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

[Raimo Niskanen]

On Tue, Aug 29, 2017 at 10:35:28AM +0200, Raimo Niskanen wrote:
> 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.

Sorry!  That should be:     from about 1/2^64 to about 1/2^53.

/ Raimo


> 
> 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
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/ Raimo Niskanen, Erlang/OTP, Ericsson AB