PHP pseudo random numbers generator

Question

I am using a linear conguential generator in my javascript code but now I need to validate the results server side (generate the same numbers from the same seed). I have translated my javascript code in PHP but it does not work as expected. The first few numbers are close to the javascript ones but with less precision and the sequence contains some negative numbers which are not present in the javascript version. I think this is because of PHP's different floating point precision but I am confused by the negative numbers.

If there is no easy way to make this work in PHP what other methods could I use to generate the same sequence of pseudo-random numbers both in javascript and in PHP?

Javascript

function SeededRandom(newSeed) {
    this.seed = newSeed;
    this.Random = function (min, max) {
        this.seed = (this.seed * 9301 + 49297) % 233280;
        return Math.floor(min + (this.seed / 233280) * (max - min + 1));
    }
}

PHP

class SeededRandom {
    private $seed;
    public function __construct($newSeed) {
        $this->seed = $newSeed;
    }
    public function Random($min, $max) {
        $this->seed = ($this->seed * 9301 + 49297) % 233280;
        return floor($min + ($this->seed / 233280) * ($max - $min + 1));
    }
}

score 0 · Accepted Answer · answered Jun 19 '18 at 02:06

Got it! With these numbers it works both in javascript and in PHP.

function SeededRandom2(newSeed) {
    this.seed = newSeed;
    this.Random = function () {
        this.seed = (this.seed * 20077 + 12345) % 32768;
        return this.seed;
    }
}

The negative numbers were probably caused by integer overflow and the divergence between the javascript and PHP numbers was caused by the division.

I found this version in the answer to this question: Was there a a time when PHP's rand() function was used as an exploit?

PHP pseudo random numbers generator

1 Answers1