On 04/12/2025 19:05, Juan Lucas Rey via Std-Proposals wrote:
> in the current proposal,
> https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p0952r2.html
>
> it is mentioned "In particular, code that depends on a specific
> sequence of results from repeated invocations, or on a particular
> number of calls to the URBG argument, will be broken."
>
> This solution avoids breaking this.

In the attached proposal within the implementation of exponential_distribution you have:

const auto u
= std::generate_canonical_centered<
     result_type,
     std::numeric_limits<result_type>::digits
>(g);
if (std::copysign(1.0, u) > 0) {
   return -std::log(result_type(1) - u) * lambda_inv;
} else {
   return -std::log(-u) * lambda_inv;
}

This is exactly equivalent to:

const auto u = ...
const auto u2
= std::copysign(1.0, u) > 0
   ? result_type(1) - u
   : -u;
return -std::log(u2) * lambda_inv;

Generating the centered uniform distribution seems redundant, you effectively map it to a
uniform distribution on [0, 1) anyway with u2. So this also boils down to fixing the
distribution on [0, 1), and centering doesn't give you anything for this particular use
case. You throw away your increased resolution at `result_type(1) - u`.

>
>
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