Re: [PATCH] Add example to rand.3

[Alejandro Colomar <alx.manpages@gmail.com>]

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Hi Branden,

On 12/28/22 01:00, G. Branden Robinson wrote:
> At 2022-12-28T00:33:13+0100, Alejandro Colomar wrote:
>> The only problem with arc4random() is the lack of repeatability.  When
>> testing a program with random data, you'll need repeatable results.
>> For that, rand(3) Just Works.  When you want unpredictable results,
>> you just seed it with some really random value, and you're fine.  You
>> need to be careful to not introduce bias, but there's nothing better
>> in libc.  It would be nice if libc provided a rand_uniform(3) variant
>> of rand(3), BTW.
> 
> Permit me to counsel against that last proposed name.  In probability
> theory "uniform" is already widely and well understood to indicate the
> nature of the distribution.  A "uniform" distribution is one in which
> each outcome is precisely equally likely.

That's exactly what I miss from the rand(3) interface; a variant that produces a 
value with uniform probability up to some maximum.  There's the same for the 
arc4random() family of functions:

<https://www.gnu.org/software/libc/manual/html_mono/libc.html#index-arc4random_005funiform>

For example I used something similar to this a long time ago:

     #include <stdbit.h>
     #include <stdint.h>
     #include <stdlib.h>

     uint32_t
     rand_uniform(uint32_t upper_bound)
     {
         uint32_t  r;

         do {
             r = rand();
             r %= stdc_bit_ceil(upper_bound);
         } while (r >= upper_bound);

         return r;
     }

(I used C23 syntax for rounding up to a power of 2, to avoid some magic macro. 
That line is not necessary, but improves performance considerably for small 
upper bounds.  I don't know if it's available already in GCC or Clang.)

That function is API-compatible with arc4random_uniform(), but it works with 
rand(3), so it can give repeatable results.  If I didn't screw it, it gives 
uniform results.

> 
> rand_deterministic() would get the right idea across but is lengthy.
> 
> Regards,
> Branden

Cheers,

Alex

> 
> P.S. I asked Bertrand to tag groff 1.23.0.rc2 earlier today.  Happy
>       belated Boxing Day.  ;-)

I saw :)  BTW, the Ada thingy, if it's opinionated as you said, it will help 
considerably to make it easier to read ;)

-- 
<http://www.alejandro-colomar.es/>

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