#!/usr/bin/perl

use strict;
use warnings;

sub pair_hash { return $_[0][0] . ',' . $_[0][1]; }

sub array_union {
  my ($a, $b) = @_;
  my %bhash = map { &pair_hash($_) => 1 } @{$b};
  return grep { defined $bhash{ &pair_hash($_) } } @{$a};
}

sub array_diff {
  my ($a, $b) = @_;
  my %bhash = map { &pair_hash($_) => 1 } @{$b};
  return grep { !defined $bhash{ &pair_hash($_) } } @{$a};
}

my @pairs_in_db = ([1,2],[1,9],[3,4],[5,6]);
my @pairs_from_somewhere_else = ([1,2],[5,6],[7,18]);

my @was_in_db = &array_union(\@pairs_in_db, \@pairs_from_somewhere_else);
my @pairs_to_delete = &array_diff(\@pairs_in_db, \@pairs_from_somewhere_else);
my @pairs_to_create = &array_diff(\@pairs_from_somewhere_else, \@pairs_in_db);

foreach my $ext (@was_in_db) {
  printf("<%s, %s> was in db\n", @{$ext});
}

foreach my $ins (@pairs_to_create) {
  printf("<%s, %s> has to be inserted into the databse\n", @{$ins});
}

foreach my $del (@pairs_to_delete) {
  printf "<%s, %s> will be removed\n", @{$del}; 
}