https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82554
Bug ID: 82554
Summary: uniform_real_distribution can generate the upper
endpoint
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: b.r.longbons at gmail dot com
Target Milestone: ---
This is related to #64351 and #63176, for which a totally-bogus fix was
applied.
There is actually a defect in the standard: it requires `generate_canonical` to
return at *least* `bits` (e.g. 53), but correct operation is only possible when
it generates *exactly* `bits` since floating-point math rounds rather than
truncating like integer math.
Furthermore, when `std::uniform_real_distribution`'s parameters have different
exponents, the difference must be *subtracted* from the number of bits
requested (not possible since `bits` is only a template parameter).
I have not considered *all* the details when uniform_real_distribution crosses
0 or its parameters are not exact powers of 2, but the principle of requesting
less-than-53 bits certainly still applies.
Consider the following horrible, but valid RNG (for easy testing):
#include <iomanip>
#include <iostream>
#include <limits>
#include <random>
int main()
{
// seed with -2 instead of -1 because it returns the post-advance state
// produces -1, 0, 1, 2, ...
std::linear_congruential_engine<uint64_t, 1, 1, 0> engine(-2ULL);
std::cout << std::setprecision(std::numeric_limits<double>::max_digits10);
std::uniform_real_distribution<double> dist(1.0, 2.0);
for (int i = 0; i < 2; ++i)
std::cout << dist(engine) << std::endl;
// just to prove that yes, they are exact, here are the next values
// the different discards are because 1.0 has a different exponent
engine.discard(1 << 11);
std::cout << dist(engine) << std::endl;
engine.discard(1 << 12);
std::cout << dist(engine) << std::endl;
}
