[SOLVED] generate random string that isn't already in DB

[BandonRandon]

Hello,

 

I have found many functions that generate random strings i am currently using one that works well. But here is a potential problem I would like to stop before it happens. I'm inserting the random string into a db and need it to be unique if the string exist I need it to create another one. Here is the random string code:

 

function genRandomString() {
    $length = 5;
    $characters = '023456789abcdefghijkmnopqrstuvwxyz';
    $string = '';    

    for ($p = 0; $p < $length; $p++) {
        $string .= $characters[mt_rand(0, strlen($characters))];
    }
    return $string;
}

 

which works great. However, when I insert it in the DB i need to make sure that a string that looks like that doesn't already exist (sure there are  45435424 combination but just to make sure) What i have now will check the current generated string and if its in the DB make a new one. I would assume that if the new string is in the DB this code wouldn't check that. I think i need to loop to the code somehow and say if "new string == randomstring" then try again. Any help on this would be great.

 

Here is the db code.

 

	//generate random string
			$random_string = $this->genRandomString();
			//select all random strings from the database
			$check_random_string_q = 'SELECT rand_string FROM '.URL_TABLE.' WHERE (rand_string="'.$random_string.'")';
			$check_random_string_result = mysql_query($check_random_string_q);
			$check_random_string_row = mysql_fetch_array($check_random_string_result);
			//if the string is in the database generate a new one				
			if($random_string == $check_random_string_row['rand_string']){
			$random_string = $this->genRandomString();
			}

			$q = 'INSERT INTO '.URL_TABLE.' (url, rand_string, date) VALUES ("'.$url.'","'.$random_string.'", NOW())';

 

Thank you!

Brooke

[rhodesa]

so, you can always put it in a do/while loop to keep generating a string and testing with a SELECT...but there are probably better solutions. can you elaborate on what you are trying to do and what parts are set in stone? if you are just looking for a unique id for a record, most DBs have some sort of auto_increment functionality to give each record a unique id. if you don't want that, can the length be increased? if so, i have another alternative

[BandonRandon]

Thanks rhodesa,

 

What I am trying to do is re-write parts of an open source tinyurl application. Each link in the database is already assigned a unique id but what i need to do is also assign it a unique "string" for example if someone goes to site.com/a1b3c it will go into the database find the row with the rand string matching "a1b3c" and then redirect to that url. All that is working fine. but because I'm using a function to generate the random string I need to make sure that it's unique. That way 2 links can't both have the rand_sting of "a1b3c" as that would only link to the most recent url with that rand_string.

 

I hope that is a little bit clearer. I was thinking of doing a do/while loops but i get confused when it comes to loops

 

Brooke 

[The Little Guy]

This will create a random string:

 

echo time().md5(rand(0,1000));

[BandonRandon]

Thanks The little guy,

 

I am already generating a random string just fine using the "genRandomString()" function posted above. What I now need to do is check to make sure the string that is generated wasn't already generated.

 

Brooke

[The Little Guy]

If you use what I used, you will have a 99% chance that the string isn't in the database, unless you have over 1000 of the same query in one second...

[The Little Guy]

In fact, if you use microtime, you will have an even stronger chance of it being unique than means you would have to create over 1000 strings in a millisecond.

 

echo microtime().md5(rand(0,1000));

[BandonRandon]

yes, but won't the sting also be 32 characters long? I would like to try to keep the sting short also won't it also all be numerical and not alphanumerical?

 

 

[The Little Guy]

then...

 

I guess you would do this (to keep the function you have)

 

1. Turn the rand_string column to unique

 

2. do an insert into the database, it will insert if it isn't in the database, otherwise it won't.

 

3. after the insert, use mysql_affected_rows, if you get 1, the insert was successfully otherwise, create a new random number.

[rhodesa]

The Little Guy, to get a really unique string, use:

microtime(true).getmypid();

...feel free to md5() that if you so desire

 

BandonRandon, as TLG said, you can add a UNIQUE attribute to the 'rand_string' column, or just run a SELECT to test it. It's up to you, but the SELECT method (since you are already on that track) would look like:

 

do {
  $random_string = $this->genRandomString();
} while (mysql_num_rows(mysql_query("SELECT rand_string FROM ".URL_TABLE." WHERE rand_string='{$random_string}'")));
$q = "INSERT INTO ".URL_TABLE." (url, rand_string, date) VALUES ('{$url}','{$random_string}', NOW())";
$result = mysql_query($q);

[The Little Guy]

You could possibly eliminate one query this way:

 

do {
     $q = "INSERT INTO ".URL_TABLE." (url, rand_string, date) VALUES ('{$url}','{$random_string}', NOW())";
     $result = mysql_query($q);
} while (mysql_affected_rows() < 1);

[rhodesa]

don't forget the random string line:

 

do {
   $random_string = $this->genRandomString();
   $q = "INSERT INTO ".URL_TABLE." (url, rand_string, date) VALUES ('{$url}','{$random_string}', NOW())";
   $result = mysql_query($q);
} while (mysql_affected_rows($result) < 1);

 

edit: and the $result in mysql_affected_rows()

 

[The Little Guy]

don't forget the random string line:

 

do {
   $random_string = $this->genRandomString();
   $q = "INSERT INTO ".URL_TABLE." (url, rand_string, date) VALUES ('{$url}','{$random_string}', NOW())";
   $result = mysql_query($q);
} while (mysql_affected_rows($result) < 1);

 

edit: and the $result in mysql_affected_rows()

 

 

Oops forgot the random string... but $result is optional, but it is a good thing to add.

[BandonRandon]

Thank you both very much for your help. I was able to do it using rhodesa original way of:

do {
  $random_string = $this->genRandomString();
} while (mysql_num_rows(mysql_query("SELECT rand_string FROM ".URL_TABLE." WHERE rand_string='{$random_string}'")));
$q = "INSERT INTO ".URL_TABLE." (url, rand_string, date) VALUES ('{$url}','{$random_string}', NOW())";
$result = mysql_query($q);

 

The second way created an infinite loop, I may try the alternative method again later.

 

Thanks again,

Brooke