Generate unique random alphanumeric characters that are 7 characters long

Question

It need not be meaningful words - more like random password generation, but the catch is - they should be unique. I will be using this for some kind of package / product code. Which is the best method available? :)

Answer 1

It is generally not possible to generate sequences with both unique and random elements: obviously to be unique the algorithm has to take into account the previously generated elements in the sequence, so the next ones will not really be random.

Therefore your best bet would be to detect collisions and just retry (which could be very expensive in your particular case).

If you are constrained to just 7 chars, there's not much you can do above:

$allowed_chars = 'abcdefghijklmnopqrstuvwxz';
$allowed_count = strlen($allowed_chars);
$password = null;
$password_length = 7;

while($password === null || already_exists($password)) {
    $password = '';
    for($i = 0; $i < $password_length; ++$i) {
        $password .= $allowed_chars{mt_rand(0, $allowed_count - 1)};
    }
}

This should eventually give you a new password.

However, in similar cases I have encountered I usually pick a larger password size which also happens to be the size of the hex representation of a popular hash function (e.g. md5). Then you can make it easier on yourself and less error prone:

$password = time(); // even better if you have some other "random" input to use here

do {
    $password = md5(time().$password);
}
while (already_exists($password));

This also has the added advantage that the sequence space is larger, hence there will be less collisions. You can pick the size of the hash function according to the expected numbers of passwords you will generate in the future to "guarantee" a low collision probability and thus less calls to the possibly expensive already_exists function.

Answer 2

Here's a way you could do it without hashes or loops:

$password = sprintf(
    "%04s%03s",
    base_convert(mt_rand(0, pow(36, 4) - 1), 10, 36),
    base_convert(mt_rand(0, pow(36, 3) - 1), 10, 36)
);

As a few others have mentioned, ensuring uniqueness is more complicated, and should be unneeded. The simplest way you could do it would be to add extra characters at the end, incrementing with each password generated.