sub packVars
{
    my $varsref = $_[0];

    # current format version: 01
    return join "&", "==01",
        map {
            my $typ;
            my $v = $varsref->{$_};

            # special data to pack?
            if (ref $v) {

                # only hash refs supported now
                $typ = "H";
                $v = packVars( $v );
            }
            # undef becomes empty, protect empty values
            elsif (!$v) {
                $v = $typ = '';
            }

            join '=',
                map {s/([%&=])/ sprintf '%%%02x', ord($1) /ge; $_}
                    $_, $v, (defined $typ ? $typ : ());
        } sort keys %$varsref;
}

sub unpackVars
{
    my $vars_str = $_[0];
    my $format_version = "00";

    # version 00: original format
    # version 01: keys are escaped, not just values
    $format_version = $1 if $vars_str =~ s/^==(\d\d)&//;

    return {} unless $vars_str;

    my %vars;

    if ($format_version eq "01") {
        for (split /&/, $vars_str) {
            my ($k,$v,$typ) =
                map { s/%(\w\w)/ chr(hex($1)) /ge; $_ } split /=/, $_, -1;

            # special data to unpack?
            if ($typ) {
                # nested hash
                if ($typ eq "H") {
                    $v = unpackVars($v);
                }
            }
            $vars{$k} = $v;
        }
    }
    # format version "00"
    else {
        %vars = map split(/=/, $_, 2), split /&/, $vars_str;
        unescape( values %vars );
        $vars{$_} eq ' ' and $vars{$_} = '' for keys %vars;
    }

    return \%vars;
}

sub cmpVars
{
    my ($var1str, $var2str) = @_;

    # return false immediately if strings match,
    # otherwise return true if both are current format version,
    # otherwise compare current format version.

    return $var1str cmp $var2str
        && ( $var1str =~ /^==01&/ && $var2str =~ /^==01&/ )
        || packVars( unpackVars ( $var1str ) )
        cmp packVars( unpackVars ( $var2str ) );
}