sub shrink {
    my $str = '';
    for ( 0 .. length( $_[0] ) - 1 ) {
        my $bit = 7 * $_;
        vec($str, $bit++, 1) = $_ for split //, unpack('b8', substr($_[0], $_, 1));
    }
    return $str; 
}

sub grow {
    my ($str, $chr) = ('', '');
    my $end = length( $_[0] ) * 8;
    $end -= ($end % 7) + 1;
    for ( 0 .. $end ) {
        vec($chr, $_ % 7, 1) = vec($_[0], $_, 1);
        if ( $_ % 7 == 6 ) {
            vec($chr, 7, 1) = 0;
            $str .= $chr;
            $chr = '';  
        }
    }
    return $str;
}