using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Net;
using System.Net.Sockets;

namespace NtpClient2B
{
    class Program
    {
        static uint ParseBigEndianUint32(byte[] data, int offset)
        {
            uint value = 0;
            uint byte0 = data[offset + 3];
            uint byte1 = data[offset + 2];
            uint byte2 = data[offset + 1];
            uint byte3 = data[offset];
            value = (byte3 << 24) | (byte2 << 16) | (byte1 << 8) | byte0;
            return value;
        }

        static DateTime GetUTC(byte[] data, int offset)
        {
            DateTime baseUTC = new DateTime(1900, 1, 1);
            uint seconds = ParseBigEndianUint32(data, offset);
            ulong fraction = ParseBigEndianUint32(data, offset + 4);
            // convert to milliseconds
            fraction *= 1000;
            // get the true fraction
            fraction /= uint.MaxValue;
            return baseUTC.AddSeconds(seconds).AddMilliseconds(fraction);
        }

        static void Main(string[] args)
        {
            byte[] packet = new byte[48];
            const byte leap = 0;
            const byte version = 3;
            const byte mode = 3;
            packet[0] = (leap << 6) | (version << 3) | mode;

            Socket socket = new Socket(SocketType.Dgram, ProtocolType.Udp);
            IPEndPoint endPoint = new IPEndPoint(IPAddress.Parse("216.239.35.8"), 123);

            socket.SendTo(packet, endPoint);

            int rlen = socket.Receive(packet);
            Console.WriteLine(rlen);

            Console.WriteLine("leap: {0} version: {1} mode: {2}", packet[0] >> 6, (packet[0] >> 3) & 7, packet[0] & 7);
            Console.WriteLine("stratum: {0}", packet[1]);
            Console.WriteLine("refid: {0}", Encoding.ASCII.GetString(packet, 12, 4));

            DateTime reference = GetUTC(packet, 16);
            DateTime origin = GetUTC(packet, 24);
            DateTime receive = GetUTC(packet, 32);
            DateTime transmit = GetUTC(packet, 40);

            Console.WriteLine("reference TS: {0} {1}", reference, reference.Millisecond);
            Console.WriteLine("origin TS: {0} {1}", origin, origin.Millisecond);
            Console.WriteLine("receive TS: {0} {1}", receive, receive.Millisecond);
            Console.WriteLine("transmit TS: {0} {1}", transmit, transmit.Millisecond);

            Console.ReadLine();
        }
    }
}