What is the use case of RandomCover?

[Ivan Kazmenko]

Hi!

Thank you for the reply.

> Hum... randomShuffle and randomSample actually have nothing to 
> do with each other.
> <snip>

I'd like to note that my post is about randomCover, not 
randomSample.  I do see the difference between the purpose of 
randomSample and randomShuffle.  But randomCover's effect is, at 
the first glance, just a slower version of randomSample wrapped 
as a lazy generator.

> I also want to comment on your "randomSample" vs "randomSuffle" 
> implementation suggestion. Keep in mind that:
> a) randomSample doesn't allocate, whereas yours suggestion 
> doesn't
> b) randomSample gives direct access to the elements, whereas 
> your suggestion doesn't.
>
> If you don't care about a) and b), then by all means, dup away, 
> and get better performance!
>
> But think about the fact that you wouldn't be able to do 
> something like this...
> <snip>
>         auto sample = randomSample(arr[], 5);
>         foreach(ref a; sample)
>             ++a;

That stands for randomCover, too.  Well, thank you, perhaps 
that's the difference I was seeking.

If this is the intended difference, well, my proposition to 
enhance randomCover's performance and usefulness transforms into:

1. Document the usage of randomCover with an example such as 
above, and refer to randomShuffle as a faster version for simpler 
use cases.

2. Optimize the performance by putting Fenwick trees to good use. 
  Currently, randomCover'ing 10,000 elements takes time on the 
order of one second, and for 100,000 or more elements, it is 
hardly usable.

> Last but not least, be warned there is an old-standing bug with 
> anything in phobos that takes a PRNG by value. Basically, the 
> PRNG will be duplicated, and generate the same sequence over 
> and over. Ergo, do NOT pass a specific random generator to 
> things like randomSample or randomSuffle.
>
> This problem is one of the major reason we are currently (and 
> slowly) re-designing random into random2.

So, there is a general agreement that in random2, RNG should by 
default get passed by reference everywhere?  That's nice to hear.

-----
Ivan Kazmenko.