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Date: Tue, 15 Feb 2022 14:09:10 +0100
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To: Go Kudo <g-kudo@colopl.co.jp>
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Subject: Re: [PHP-DEV] [RFC] [Under Discussion] Random Extension 4.0
From: tim@bastelstu.be (=?UTF-8?Q?Tim_D=c3=bcsterhus?=)

Hi

On 2/15/22 12:48, Go Kudo wrote:
> At first, I updated the RFC to the latest status.
> 
> https://wiki.php.net/rfc/rng_extension

Thank you, the examples are useful.

> I need some time to think about the current issue. I understand its
> usefulness, but I feel uncomfortable with the fact that the NumberGenerator
> generates a string.

Would you feel more comfortable if the interface would be called 
\Random\Engine or \Random\Generator (i.e. leaving out the "Number" from 
the interface name)?

Engine is the term used by C++: 
https://en.cppreference.com/w/cpp/numeric/random/mersenne_twister_engine
Generator is the term used by Java: 
https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/random/RandomGenerator.html

----------

With the 'FixedForUnitTest' example you mentioned in the RFC: While for 
that specific implementation it appears pretty obvious that increasing 
numbers are generated, in practice:

1. This will result in inconsistent behavior based on the architecture. 
I can't test it due to the lack of the necessary architectures, but I 
believe the following to be accurate:

     $g = new FixedForUnitTest();

     $r = new Randomizer($g);

     // 0100000000000000 in 64 Bit little endian
     // 0100000002000000 in 32 Bit little endian
     // 0000000000000001 in 64 Bit big endian
     var_dump(bin2hex($r->getBytes(8)));

2. This analogy completely breaks down for the 'shuffle' functions which 
call the generator internally an unspecified number of times:

     $g = new FixedForUnitTest();

     $r = new Randomizer($g);

     var_dump($r->shuffleString("abcdefghijklmnopqrstuvwxyz")); // 
wosqyrupatvxznmlkjihgfedcb
     var_dump($r->shuffleString("abcdefghijklmnopqrstuvwxyz")); // 
fwrtjndlsyvpzuhxbqomkigeca

The resulting strings look somewhat ordered, but there is no clear 
pattern, despite the underlying generator being completely predictable!

> I also wonder about the point of changing RNG to XorShift128Plus. There are
> a number of derived implementations, which RNG do you think is more
> suitable?
> 

I'm not an expert in RNGs, but based off this page: 
https://prng.di.unimi.it/ and the linked papers it appears that 
xoshiro256** is the overall best choice if memory usage is not a concern.

Best regards
Tim Düsterhus