#!/usr/bin/perl
# vim: shiftwidth=4 tabstop=4
#
# This CGI program is a Prometheus exporter for pppd's lcp-rtt-file feature.
#
# Copyright (C) Marco d'Itri <md@linux.it>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.

use v5.14;
use warnings;
use autodie;

use List::Util qw(sum max min);

{
	my $data = read_data('/run/ppp-rtt.data');
	my $stats = compute_statistics($data, 60);

	my $s = metrics($stats);
	my $length = length($s);

	print "Content-type: text/plain\n";
	print "Content-length: $length\n\n$s";
	exit;
}

sub metrics {
	my ($stats) = @_;

	my $s = <<END;
# TYPE lcp_rtt_status gauge
# HELP LCP RTT status
lcp_rtt_status $stats->{status}
END
	foreach (qw(average min max loss)) {
		next if not exists $stats->{$_};
		$s .= <<END;
# TYPE lcp_rtt_$_ gauge
# HELP LCP RTT $_
lcp_rtt_$_ $stats->{$_}
END
	}

	return $s;
}

sub compute_statistics {
	my ($data, $length) = @_;

	my $cutoff = time() - $length;
	my @e = grep { $_->[0] >= $cutoff } @{ $data->{data} };
	return { status => -1 } if not @e; # no data

	my $average = (sum map { $_->[1] } @e) / scalar(@e);
	my $min = min map { $_->[1] } @e;
	my $max = max map { $_->[1] } @e;
	my $loss = sum map { $_->[2] } @e;

	return {
		status	=> $data->{status},
		average	=> $average,
		min		=> $min,
		max		=> $max,
		loss	=> $loss,
	};
}

sub read_data {
	my ($file) = @_;

	my $data;
	open(my $fh, '<', $file);
	binmode($fh);
	my $bytes_read;
	do {
		$bytes_read = sysread($fh, $data, 8192, length($data));
	} while ($bytes_read == 8192);
	close($fh);

	my ($magic, $status, $position, $echo_interval, $rest)
		= unpack('NNNN a*', $data);
	return undef if $magic != 0x19450425;

	# the position is relative to the C array, not to the logical entries
	$position /= 2;

	my @rawdata = unpack('(N C a3)*', $rest);
	my @data;
	while (my ($time, $loss, $rtt) = splice(@rawdata, 0, 3)) {
		push(@data, [ $time, unpack('N', "\000$rtt"), $loss ]);
	}

	@data =
		# skip any "empty" (null) entries
		grep { $_->[0] }
		# rearrange the list in chronological order
		(@data[$position+1 .. $#data], @data[0 .. $position]);

	return {
		status			=> $status,
		echo_interval	=> $echo_interval,
		position 		=> $position,
		data			=> \@data,
	};
}