check random string not in database - php

Question

I have the following function which seems to run forever. Its creating a random string then checking if its in the database. If it is it should run again and again until it has one that is new. It should then return the value to me
public function checkPromo(){
$continue = true;
while ($continue){
$promo = $this->getRandString(6);
$query = sprintf("SELECT * FROM table WHERE field=%s",
$this->db->cleanCode($promo, "text"));
$result = $this->db->query($query);
if($this->db->num_rows($result) >= 1){
$continue = false;
}
}
return $promo;
}

Answer 1

Your specific problem is that your check is backwards. You're looping until you find a duplicate, which will likely take quite a while or forever.
Even if you fixed it, it's a bad algorithm. It will take exponentially longer to generate a new unique code the more codes you have in your database, since the chance of duplicates increases. Especially if your codes are only 6 characters long the chance for duplication increases a lot.
Note that you're also prone to race conditions with your code (think it through from that perspective).
There are two approaches to generating unique ids:
use an incrementing counter, which means you're using a central generator which keeps track of existing ids
use a decentralised approach where you're not keeping track of your ids, but you're using an algorithm that is random enough and has a large enough space that collisions are so unlikely as to be irrelevant in practice
You're combining the worst of both worlds: you're using a central system to keep track of your ids, but you're using random id generation with it. Use one or the other, not both.
If you're going to use a database anyway, just use a standard auto_increment id. If you want it to look a bit random, hash it with MD5 or such.
Alternatively, simple pre-generate all possible codes (just 6 characters isn't a lot), and pick one at random using a method which is not prone to race conditions. Something along the lines of this:
UPDATE codes
SET claimed_user_id = %d
WHERE claimed_user_id IS NULL
ORDER BY RAND()
LIMIT 1
Otherwise, if you want decentralised random ids, use an appropriate algorithm, which is pretty much UUID.

Answer 2

while (true) is a dangerous thing. As you noted, its defaulted to run again and again creating an infinite loop.
You'd be better offer utilizing an auto-incremented value(link assumes MySQL) or generate a pseudo unique id with PHP (such as uniqid()).
You could expand on these values further by prefixing or suffixing a common word, say promo, or padding or hashing them to create a standard length.

Answer 3

Assuming you simply want to generate a promocode that was not recorded in the database and taking into account the context, I advise you change the attitude a little bit:
public function generatePromo(){
$promo = $this->getRandString(6);
$query = sprintf("SELECT * FROM table WHERE field=%s", $this->db->cleanCode($promo, "text"));
$result = $this->db->query($query);
if($this->db->num_rows($result) > 0){ # in case a record with this "text" already exists, run this method again
$this->generatePromo();
} else {
return $this->promo = $promo; # otherwise return the value/store it in the object
}}
If you expect your table to contain only unique values, then I suggest you make it UNIQUE. That will significantly reduce query time.