How to be sure UUID is not duplicated [duplicate] - php

Question

This question already has answers here:
PHP function to generate v4 UUID
(18 answers)
Closed 9 years ago.
I'm working on a video share project and I would like to generate "characters" id's for each video similar to how youtube does it. for example tgax-1sCgIs
Is it safe to use the following function to generate UUIDs, If for example I've 100000000 videos and I need to add new uuid, how can I be sure it's not duplicated?
function generateRandomString($length = 11) {
$characters = '0123456789abcdefghijklm-_nopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
$randomString = '';
for ($i = 0; $i < $length; $i++) {
$randomString .= $characters[rand(0, strlen($characters) - 1)];
}
return $randomString;
}

Answer 1

The following strictly addresses UUIDs. The URLs used by Youtube are not UUIDs and cannot be compared as such. They are much smaller (over 293 times smaller!) and do not have the same guarantees of such an immensely huge domain as a UUID. In this case (for "short hash tags"), duplicate checking must be used - but it need not differ than any other kind of duplicate checking.
If you create a UUID from a proper generator (e.g. a random UUIDv4 generator), then you can be assured that the probability of duplicates is "so low that it just doesn't matter".
As such, while I normally suggest not checking for duplicate UUIDs, there are cases when doing so is pertinent:
During re-mergering (i.e. a cyclic merge operation) where duplicates from prior data are expected and will occur;
The UUID comes from an untrusted generator (i.e. the UUID values be subverted/injected by an attacker or from other manual human intervention);
If used as an SQL Column/Index there is no reason why a Unique Constraint should not be applied as it is required anyway to maintain proper multiplicities.
On the other hand, while I find UUIDs very good for inter-boundary identification (such as transporting information between systems or providing "long" unique resource handles), I find UUIDs very poor to be used as a standard database "record identifier". Where I need a surrogate PK, I merely use a traditional auto-incremement column which is much easier on physical layout. (SQL Server provides a special UUID generator that is much better for indexing - but less secure - than a truly random v4 UUID.)
Unfortunately, PHPs standard uniqid (a "custom" format?) function does not provide the best guarantees. In any case, see PHP function to generate v4 UUID that shows a UUIDv4(-ish?) implementations that is much better than the posted code as they conform to a common generation technique and use a much higher grade random source. (However, please see the comments relating to how mt_rand is seeded - or not seeded - in the answers.)

Answer 2

If you're using a database you have a couple options:
Just use the auto increment column of the table you're storing the videos in. The number will always be unique.
Each time you generate an id, check the database to see if it exists. If it exists, re-run the function to generate a new uuid and check the database again. Do it until you query the database and no rows are returned with that id.
There are a few other posts you should look at that have a better approach at generating a true uuid:
Nice UUID class
PHP function to generate v4 UUID
How to create a UUID in php

Answer 3

I'm pretty sure YouTube is just encoding integer IDs in a base-X system. There's just so many, and they are created so fast, that they seem random.
The code would look something like:
<?php
$base_str = '0123456789abcdefghijklmnopqrstuvwxyz-_';
$base = strlen($base_str);
// generate a number if no input
if( ! isset($argv[1]) ) {
$number = rand(1000,1000000);
} else {
$number = intval($argv[1]);
}
printf("Input: %d\n", $number);
printf("Base: %d\n", $base);
// will hold the base-X encoded representation of the number
$repr = '';
for( $i=$number; $i>0; ) {
$remainder = $i % $base;
$digit_repr = substr($base_str, $remainder, 1);
$repr = $digit_repr . $repr;
printf("Rem: %2d Repr: %s Cur: %16d Progress: %s\n", $remainder, $digit_repr, $i, $repr);
$i = ($i - $remainder) / $base;
}
Example output:
Input: 2000000
Base: 38
Rem: 22 Repr: m Cur: 2000000 Progress: m
Rem: 1 Repr: 1 Cur: 52631 Progress: 1m
Rem: 17 Repr: h Cur: 1385 Progress: h1m
Rem: 36 Repr: - Cur: 36 Progress: -h1m
If you want to introduce a little more "randomness" into how the IDs look you can always scramble $base_str. Just keep in mind that you can only scramble it once before you start encoding IDs.
Decoding
I guess that's important, right?
<?php
$base_str = '0123456789abcdefghijklmnopqrstuvwxyz-_';
$base = strlen($base_str);
if( ! isset($argv[1]) ) {
$input = '-h1m';
} else {
$input = $argv[1];
}
printf("Input: %s\n", $input);
printf("Base: %d\n", $base);
$repr = str_split($input);
$number = 0;
for( $i=0; $i<count($repr); $i++) {
$number = $number * $base;
$value = strpos($base_str, $repr[$i]);
$number += $value;
printf("Char: %s Value: %2d Cur: %12d\n", $repr[$i], $value, $number);
}
Example output:
Input: -h1m
Base: 38
Char: - Value: 36 Cur: 36
Char: h Value: 17 Cur: 1385
Char: 1 Value: 1 Cur: 52631
Char: m Value: 22 Cur: 2000000