Random string gets repeated?

Question

I have a service on the internet where people post pictures and a short string is generated. Only one can be used ever. However, I am getting into duplicates in the database and I am seeing major problems.

Here's what I am using:

$id=rand(10000,99999);
$short_string = base_convert($id,20,36);

What would be the best way to fix it? Check from the database and keep looping till it doesn't match one? What if every possible solution and it goes in an infinite loop?

by the code you posted, you can only have 89999 numeric string ids. Not very many. Are these permanently in use? or do you remember and recycle them? — Mitch Wheat, May 21 '11 at 02:24
question: base_convert( string $number, int $frombase, int $tobase ). I may be missing something but isn't your frombase, base 10 and not 20? — Mitch Wheat, May 21 '11 at 02:42
@Mitch Wheat: That's the code I use, exactly. What does changing the base from 10 to 20 do? — iosfreak, May 21 '11 at 02:56

score 1 · Answer 1 · answered May 21 '11 at 01:41

1

Put your PK through an algorithm that generates a unique number from it and put that through your function.

answered May 21 '11 at 01:41

Ignacio Vazquez-Abrams

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I do have ID for each image. So can I do something with that? Like generate something special like md5 but for really short. I could be using the wrong vocabulary, but you know what I mean. Remember: I need to keep it short as possible. – iosfreak May 21 '11 at 01:47
Zero-pad it to, say, 8 digits, reverse the digits, turn that back into a number, then convert it. – Ignacio Vazquez-Abrams May 21 '11 at 01:49
I get what all you are saying. Can you please post a short example or link that I can look at to get this working? – iosfreak May 21 '11 at 01:51
http://www.php.net/manual/en/function.sprintf.php http://www.php.net/manual/en/function.strrev.php – Ignacio Vazquez-Abrams May 21 '11 at 01:53
Will this ensure I will get a random one each time? – iosfreak May 21 '11 at 01:56
No, since it will derive it from the PK of the entry. A more complex algorithm will result in less predictable values, but they will *always* be deterministic. – Ignacio Vazquez-Abrams May 21 '11 at 01:57
This is not to say that you can't use them, though. They *will* be unique per entry (depending on the algorithm, of course). – Ignacio Vazquez-Abrams May 21 '11 at 02:07

score 1 · Answer 2 · answered May 21 '11 at 01:46

1

The best bet would be to make sure the image doesn't exist by using the random number generator against the list of images before writing a new image with that number in it.

Try to increase the amount and type of characters by using an algorithm that uses numbers, letters so it increases the combinations that you can have.

answered May 21 '11 at 01:46

BRampersad

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But doesn't that still put it at risk for getting the same result more than once? – iosfreak May 21 '11 at 01:49
You can count in order of increasing numbers instead of random numbers if you truely want to never have repeating numbers but if you choose to go the random way there will always be a chance of having dups. – BRampersad May 21 '11 at 02:34
That would probably introduce a race condition if two processes were racing to insert the same value; both checks could succeed. – MarkR May 21 '11 at 06:22

score 1 · Accepted Answer · answered May 21 '11 at 02:05

1

Increment the last value by a random amount instead of using a random value. Like so:

$results = mysql_query("SELECT * FROM thetable ORDER BY theId DESC LIMIT 1");
$keyToUse = 1;
if($row = mysql_fetch_array($results)) {
    $keyToUse = (int)$row['theId'] + rand(1, 100);
}

Then convert the integer key to and from any format, say using base_convert.

answered May 21 '11 at 02:05

Ry-

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This is exactally what I did. Except I added 10 random numbers to it. I don't think it will get repeated any time soon. – iosfreak May 21 '11 at 03:03

score 1 · Answer 4 · answered May 21 '11 at 02:14

1

Did you try using mt_rand() http://www.php.net/manual/en/function.mt-rand.php. Also you should be increasing the range.

answered May 21 '11 at 02:14

Shameer

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score 0 · Answer 5 · answered May 21 '11 at 01:40

0

Use md5 http://php.net/manual/en/function.md5.php The strings produced have no conflict (with large probability).

answered May 21 '11 at 01:40

George Kastrinis

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It needs to be URL-friendly. 1-6 characters long. – iosfreak May 21 '11 at 01:41
You can user urlEncode for the first part, but I am not sure if there is something robust for only 6 characters. – George Kastrinis May 21 '11 at 01:42
Here's a example URL: http://myflashpics.com/picture/8292 ... I cannot have a 20 - 40 character string. – iosfreak May 21 '11 at 01:42
Why not use an auto increment number? Is there really a need for randomness? – George Kastrinis May 21 '11 at 01:47
I have that. But I don't want users to have access to it -- I want a short string. – iosfreak May 21 '11 at 01:48

Random string gets repeated?

5 Answers5