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

[Kenji Rikitake]

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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