[erlang-questions] On OTP rand module difference between OTP 19 and OTP 20

[Raimo Niskanen]

Thank you Kenji for reviewing the changes and summarizing the implications.

Sorry about not getting you into the loop during the rewrite!

As you guessed I was focused on plugging in 'crypto', fixing the flaws,
incorporating the new algorithm from Prof. Vignia and keeping it
as backwards compatible as possible, so I forgot about you...



Regarding the changed uniform float behaviour: it is the functions
rand:uniform/0 and rand:uniform_s/1 this concerns.  They were previously
(OTP-19.3) documented to output a value 0.0 < X < 1.0 and are now
(OTP-20.0) documented to return 0.0 =< X < 1.0.

Previously they _could_ return exactly 0.0, even though not stated in the
documentation.  But the probability for exactly 0.0 was about 2048 times
less than it is now.  This is because the float generation has been fixed
to generate equidistant floats on the form K * 2^-53 so exactly 0.0 has now
got the same probability as all other possible float values.

I'd rather say that the documentation of rand:uniform/0 and rand:uniform_s/1
has been corrected to match their behaviour, and that the probability for
exactly 0.0 has increased from about 1/64 to about 1/53.

Despite this the distribution of generated numbers has actually not changed
- it is still uniform over the range 0.0 =< X < 1.0.

That is my view of the float changes, of which I am fairly certain. :-)


/ Raimo



On Tue, Aug 29, 2017 at 10:53:17AM +0900, Kenji Rikitake wrote:
> I finally reviewed OTP rand module change between 19.3 and 20.0.
> While most of users won't recognize the changes,
> the details are considerably different.
> 
> A good news is that if you're not aware of the PRNG algorithm
> you can keep the code untouched. The new default algorithm
> works ok and even better than the OTP 19 with less bugs.
> 
> On the other hand, if you explicitly specify the algorithm in rand:seed/1,
> you are suggested to use a newly-available algorithms
> described in the rand module manual of OTP 20.0. Actually
> all the old (OTP 18 and 19) algorithms are now deprecated.
> See rand module manual of OTP 20. Use exrop (default of OTP 20) in most
> cases;
> if you use exs1024 that should be changed to exs1024s;
> if you stick with exsplus (default until OTP 19), use exsp instead.
> If you use other algorithms, consider converting to exrop.
> 
> Also, if your code depends on the output range of
> rand:uniform/1 or rand:uniform_s/1, it has been changed as follows:
> OTP 18 and 19: 0.0 < X < 1.0
> OTP 20: 0.0 =< X < 1.0 (note the =< operator)
> where X is the output of the functions.
> In short, since OTP 20, the functions may output 0.0.
> 
> I noticed the changes shortly after OTP 20.0 release.
> These changes were in the last minute (19-MAR-2017 - 26-APR-2017)
> just before the OTP 20 release
> and after my Erlang and Elixir Factory 2017 trip so I didn't notice.
> I was not notified at all by Raimo Niskanen and other contributors
> about these changed either,
> so I had to take time and chance to review the code again.
> I know OTP is open source software and the OTP Team can modify
> the code for better features and removing bugs without notifying to anyone,
> so nobody is to blame on these changes.
> I'm assured that Raimo and the other contributors have done a great job
> on changing the details while maintaining the compatibility
> and fixing the trivial bugs which I left in the code.
> 
> I would also like to note that crypto module also utilizes
> the rand module API to simplify access to cryptographically strong
> random number module since OTP 20. See crypto:rand_seed/{0,1}
> and crypto:rand_uniform/2. This is a good example to make use of
> rand module's extensible plugins.
> 
> Thanks to Tuncer Ayaz for giving me a chance to review this.
> 
> And always remember: if you are still dependent on random module,
> migrate to rand module now.
> 
> Regards,
> Kenji Rikitake

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-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB