using UnityEngine;
using System.Collections;
using System;
using System.Net;
using System.Net.Sockets;

public class NtpClient {
	
	public DateTime GetNetworkTimeUTC() {

		//default Windows time server
		const string ntpServer = "ntp.ntu.edu.tw";
//		const string ntpServer = "time.windows.com";

		// NTP message size - 16 bytes of the digest (RFC 2030)
		var ntpData = new byte[48];

		//Setting the Leap Indicator, Version Number and Mode values
		ntpData[0] = 0x1B; //LI = 0 (no warning), VN = 3 (IPv4 only), Mode = 3 (Client Mode)

		var addresses = Dns.GetHostEntry(ntpServer).AddressList;

		//The UDP port number assigned to NTP is 123
		var ipEndPoint = new IPEndPoint(addresses[0], 123);
		//NTP uses UDP
		var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);

		socket.Connect(ipEndPoint);

		//Stops code hang if NTP is blocked
		socket.ReceiveTimeout = 3000;     

		socket.Send(ntpData);
		socket.Receive(ntpData);
		socket.Close();

		//Offset to get to the "Transmit Timestamp" field (time at which the reply 
		//departed the server for the client, in 64-bit timestamp format."
		const byte serverReplyTime = 40;

		//Get the seconds part
		ulong intPart = BitConverter.ToUInt32(ntpData, serverReplyTime);

		//Get the seconds fraction
		ulong fractPart = BitConverter.ToUInt32(ntpData, serverReplyTime + 4);

		//Convert From big-endian to little-endian
		intPart = SwapEndianness(intPart);
		fractPart = SwapEndianness(fractPart);

		var milliseconds = (intPart * 1000) + ((fractPart * 1000) / 0x100000000L);

		MonoBehaviour.print ("Milliseconds: " + milliseconds);

		//**UTC** time
		var networkDateTime = (new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc)).AddMilliseconds((long)milliseconds);
		MonoBehaviour.print("NTP Time: " + networkDateTime.ToUniversalTime().ToString("yyyyMMdd-hh:mm:ss_fff"));

		return networkDateTime;

	}

	public double GetNetworkTimeOffset() {
		DateTime networkDateTimeUTC = GetNetworkTimeUTC ();

		MonoBehaviour.print ("Curr Time: " + DateTime.UtcNow.ToString ("yyyyMMdd-hh:mm:ss_fff"));
		var offset = DateTime.UtcNow - networkDateTimeUTC;

		MonoBehaviour.print ("offset: " + offset.TotalMilliseconds);
		return offset.TotalMilliseconds;
	}

	static uint SwapEndianness(ulong x)
	{
		return (uint)(((x & 0x000000ff) << 24) +
		((x & 0x0000ff00) << 8) +
		((x & 0x00ff0000) >> 8) +
		((x & 0xff000000) >> 24));
	}
}