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.
