CODEWITHSUNDEEP
phprandomgd
Ákos Nikházy
Ákos Nikházy

Reputation: 1306

Why rand() isn't really random?

I wanted to put random points on an image (stars in space for some little fun side project)

I have this simple script. 

<?php
$gd = imagecreatetruecolor(1000, 1000);
$white = imagecolorallocate($gd, 255, 255, 255);

for ($i = 0; $i < 100000; $i++) 
{
    $x = rand(1,1000);
    $y = rand(1,1000);

    imagesetpixel($gd, round($x),round($y), $white);
}

header('Content-Type: image/png');
imagepng($gd);
?>

Keep in mind this is just for testing, that is why I put 100000 in for loop so it shows the pattern I noticed emerging. We have 1 million pixel to use, still random X and Y creates this pattern instead: enter image description here

So it is far from random. I know rand is not real random, that is why it isn't good for cryptography. But I find no information about how it works and what should I do to avoid patterns like this.

Upvotes: 11

Views: 2244

Answers (3)

Christian
Christian

Reputation: 11

We used in our company mt_rand() to generate a 16 character long random ident. We had 3500000 records in our database and idents were colliding.

We used "abcdefghijklmnopqrstuvwxyz0123456789" as our charset and were picking "random" chars with mt_rand() for our ident.

The possibilities are theoretically 16^32. Thats 340282366920938463463374607431768211456 (340 undecillion)

And yet there were collisions...

we switched to random_int() and hopefully never get collisions again

Upvotes: 1

theking2
theking2

Reputation: 2803

Rest assured: As of PHP 7.1.0, rand() uses the same random number generator as mt_rand().

This only is an issue for PHP < 7.1 see the Docs

Upvotes: 1

Bathsheba
Bathsheba

Reputation: 234655

Linear congruential random number generators (which is what PHP rand uses) will always display autocorrelation effects on an x-y plot.

You will have better results with mt_rand. This is a Mersenne Twister generator.

Upvotes: 11

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