sub id2path {
    my $id = shift;
    return q() unless $id;

    my $path;
    if ($id > 999_999) {
        $path = sprintf '%03d/%03d/%03d', $id / 1_000_000,
                                          ($id / 1_000) % 1_000,
                                          $id % 1_000;
    } else {
        $path = sprintf '%02d/%02d/%02d', $id / 10_000,
                                          ($id / 100) % 100,
                                          $id % 100;
    }
    return $path;
}

##</code><code>##

sub id2path_new {
    my $id = shift or return q();

    my $chunk_length = length $id > 6 ? 3 : 2;

    $id = sprintf '%0' . ($chunk_length * 3) . 'd', $id;

    my $chunk = ".{$chunk_length}";
    my $path  = join '/', $id =~ / ^ (.*) ($chunk) ($chunk) $ /xg;
    return $path;
}

##</code><code>##

use Test::More tests => 23;
is(id2path_new($_), id2path($_), "id=$_") for q(), 0, 1, 9, 10, 99,
                                          100, 999, 1000, 9999, 10000,
                                          99999, 100000, 999999, 1e6,
                                          1e7-1, 1e7, 1e8-1, 1e8, 1e9,
                                          1e10, 1e11, 1e12;