sub sieve_s2 {
    my ($n, $i, $s, $d, @primes) = (shift, 7);

    my $sieve = '110010101110101110101110111110' .
                '101111101110101110101110111110' x ($n/30);

    for ($sieve) {
        until (($s = $i*$i) > $n) {
           $d = 2*$i;
           do { substr($_, $s, 1, '1') } until ($s += $d) > $n;
           1 while substr($_, $i += 2, 1);
        }
        $_ = substr($_, 1, $n);
        push @primes, pos while m/0/gogo;
        return @primes;
    }
}