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Date: Sun, 31 Jul 2022 19:52:21 +0200
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To: Anton Smirnov <sandfox@sandfox.me>,
 PHP Internals List <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Negative values in PcgOneseq128XslRr64::jump($advance)
From: tim@bastelstu.be (=?UTF-8?Q?Tim_D=c3=bcsterhus?=)

Hi

On 7/31/22 17:50, Anton Smirnov wrote:
> I'm writing a polyfill for ext-random and I noticed a small weirdness.
> PcgOneseq128XslRr64::jump() accepts negative $advance where it
> automagically becomes unsigned positive.

Yes, this is what happens when casting a negative signed integer to an 
unsigned integer. It will wrap around to the maximum positive number 
minus whatever the negative value was.

> Does it make sense or maybe it's better to throw a ValueError there?

No it doesn't, especially since:

$a = $pcg->generate();
$pcg->jump(-1); // jumps back 2^64 - 1 steps in a 64-bit system
var_dump($pcg->generate() === $a); // bool(false)

The PCG state is 128 Bits, so a negative jump does not work as expected 
and thus should throw.

Would you mind filing an issue in the tracker, so that it can be 
properly referenced (and thus you properly credited for this find)?

https://github.com/php/php-src/issues

Best regards
Tim Düsterhus