#! /usr/bin/perl
use strict;
use warnings;
local *F;
my $ta="/tmp/upsc-log-diff.".$$."a";
my $tb="/tmp/upsc-log-diff.".$$."b";
unlink $ta; unlink $tb;
END {
  unlink $ta; unlink $tb;
}
my $F="/var/log/upsc.log";
open F,$F or die "$F: $!";
my $prev;
my $this="";
my $line;
while (<F>) {
  $line=$.;
  chomp;
  if (!/^$/) {
    $this.="$_\n";
    next;
  }
  if ($prev) {
    local *TA;
    local *TB;
    open TA,">$ta" or die "$ta: $!";
    open TB,">$tb" or die "$tb: $!";
    print TA $prev or die "$ta: $!";
    print TB $this or die "$tb: $!";
    close TA or die "$ta: $!";
    close TB or die "$ta: $!";
    local *TD;
    my $td="diff -u $ta $tb|";
    open TD,$td or die "$td: $!";
    <TD>=~/^\Q--- / or die "line $line: $_";
    <TD>=~/^\Q+++ / or die "line $line: $_";
    while (<TD>) {
      next if /^[@ ]/;
      print;
    }
    close TD; # or die "$td: $!";
    print "\n";
  }
  $prev=$this;
  $this="";
}
die $this if $this;