[Bug libstdc++/60441] New: Incorrect textual representation for std::mersenne_twister_engine

[Bug libstdc++/60441] New: Incorrect textual representation for std::mersenne_twister_engine

[koherde at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60441

            Bug ID: 60441
           Summary: Incorrect textual representation for
                    std::mersenne_twister_engine
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: koherde at hotmail dot com

The textual representation of std::mersenne_twister_engine should consist of n
values (n being the state size). libstdc++ uses a textual representation that
consists of n+1 values.
----------------------------------
#include <iostream>
#include <random>
#include <sstream>

int main()
{
    std::mt19937 r;
    std::stringstream ss;
    ss << r;
    int valueCount = 0;
    std::uint32_t val;
    while (ss >> val)
    {
        ++valueCount;
    }
    std::cout << r.state_size << std::endl;
    std::cout << valueCount << std::endl;
}
----------------------------------
Output:
    624
    625
Expected output:
    624
    624

[Bug libstdc++/60441] Incorrect textual representation for std::mersenne_twister_engine

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60441

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2018-08-07 00:00:00         |2023-5-23

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The fix is quite simple, but it breaks compatibility between releases:

diff --git a/libstdc++-v3/include/bits/random.tcc
b/libstdc++-v3/include/bits/random.tcc
index 24a5987db0d..8f53d5c5f53 100644
--- a/libstdc++-v3/include/bits/random.tcc
+++ b/libstdc++-v3/include/bits/random.tcc
@@ -486,9 +486,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __os.flags(__ios_base::dec | __ios_base::fixed | __ios_base::left);
       __os.fill(__space);

-      for (size_t __i = 0; __i < __n; ++__i)
+      for (size_t __i = __x._M_p; __i < __n; ++__i)
+       __os << __x._M_x[__i] << __space;
+      for (size_t __i = 0; __i < __x._M_p; ++__i)
        __os << __x._M_x[__i] << __space;
-      __os << __x._M_p;

       __os.flags(__flags);
       __os.fill(__fill);
@@ -512,7 +513,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

       for (size_t __i = 0; __i < __n; ++__i)
        __is >> __x._M_x[__i];
-      __is >> __x._M_p;
+      __x._M_p = 0;

       __is.flags(__flags);
       return __is;

[Bug libstdc++/60441] Incorrect textual representation for std::mersenne_twister_engine

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60441

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Ignore that, it doesn't work. Due to the way we've implemented the mersenne
twister state, we really do need to remember how much of the state has been
used already, which is why we serialize the _M_p index into the textual
representation.

[Bug libstdc++/60441] Incorrect textual representation for std::mersenne_twister_engine

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60441

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We have the same problem with std::subtract_with_carry_engine.