"You keep saying "canonical_distribution" ... do you mean
std::generate_canonical, or one of the random number distributions in
<random>, or some non-standard random number distribution in your own
code?"

I mean std::generate_canonical, yes.

"But your proposal returns negative numbers for those 10 values. It's
highly debatable whether that is a "better distribution" given that
those values are outside the [0,1) range!
Your results might be more uniformly distributed over some range, but
it's a different range!"

My suggestion to use "generate_canonical_centered" inside
"std::exponential_distribution" (as proposed in the sample file I
sent) does return 10 different values for the extremes. what libstd++
is proposing is to return the same value for those 10 cases. As
explained before, the purpose here is to have that different range,
containing better precision, especially in the right limit, being
properly handled in the other distributions.

Lucas



On Fri, 5 Dec 2025 at 13:59, Jonathan Wakely <cxx_at_[hidden]> wrote:
>
>
>
> On Fri, 5 Dec 2025 at 13:51, Juan Lucas Rey <juanlucasrey_at_[hidden]> wrote:
>>
>> Hi,
>>
>> "What do you mean by "discard RNGs"? Deterministically discard values
>> produced from RNGs?"
>>
>> Yes. Assume I want to simulate 10.000 Monte Carlo paths in one process
>> and the next 10.000 Monte Carlo paths in another process. Assuming I
>> know that each path uses Y RNGs, then I would have to call
>> rng.discard(10000 * Y) on the second process before using it.
>> With p0952r2 now I need to actually produce the numbers with
>> "canonical_distribution", and if any of those gives 1.0 I have to add
>> 1 to 10000 *Y. This defeats the purpose of having fast discard
>> methods.
>
>
> You keep saying "canonical_distribution" ... do you mean std::generate_canonical, or one of the random number distributions in <random>, or some non-standard random number distribution in your own code?
>
>>
>>
>> "That could be achieved the same way libstdc++ does it: when
>> generate_canonical would return 1.0, just return
>> std::nextafter(RealType(1), RealType(0)) instead. We don't need a
>> completely different function that doesn't return values in the range
>> [0,1) and which needs every caller to be adjusted for the new output
>> range."
>>
>> My proposal actually allows for more control. say for example all 10
>> values [RNG::max() -10, RNG::max()] return 1.0 with
>> canonical_distribution. libstd++ proposes to return
>> std::nextafter(RealType(1), RealType(0)) for these 10 values. What I
>> am proposing is to return different values for those 10 results,
>> allowing for better distribution and avoiding the loss of information
>> from multiple distinct RNG outputs being mapped to the same
>> floating-point value.
>
>
> But your proposal returns negative numbers for those 10 values. It's highly debatable whether that is a "better distribution" given that those values are outside the [0,1) range!
> Your results might be more uniformly distributed over some range, but it's a different range!
>
>>
>>
>> From what you tell me then though, it is not worth pursuing this
>> further because of timing?
>
>
>
> It's going to be very difficult to change anything now.
>
> If you want to try to convince the committee to undo the P0952 changes (maybe as a defect report against C++26) then your proposal needs to explain (in detail!) why the new spec is a problem. Not just say "here's a different function with different properties". Proposing a completely different function won't change anything about the current contents of the <random> header.
>
> And if you don't want to change anything in <random>, but just need some other function for your own uses, why does that need to be in the standard? You can already use the function yourself.
>