=pod

extract_key takes an element from a list and returns a scalar that is the key element. Think of MD5.

=cut

sub extract_key {
  # blindly return the item itself, stringified.
  return "@_";
};

sub compare_items {
  # plain string identity comparision
  $_[0] eq $_[1]
};

sub compare_lists {
  my ($list_a, $list_b) = @_;
  my (%dict);
  my %result = (
    equal => [],
    different => [],
    only_a => [],
    only_b => [],
  );

  for my $item_a (@$list_a) {
    my $key = extract_key($item_a);
    $dict{$key} = []
      unless exists $dict{$key};
    push @$dict{$key}, $item_a;
  };

  my @found;
  for my $item_b (@$list_b) {
    my $key = extract_key($item_b);
    if (exists $dict{$key}) {
      if (@$dict{$key}) {
        my $item_a = shift @$dict{$key};
        push @found, [ $item_a, $item_b ];
      } else {
        push $result{only_b}, $item_b;
      };
    };
  };

  push $result{only_a}, @$dict{$key}
    for my $key (keys %dict);

  for my $pair (@found) {
    if (compare_items( $pair->[0], $pair->[1] )) {
      push @$result{equal}, $pair;
    } else {
      push @$result{different}, $pair;
    };
  };

  return %result;
};