#!/usr/bin/perl

# see http://www.lll.lu/~edward/edward/adsb/DecodingADSBposition.html

use strict;
use warnings;
use Time::HiRes qw ( setitimer ITIMER_VIRTUAL ITIMER_REAL time );;

sub readMessage {
	my $result = undef;
	my %msg = ();
	while (my $line = <>) {
		# encoded message, like '*02e19837a89cb6;'
		if ($line =~ /^\*(.*);/) {
			$msg{"enc"} = $1;
		}
		
		# downlink format, like 'DF 17: ADS-B message.'
		# we just take the numerical value
		if ($line =~ /^DF (\d*): /) {
			$msg{"df"} = $1;
		}
		
		# every line starting with blanks is a payload 
		if ($line =~ /^\s+/) {
			unless (exists($msg{"payload"})) {
				my @payload = ();
				$msg{"payload"} = \@payload;
			}
			push(@{$msg{"payload"}}, $line);
		}
		
		# messages are sparated by an empty line
		if ($line =~ /^$/) {
			# make sure that we don't return an incomplete message
			$result = \%msg;
			last;
		}
	}
	return $result;
}

sub parseMessage {
	my ($msg) = @_;
	
	# we are just interested in downlink format == 17
	if ($msg->{"df"} == 17) {
		parseADSBMessage($msg->{"payload"});
	}
}

sub parseADSBMessage{
	my ($payload) = @_;
	my %data;
	while (my $line = shift(@$payload)) {
		chomp($line);
		
		# icao code
		if ($line =~ /^\s*ICAO Address\s*:\s*(\w+)\s*$/) {
			$data{"icao"} = $1;
		}
		
		# altitude
		# TODO: be careful with "feet"
		if ($line =~ /^\s*Altitude\s*:\s*(\d+)\s*feet\s*$/)	{
			$data{"alt"} = $1;
		}
		
		# latitude
		if ($line =~ /^\s*Latitude\s*:\s*(\S+)\s*$/)	{
			$data{"lat"} = $1;
		}

		# longitude
		if ($line =~ /^\s*Longitude\s*:\s*(\S+)\s*$/)	{
			$data{"long"} = $1;
		}
		
		# speed is divided into two vectors
		if ($line =~ /^\s*EW velocity\s*:\s*(-?\d+)\s*$/)	{
			$data{"speed_ew"} = $1;
		}
		
		if ($line =~ /^\s*NS velocity\s*:\s*(-?\d+)\s*$/)	{
			$data{"speed_ns"} = $1;
		}
		
		# TODO: unsure about vertical rate, needs confirmation
		if ($line =~ /^\s*Vertical rate\s*:\s*(-?\d+)\s*$/)	{
			$data{"vertical"} = $1;
		}
	}
	
	storeInCache(\%data);
}

my %cache;
sub storeInCache {
	my ($data) = @_;
		
	my $icao = $data->{"icao"};
	foreach my $key (keys(%$data)) {
		next if ($key eq "icao");
		$cache{$icao}->{$key} = $data->{$key};
	}
	
	# update the timestamp
	$cache{$icao}->{"timestamp"} = time;
		
}

sub purgeOutdated {
	my @toDelete = ();
	my $now = time;
	foreach my $icao (keys(%cache)) {
		if ($cache{$icao}->{"timestamp"} < $now - 30) {
			push @toDelete, $icao;
		}
	}
	
	foreach my $icao (@toDelete) {
		$cache{$icao} = undef;
	}
}

sub printPFLAA {
	my ($icao, $data) = @_;
	print "\$PFLAA,0";
	print "," . $data->{"lat"};
	print "," . $data->{"long"};
	print "," . $data->{"alt"};
	print ",1";
	print "," . $icao;
	print ",track_not_yet_done";
	print ",turnrate_not_needed";
	print "," . speed($data->{"speed_ew"}, $data->{"speed_ns"}); 
	print "," . $data->{"vertical"};
	print ",1";
	print "*checksum_not_needed";
	print "\n";
}

sub printCache {
	purgeOutdated();
	
	foreach my $icao (keys(%cache)) {
		printPFLAA($icao, $cache{$icao}) if (checkRequiredFields($icao, $cache{$icao}));
	}
}

sub checkRequiredFields {
	my ($icao, $data) = @_;
	foreach my $key (requiredFields()) {
		unless (exists($data->{$key})) {
			return 0;
		}
	}
	return 1;
}

sub requiredFields { return qw( lat long alt speed_ns speed_ew vertical ) }

sub speed {
	my ($speed_ew, $speed_ns) = @_;
	return sqrt($speed_ew*$speed_ew + $speed_ns+$speed_ns);
}

# we set a timer that fires every second a SIGALRM
setitimer(ITIMER_REAL, 1, 1);
$SIG{ALRM} = \&printCache;

# main loop: read from stdin
while (my $msg = readMessage()) {
	last unless defined($msg);
	parseMessage($msg);	
}

# make sure that we print out our cache at least once
printCache();