package NTP

import (
	"encoding/binary"
	"fmt"
	"log"
	"net"
	"sync"
	"time"
)

const (
	STANDARD_PACKET_SIZE = 48 // 标准NTP的报文大小
)

type NTPServer struct {
	srvAddress string

	conn *net.UDPConn
	wait sync.WaitGroup

	ntpPack      NtpPacket // NTP协议报文
	requestCount uint64    // 请求计数
}

type NtpPacket struct {
	/*
		LI: 2bit      00   Leap Indicator(0)
		VN: 3bit      100  NTP Version(4)
		Mode: 3bit    100  Mode: server(4), client(3)
	*/
	Header    uint8 // 报文头: 包含LI、VN、Mode
	Stratum   uint8 // Peer Clock Stratum: primary reference (1)
	Poll      uint8 // Peer Polling Interval: invalid (0)
	Precision uint8 // Peer Clock Precision: 0.000000 seconds

	RootDelay uint32 // Root Delay
	RootDisp  uint32 // Root Dispersion
	RefID     uint32 // Reference Identifier

	RefTS   uint64 // Reference Timestamp 参考时间戳
	OrigTS  uint64 // Originate Timestamp 起始时间戳
	RecvTS  uint64 // Receive Timestamp   接收时间戳
	TransTS uint64 // Transmit Timestamp  传输时间戳
}

func (srv *NTPServer) NewNtpPacket() *NtpPacket {
	// 初始化Header字段
	header := uint8(0)
	header |= (0 << 6) // LI: 2bit 00
	header |= (4 << 3) // VN: 3bit 100
	header |= (4 << 0) // Mode: 3bit 100

	// 创建新的NtpPacket实例
	packet := &NtpPacket{
		Header:    header,
		Stratum:   0x01,
		Poll:      0x00,
		Precision: 0x00,
		RootDelay: 0,
		RootDisp:  0,
		RefID:     0,
		RefTS:     0,
		OrigTS:    0,
		RecvTS:    0,
		TransTS:   0,
	}

	return packet
}

func (pack *NtpPacket) SetTimestamp(timestamp time.Time, field string) {
	ntpTime := ToNTPTime(timestamp)
	switch field {
	case "RefTS":
		pack.RefTS = ntpTime
	case "OrigTS":
		pack.OrigTS = ntpTime
	case "RecvTS":
		pack.RecvTS = ntpTime
	case "TransTS":
		pack.TransTS = ntpTime
	}
}

// toNTPTime 将Unix时间转换为NTP时间
func ToNTPTime(t time.Time) uint64 {
	seconds := uint32(t.Unix()) + 2208988800 // NTP时间从1900年开始计算
	fraction := uint32(float64(t.Nanosecond()) * (1 << 32) / 1e9)
	return uint64(seconds)<<32 | uint64(fraction)
}

func NewNTPServer(srvAddr string) *NTPServer {
	return &NTPServer{srvAddress: srvAddr}
}

// 启动NTP服务器
func (srv *NTPServer) Start() error {
	addr, err := net.ResolveUDPAddr("udp", srv.srvAddress)
	if err != nil {
		return err
	}
	log.Println(fmt.Sprintf("<%s:%d>", addr.IP.String(), addr.Port))

	conn, err := net.ListenUDP("udp", addr)
	if err != nil {
		return err
	}

	srv.wait.Add(1)
	srv.conn = conn

	go RecvMsg(srv)

	return nil
}

// 关闭NTP服务器
func (srv *NTPServer) Stop() {
	srv.conn.Close()
	srv.wait.Wait()
}

// 接收数据
func RecvMsg(srv *NTPServer) {
	defer srv.wait.Done()
	buffer := make([]byte, 2*1024)

	for {
		n, remoteAddr, err := srv.conn.ReadFromUDP(buffer[0:])
		if err != nil {
			fmt.Println("ReadFromUDP error:", err)
			return
		}
		log.Println(fmt.Sprintf("[Recv] %d bytes from <%s>", n, remoteAddr.String()))
		if n != STANDARD_PACKET_SIZE {
			continue
		}

		// 接收到NTP客户端消息的时间
		recvMsgTime := time.Now().UTC()

		recvHexString := BytesToHex(buffer[:n])
		log.Println(fmt.Sprintf("[Recv] %s", recvHexString))

		udpPacket, err := ParseUDPPacket(buffer[:n])
		if err != nil {
			log.Printf("Error parsing UDP packet: %v", err)
			continue
		}

		ntpPack := srv.NewNtpPacket()
		ntpPack.SetTimestamp(time.Now().UTC(), "RefTS")
		ntpPack.OrigTS = udpPacket.TransTS
		ntpPack.SetTimestamp(recvMsgTime, "RecvTS")
		ntpPack.SetTimestamp(time.Now().UTC(), "TransTS")

		sendPacket := ntpPack.Serialize()

		sendLen, err := srv.conn.WriteToUDP(sendPacket, remoteAddr)
		if err != nil {
			log.Println(err.Error())
			continue
		}

		if sendLen > 0 {
			log.Println(fmt.Sprintf("[Send] %s", BytesToHex(sendPacket)))
		}

		srv.requestCount++
	}
}

func (pack *NtpPacket) Serialize() []byte {
	packet := make([]byte, 48)

	// binary.BigEndian.PutUint32(packet[0:4], pack.Header)
	packet[0] = pack.Header
	packet[1] = pack.Stratum
	packet[2] = pack.Poll
	packet[3] = pack.Precision
	binary.BigEndian.PutUint32(packet[4:8], pack.RootDelay)
	binary.BigEndian.PutUint32(packet[8:12], pack.RootDisp)
	binary.BigEndian.PutUint32(packet[12:16], pack.RefID)
	binary.BigEndian.PutUint64(packet[16:24], pack.RefTS)
	binary.BigEndian.PutUint64(packet[24:32], pack.OrigTS)
	binary.BigEndian.PutUint64(packet[32:40], pack.RecvTS)
	binary.BigEndian.PutUint64(packet[40:48], pack.TransTS)

	return packet
}

// BytesToHex 将字节数组转换为16进制字符串
func BytesToHex(data []byte) string {
	hexString := make([]byte, 3*len(data)-1)
	for i, b := range data {
		high := "0123456789ABCDEF"[(b >> 4)]
		low := "0123456789ABCDEF"[(b & 0x0F)]
		hexString[i*3] = high
		hexString[i*3+1] = low
		if i < len(data)-1 {
			hexString[i*3+2] = ' ' // 每个16进制数据之间加空格
		}
	}
	return string(hexString)
}

func ParseUDPPacket(buf []byte) (*NtpPacket, error) {
	if len(buf) < STANDARD_PACKET_SIZE { // 最小有效长度为48字节
		return nil, fmt.Errorf("Invalid UDP packet length: %d", len(buf))
	}

	packet := &NtpPacket{
		// Header:    binary.BigEndian.Uint32(buf[0:4]),
		Header:    buf[0],
		Stratum:   buf[1],
		Poll:      buf[2],
		Precision: buf[3],
		RootDelay: binary.BigEndian.Uint32(buf[4:8]),
		RootDisp:  binary.BigEndian.Uint32(buf[8:12]),
		RefID:     binary.BigEndian.Uint32(buf[12:16]),
		RefTS:     binary.BigEndian.Uint64(buf[16:24]),
		OrigTS:    binary.BigEndian.Uint64(buf[24:32]),
		RecvTS:    binary.BigEndian.Uint64(buf[32:40]),
		TransTS:   binary.BigEndian.Uint64(buf[40:48]),
	}

	return packet, nil
}