use strict;
use warnings;

sub prettyip {
  my $ip = shift;
  $ip =~ s/ (\d+) / sprintf("%03d",$1) /smgex;
  return $ip;
}

my %last_seen_ip_from_hop;   # last IP seen for key=HOP
my $section      = 1;        # section within the file
my $previous_hop = 0;        # previous HOP / new section event

while (my $line = <DATA>) {
  if ($line =~ /^(\d+),(\S+)/) { # extract HOP and IP
    my ($hop, $ip)    = ($1, $2);
    my $last_ip_seen  = $last_seen_ip_from_hop{$hop};
    my ($changemark_pre, $changemark_pos)  = ("", "");

    # detect a new section (A/B/C)
    $section++, print "\n" if $previous_hop > $hop; # new file/section
    $previous_hop = $hop;

    # notify if a change occured for a given hop since last seen
    if (defined $last_ip_seen and $ip ne $last_ip_seen) {
      $changemark_pre = 'changed to';
      $changemark_pos = '(was: ' . prettyip($last_ip_seen) . ')';
    }
    $last_seen_ip_from_hop{$hop} = $ip; # init or update current HOP/IP

    printf "sect.%2d / hop %2d: %15s %15s %s\n",
      $section, $hop, $changemark_pre, prettyip($ip), $changemark_pos;
  }
}


__DATA__
13,4.69.137.70
14,4.69.134.70
15,4.69.134.113
16,4.69.135.185
17,4.69.134.246
18,4.68.18.75
19,4.59.0.10
20,124.211.34.129
21,203.181.100.61
22,118.155.197.140
23,124.211.10.66
24,163.139.130.138
25,163.139.124.57
26,202.215.179.1
27,202.215.179.11

13,4.69.137.74
14,4.69.134.70
15,4.69.134.113
16,4.69.135.185
17,4.69.134.246
18,4.68.18.11
19,4.59.0.10
20,124.211.34.121
21,203.181.100.61
22,118.155.197.140
23,124.211.10.66
24,163.139.130.138
25,163.139.124.57
26,202.215.179.1
27,202.215.179.11

13,4.69.137.70
14,4.69.134.78
15,4.69.134.125
16,4.69.135.185
17,4.69.134.250
18,4.68.18.139
19,4.59.0.10
20,124.211.34.121
21,203.181.100.189
22,118.155.197.140
23,124.211.10.66
24,163.139.130.138
25,163.139.124.57
26,202.215.179.1
27,202.215.179.11