#!/usr/bin/perl
use strict;
use warnings;
use IO::Socket::INET;
use Time::HiRes qw(gettimeofday);

use constant TRUE      => scalar 1;
use constant PORT      => scalar 12345;
use constant MAXBYTES  => scalar 1024;
use constant ARGUMENTS => scalar 0;

# flush after every initialization
$| = 1;

my ($server_socket,$received_data,$peer_address,$peer_port,$ntp_packet);

# we call IO::Socket::INET->new() to create the UDP Socket and bound

if (@ARGV > ARGUMENTS) {
    print "\nPlease no more than ".ARGUMENTS." arguments (ARGV[])!\n";
    print "\nCorrect Syntax: perl $0 \n\n";
    exit();
}
else {

    $server_socket = new IO::Socket::INET (
	LocalPort => 12345, # Default NTP port 123
	Type      => SOCK_DGRAM,
	Proto     => 'udp',
	) or die "ERROR in Socket Creation : $!\n";

    printf("\nServer is up, listens on port: ".PORT." and waiting for client...\n");

    while (TRUE) {
	
	# read operation on the socket
	$server_socket->recv($received_data,MAXBYTES) 
	    or die "Server received: $!\n";
	my ($epoc_rec_sec, $epoc_rec_nano_sec) = gettimeofday();

	$peer_address = $server_socket->peerhost();
	$peer_port = $server_socket->peerport();

        #send the data to the client at which the read/write operations done recently.
	if ($received_data) {

	    my $server_li_binary =  substr($received_data, 0, 2);
	    my $server_li = oct("0b".$server_li_binary);

	    my $server_vn_binary =  substr($received_data, 2, 3);
	    my $server_vn = oct("0b".$server_vn_binary);

	    my $server_mode_binary =  substr($received_data, 5, 3);
	    my $server_mode = oct("0b".$server_mode_binary);

	    my $server_stratum_binary =  substr($received_data, 8, 8);
	    my $server_stratum = oct("0b".$server_stratum_binary);

	    my $server_poll_interval_binary =  substr($received_data, 16, 8);
	    my $server_poll_interval = oct("0b".$server_poll_interval_binary);

	    my $server_precission_binary =  substr($received_data, 24, 8);
	    my $server_precission = oct("0b".$server_precission_binary);

	    my $server_root_delay_binary =  substr($received_data, 32, 32);
	    my $server_root_delay = oct("0b".$server_root_delay_binary);

	    my $server_root_dispersion_binary =  substr($received_data, 64, 32);
	    my $server_root_dispersion = oct("0b".$server_root_dispersion_binary);

	    my $server_root_identifier_binary =  substr($received_data, 96, 32);
	    my $server_root_identifier = oct("0b".$server_root_identifier_binary);

	    my $ref_client_epoch_binary =  substr($received_data, 128, 64);
	    my $ref_client_epoch = oct("0b".$ref_client_epoch_binary);

	    my $originate_client_epoch_binary =  substr($received_data, 192, 64);
	    my $originate_client_epoch = oct("0b".$originate_client_epoch_binary);

	    printf "\nNumber of seconds since Jan 1, 1970 - ".$epoc_rec_sec." time of message arrival.\n";

            # save exported data...prepare new packet

	    my $difference = $epoc_rec_sec - $originate_client_epoch;

	    my $dec_li;

	    if ($difference < 59) {
		
		$dec_li = "0";

	    } elsif ($difference > 61 ) {

		$dec_li = "1";

	    } else {

		$dec_li = "3";

	    }

            # $li = 0; is the client we have no waring for asynchronization
	    my $li = sprintf ("%02b",$dec_li);

            # $vn = 3; is the version 3 only IPV4
	    my $dec_vn = "3";
	    my $vn = sprintf ("%03b",$dec_vn);

            # $mode = 4; is the server (mode 4) 
	    my $dec_mode = "4";
	    my $mode = sprintf ("%03b",$dec_mode);

            # $stratum = 1; is the server clock which is radio or clock (Read RFC)
	    my $dec_stratum = "1";
	    my $stratum = sprintf ("%08b",$dec_stratum);

            # $poll = 6; maximum interval between messages 6 = 64 sec
	    my $dec_poll = "6";
	    my $poll = sprintf ("%08b",$dec_poll);

            # $precision = 0; is the precission interval undefined local clock pecision
	    my $dec_precision = "0";
	    my $precision = sprintf ("%08b",$dec_precision);

            # $root_delay = 0; is the roundtrip delay 32 bit client is 0
	    my $dec_root_delay = "0";
	    my $root_delay = sprintf ("%032b",$dec_root_delay);

            # $root_dispersion = 0; is the nominal error relative to the primary reference
	    my $dec_root_dispersion = "0";
	    my $root_dispersion = sprintf ("%032b",$dec_root_dispersion);

            # $reference_identifier = 0; is the nominal error relative to the primary reference
	    my $reference_identifier_str = "LOCL";
	    my $reference_identifier .= unpack("B32", $reference_identifier_str);

	    my ($dec_epoc_ref, $epoc_ref_nano_sec) = gettimeofday();
	    my $epoc_ref = sprintf ("%064b",$dec_epoc_ref);

	    # $ref_client_epoch is the time that the message departed the client
	    my $epoc_send = sprintf ("%064b",$ref_client_epoch);

            # Receive time of msg client
	    my $epoc_rec_sec_msg = sprintf ("%064b",$epoc_rec_sec);

	    #printf "\$epoc_rec_sec_msg: ".$epoc_rec_sec."\n";

	    my ($dec_epoc_sec_send, $dec_epoc_nano_sec_send) = gettimeofday();
	    my $epoc_send_msg = sprintf ("%064b",$dec_epoc_sec_send);

            #get the peerhost and peerport at which the recent data received.

            #print "\n(".$time." \, ".$peer_address.") data received at port number: ".$peer_port." Client said: ".$received_data."\n";

	    printf "Number of seconds since Jan 1, 1970 - ".$dec_epoc_sec_send." time of message departure.\n";

	    $ntp_packet = $li . $vn . $mode . $stratum . $poll . $precision . $root_delay . $root_dispersion . $reference_identifier . $epoc_ref . $epoc_send . $epoc_rec_sec_msg . $epoc_send_msg;

	} # End of if($received_data)
	else {
	    $ntp_packet = "Invalid Request";
	}

	$server_socket->send($ntp_packet)
	    or die "Server Send: $!\n";
    } # End of while loop

} #End of else ARGV
$server_socket->close(); # Close socket