[Boost-bugs] [Boost C++ Libraries] #7951: ranlux24_base(0) should be equal to ranlux24_base()

Subject: [Boost-bugs] [Boost C++ Libraries] #7951: ranlux24_base(0) should be equal to ranlux24_base()
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-01-30 02:58:51

#7951: ranlux24_base(0) should be equal to ranlux24_base()
----------------------------------------------------+-----------------------
 Reporter:  Stephan T. Lavavej <stl@…>              |       Owner:  no-maintainer
     Type:  Bugs                                    |      Status:  new          
Milestone:  To Be Determined                        |   Component:  random       
  Version:  Boost 1.52.0                            |    Severity:  Problem      
 Keywords:                                          |  
----------------------------------------------------+-----------------------
 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

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/7951>
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