[Bug libstdc++/86880] Incorrect mersenne_twister_engine equality comparison between rotated states

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86880

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This would fix the equality operator to correctly compare rotated states:

--- a/libstdc++-v3/include/bits/random.h
+++ b/libstdc++-v3/include/bits/random.h
@@ -601,8 +601,37 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       friend bool
       operator==(const mersenne_twister_engine& __lhs,
                 const mersenne_twister_engine& __rhs)
-      { return (std::equal(__lhs._M_x, __lhs._M_x + state_size, __rhs._M_x)
-               && __lhs._M_p == __rhs._M_p); }
+      {
+       const _UIntType* const __lx = __lhs._M_x;
+       const _UIntType* const __rx = __rhs._M_x;
+       size_t __lp = __lhs._M_p % state_size;
+       size_t __rp = __rhs._M_p % state_size;
+       size_t __n1, __n2;
+       if (__lp > __rp)
+         {
+           __n1 = state_size - __lp;
+           __n2 = __lp - __rp;
+         }
+       else
+         {
+           __n1 = state_size - __rp;
+           __n2 = __rp - __lp;
+         }
+       if (!std::equal(__lx + __lp, __lx + __lp + __n1, __rx + __rp))
+         return false;
+       if (__n1 == state_size) // i.e. __lhs._M_p == 0 && __rhs._M_p == 0
+         return true;
+       __lp = (__lp + __n1) % state_size;
+       __rp = (__rp + __n1) % state_size;
+       if (!std::equal(__lx + __lp, __lx + __lp + __n2, __rx + __rp))
+         return false;
+       __lp = (__lp + __n2) % state_size;
+       __rp = (__rp + __n2) % state_size;
+       size_t __n3 = state_size - __n1 - __n2;
+       if (!std::equal(__lx + __lp, __lx + __lp + __n3, __rx + __rp))
+         return false;
+       return true;
+      }

       /**
        * @brief Inserts the current state of a % mersenne_twister_engine


But the testcase above still fails, because mteB and mteA have different
content in the array, even though they produce the same sequence of numbers.