Re: [boost] [config] std::shuffle support

Subject: Re: [boost] [config] std::shuffle support
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2016-11-07 11:36:31

On 11/07/16 17:54, Edward Diener wrote:
> On 11/7/2016 7:28 AM, Peter Dimov wrote:
>> Andrey Semashev wrote:
>>
>>> FWIW, in Boost.Log I'm using my local random_shuffle[1] regardless of
>>> the standard library version. Maybe that would be a better solution
>>> than conditioning your code on config macros.
>>>
>>> [1]: https://github.com/boostorg/log/blob/develop/src/core.cpp#L60
>>
>> The standard random_shuffle doesn't use rng() % n though, it uses
>> rng(n). rng() % n is non-uniform in general.
>
> So substituting:
>
> Iterator where = begin + rng(it - begin + 1u);
>
> in Andrey's code would be fine according to the current random_shuffle
> implementations ?

In that particular code, rng does not implement operator() with an 
argument (it's an instance of random::taus88).

Yes, the code could be changed to use the canon operator() and use 
random::uniform_int_distribution internally to guarantee uniform 
distribution. I just chose not to waste CPU cycles because that level of 
correctness is not needed in that particular code.

I'll add that at least libstdc++ uses modulus division, like in 
Boost.Log, in std::random_shuffle that doesn't take random number 
generator as an argument. Any place that uses that overload of 
std::random_shuffle is already susceptible to the uniformity error.