy {
	return &DNSProxy{
		upstreamDNS: upstreamDNS,
		localPort:   localPort,
		cache: &dnsCache{
			entries: make(map[string]*cacheEntry),
		},
	}
}

// Start 启动DNS代理服务器
func (d *DNSProxy) Start() error {
	d.mutex.Lock()
	defer d.mutex.Unlock()

	if d.running {
		return fmt.Errorf("DNS proxy is already running")
	}

	addr, err := net.ResolveUDPAddr("udp", fmt.Sprintf(":%d", d.localPort))
	if err != nil {
		return fmt.Errorf("failed to resolve UDP address: %v", err)
	}

	d.server, err = net.ListenUDP("udp", addr)
	if err != nil {
		return fmt.Errorf("failed to start DNS proxy server: %v", err)
	}

	d.running = true
	logger.Info("DNS proxy started on port %d", d.localPort)

	// 启动清理缓存的goroutine
	go d.cleanupCache()

	// 处理DNS请求
	go d.handleRequests()

	return nil
}

// Stop 停止DNS代理服务器
func (d *DNSProxy) Stop() error {
	d.mutex.Lock()
	defer d.mutex.Unlock()

	if !d.running {
		return nil
	}

	d.running = false
	if d.server != nil {
		d.server.Close()
	}

	logger.Info("DNS proxy stopped")
	return nil
}

// handleRequests 处理DNS请求
func (d *DNSProxy) handleRequests() {
	buffer := make([]byte, 512) // DNS消息最大512字节（UDP）

	for d.isRunning() {
		n, clientAddr, err := d.server.ReadFromUDP(buffer)
		if err != nil {
			if d.isRunning() {
				logger.Error("Failed to read DNS request: %v", err)
			}
			continue
		}

		go d.processRequest(buffer[:n], clientAddr)
	}
}

// processRequest 处理单个DNS请求
func (d *DNSProxy) processRequest(request []byte, clientAddr *net.UDPAddr) {
	// 简单的DNS请求解析
	domain := d.extractDomain(request)
	logger.Debug("DNS request for domain: %s", domain)

	// 检查缓存
	if cachedResponse := d.getFromCache(domain); cachedResponse != nil {
		logger.Debug("Serving %s from cache", domain)
		d.server.WriteToUDP(cachedResponse, clientAddr)
		return
	}

	// 转发到上游DNS服务器
	response, err := d.forwardToUpstream(request)
	if err != nil {
		logger.Error("Failed to forward DNS request: %v", err)
		return
	}

	// 缓存响应
	d.addToCache(domain, response)

	// 返回响应给客户端
	d.server.WriteToUDP(response, clientAddr)
}

// extractDomain 从DNS请求中提取域名（简化实现）
func (d *DNSProxy) extractDomain(request []byte) string {
	if len(request) < 12 {
		return ""
	}

	// 跳过DNS头部（12字节）
	offset := 12
	var domain strings.Builder

	for offset < len(request) {
		length := int(request[offset])
		if length == 0 {
			break
		}

		offset++
		if offset+length > len(request) {
			break
		}

		if domain.Len() > 0 {
			domain.WriteByte('.')
		}
		domain.Write(request[offset : offset+length])
		offset += length
	}

	return domain.String()
}

// forwardToUpstream 转发DNS请求到上游服务器
func (d *DNSProxy) forwardToUpstream(request []byte) ([]byte, error) {
	conn, err := net.Dial("udp", d.upstreamDNS)
	if err != nil {
		return nil, fmt.Errorf("failed to connect to upstream DNS: %v", err)
	}
	defer conn.Close()

	// 设置超时
	conn.SetDeadline(time.Now().Add(5 * time.Second))

	// 发送请求
	if _, err := conn.Write(request); err != nil {
		return nil, fmt.Errorf("failed to send DNS request: %v", err)
	}

	// 读取响应
	response := make([]byte, 512)
	n, err := conn.Read(response)
	if err != nil {
		return nil, fmt.Errorf("failed to read DNS response: %v", err)
	}

	return response[:n], nil
}

// getFromCache 从缓存获取DNS响应
func (d *DNSProxy) getFromCache(domain string) []byte {
	d.cache.mutex.RLock()
	defer d.cache.mutex.RUnlock()

	entry, exists := d.cache.entries[domain]
	if !exists || time.Now().After(entry.expiry) {
		return nil
	}

	return entry.response
}

// addToCache 添加DNS响应到缓存
func (d *DNSProxy) addToCache(domain string, response []byte) {
	d.cache.mutex.Lock()
	defer d.cache.mutex.Unlock()

	// 设置缓存过期时间（5分钟）
	expiry := time.Now().Add(5 * time.Minute)

	d.cache.entries[domain] = &cacheEntry{
		response: make([]byte, len(response)),
		expiry:   expiry,
	}
	copy(d.cache.entries[domain].response, response)
}

// cleanupCache 清理过期的缓存条目
func (d *DNSProxy) cleanupCache() {
	ticker := time.NewTicker(1 * time.Minute)
	defer ticker.Stop()

	for {
		select {
		case <-ticker.C:
			if !d.isRunning() {
				return
			}

			d.cache.mutex.Lock()
			now := time.Now()
			for domain, entry := range d.cache.entries {
				if now.After(entry.expiry) {
					delete(d.cache.entries, domain)
				}
			}
			d.cache.mutex.Unlock()
		}
	}
}

// isRunning 检查DNS代理是否在运行
func (d *DNSProxy) isRunning() bool {
	d.mutex.RLock()
	defer d.mutex.RUnlock()
	return d.running
}