ranlux24_base(0) should be equal to ranlux24_base()

[Stephan T. Lavavej <stl@…>]

After fixing a bug in VC11's <random> (see [1]), I compared VC's fixed output to Boost 1.52.0's output and I believe I found a bug in Boost. Here's a self-contained test case:

C:\Temp>type meow.cpp #include <ios> #include <iostream>

#ifdef USE_BOOST

#include <boost/random.hpp> using std::boolalpha; using std::cout; using std::endl; using boost::random::minstd_rand0; using boost::random::mt19937; using boost::random::ranlux24_base;

#else

#include <random> using namespace std;

#endif

template <typename Engine> typename Engine::result_type run_10k(Engine engine) {

for (int i = 0; i < 9999; ++i) {

engine();

}

return engine();

}

int main() {

cout << boolalpha;

#ifdef USE_BOOST

cout << "Using boost." << endl;

#else

cout << "Using std." << endl;

#endif

cout << "N3485 26.5.5 [rand.predef]/1 satisfied: " << (run_10k( minstd_rand0()) == 1043618065) << endl; cout << "N3485 26.5.5 [rand.predef]/3 satisfied: " << (run_10k( mt19937()) == 4123659995) << endl; cout << "N3485 26.5.5 [rand.predef]/5 satisfied: " << (run_10k(ranlux24_base()) == 7937952 ) << endl;

cout << "I believe minstd_rand0(0) == minstd_rand0() should be true: "

<< (minstd_rand0(0) == minstd_rand0()) << endl;

cout << "I believe mt19937(0) == mt19937() should be false: "

<< (mt19937(0) == mt19937()) << endl;

cout << "I believe ranlux24_base(0) == ranlux24_base() should be true: "

<< (ranlux24_base(0) == ranlux24_base()) << endl;

cout << "run_10k( minstd_rand0(0)): " << run_10k( minstd_rand0(0)) << endl; cout << "run_10k( mt19937(0)): " << run_10k( mt19937(0)) << endl; cout << "run_10k(ranlux24_base(0)): " << run_10k(ranlux24_base(0)) << endl;

}

With my fixed build of Milan/VC12, I'm getting:

C:\Temp>cl /EHsc /nologo /W4 /MTd meow.cpp meow.cpp

C:\Temp>meow Using std. N3485 26.5.5 [rand.predef]/1 satisfied: true N3485 26.5.5 [rand.predef]/3 satisfied: true N3485 26.5.5 [rand.predef]/5 satisfied: true I believe minstd_rand0(0) == minstd_rand0() should be true: true I believe mt19937(0) == mt19937() should be false: false I believe ranlux24_base(0) == ranlux24_base() should be true: true run_10k( minstd_rand0(0)): 1043618065 run_10k( mt19937(0)): 1543171712 run_10k(ranlux24_base(0)): 7937952

(Note that VC11 RTM/Update 1 will suffer the bug described by [1].)

If I use Boost's implementation with VC11 Update 1, I get:

C:\Temp>cl /EHsc /nologo /W4 /MTd /DUSE_BOOST meow.cpp /I boost-1.52.0\include meow.cpp

C:\Temp>meow Using boost. N3485 26.5.5 [rand.predef]/1 satisfied: true N3485 26.5.5 [rand.predef]/3 satisfied: true N3485 26.5.5 [rand.predef]/5 satisfied: true I believe minstd_rand0(0) == minstd_rand0() should be true: true I believe mt19937(0) == mt19937() should be false: false I believe ranlux24_base(0) == ranlux24_base() should be true: false run_10k( minstd_rand0(0)): 1043618065 run_10k( mt19937(0)): 1543171712 run_10k(ranlux24_base(0)): 14007167

Here's why I believe that ranlux24_base(0) should be equal to ranlux24_base(). ranlux24_base is a subtract_with_carry_engine. 26.5.3.3 [rand.eng.sub]/7 says:

"explicit subtract_with_carry_engine(result_type value = default_seed); [...] linear_congruential_engine<result_type, 40014u,0u,2147483563u> e(value == 0u ? default_seed : value);"

/7 doesn't refer to "value" anywhere else. Therefore, 0 should behave identically to the default_seed 19780503u.

(For comparison, VC and Boost agree that minstd_rand0(0) == minstd_rand0(). 26.5.3.1 [rand.eng.lcong]/5 says:

"explicit linear_congruential_engine(result_type s = default_seed); [...] If c mod m is 0 and s mod m is 0, sets the engine's state to 1, otherwise sets the engine's state to s mod m."

The default_seed is 1u and minstd_rand0 sets c to 0, so that's why 0 behaves identically to the default_seed.)

[1] ​http://connect.microsoft.com/VisualStudio/feedback/details/776456