Fix PR#20843: binomial_distribution<unsigned> is broken. Add test to ensure that signed and unsigned verstions produce the same sequence.

llvm-svn: 217976

2 files changed, 15 insertions, 1 deletions

diff --git a/libcxx/include/random b/libcxx/include/random
index c0db1abacc0..e7053ce4ecf 100644
--- a/libcxx/include/random
+++ b/libcxx/include/random
@@ -4009,6 +4009,8 @@ binomial_distribution<_IntType>::param_type::param_type(result_type __t, double
     }
 }
 
+// Reference: Kemp, C.D. (1986). `A modal method for generating binomial
+//           variables', Commun. Statist. - Theor. Meth. 15(3), 805-813.
 template<class _IntType>
 template<class _URNG>
 _IntType
@@ -4035,7 +4037,8 @@ binomial_distribution<_IntType>::operator()(_URNG& __g, const param_type& __pr)
             if (__u < 0)
                 return __rd - 1;
         }
-        --__rd;
+        if ( __rd != 0 )
+            --__rd;
         ++__ru;
         if (__ru <= __pr.__t_)
         {
diff --git a/libcxx/test/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.bin/eval.pass.cpp b/libcxx/test/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.bin/eval.pass.cpp
index 3bfd2af6106..85ff2ac4750 100644
--- a/libcxx/test/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.bin/eval.pass.cpp
+++ b/libcxx/test/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.bin/eval.pass.cpp
@@ -322,6 +322,17 @@ int main()
         assert(std::abs((kurtosis - x_kurtosis) / x_kurtosis) < 0.01);
     }
     {
+        const int N = 100000;
+        std::mt19937 gen1;
+        std::mt19937 gen2;
+
+        std::binomial_distribution<>         dist1(5, 0.1);
+        std::binomial_distribution<unsigned> dist2(5, 0.1);
+
+        for(int i = 0; i < N; ++i)
+            assert(dist1(gen1) == dist2(gen2));
+    }
+    {
         typedef std::binomial_distribution<> D;
         typedef std::mt19937 G;
         G g;