random.h (negative_binomial_distribution<>::…

random.h (negative_binomial_distribution<>:: negative_binomial_distribution(_IntType, double), [...]): Tweak construction of _M_gd.

2011-03-25  Paolo Carlini  <paolo.carlini@oracle.com>

	* include/bits/random.h (negative_binomial_distribution<>::
	negative_binomial_distribution(_IntType, double),
	negative_binomial_distribution<>::
	negative_binomial_distribution(const param_type&)): Tweak
	construction of _M_gd.
	* include/bits/random.tcc (negative_binomial_distribution<>::
	operator()): Adjust.

From-SVN: r171443

libstdc++-v3/ChangeLog

+2011-03-25  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	* include/bits/random.h (negative_binomial_distribution<>::
+	negative_binomial_distribution(_IntType, double),
+	negative_binomial_distribution<>::
+	negative_binomial_distribution(const param_type&)): Tweak
+	construction of _M_gd.
+	* include/bits/random.tcc (negative_binomial_distribution<>::
+	operator()): Adjust.
+
 2011-03-24  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* include/bits/random.h (negative_binomial_distribution<>::

libstdc++-v3/include/bits/random.h

@@ -3804,12 +3804,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       explicit
       negative_binomial_distribution(_IntType __k = 1, double __p = 0.5)
-      : _M_param(__k, __p), _M_gd(__k, 1.0)
+      : _M_param(__k, __p), _M_gd(__k, (1.0 - __p) / __p)
       { }
 
       explicit
       negative_binomial_distribution(const param_type& __p)
-      : _M_param(__p), _M_gd(__p.k(), 1.0)
+      : _M_param(__p), _M_gd(__p.k(), (1.0 - __p.p()) / __p.p())
       { }
 
       /**

libstdc++-v3/include/bits/random.tcc

@@ -1075,7 +1075,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __is;
     }
 
-  // This is Leger's algorithm.
+  // This is Leger's algorithm, also in Devroye, Ch. X, Example 1.5.
   template<typename _IntType>
     template<typename _UniformRandomNumberGenerator>
       typename negative_binomial_distribution<_IntType>::result_type
@@ -1085,8 +1085,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	const double __y = _M_gd(__urng);
 
 	// XXX Is the constructor too slow?
-	std::poisson_distribution<result_type> __poisson(__y * (1.0 - p())
-							 / p());
+	std::poisson_distribution<result_type> __poisson(__y);
 	return __poisson(__urng);
       }
 
@@ -1100,10 +1099,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	typedef typename std::gamma_distribution<result_type>::param_type
 	  param_type;
 	
-	const double __y = _M_gd(__urng, param_type(__p.k(), 1.0));
+	const double __y =
+	  _M_gd(__urng, param_type(__p.k(), (1.0 - __p.p()) / __p.p()));
 
-	std::poisson_distribution<result_type> __poisson(__y * (1.0 - __p.p())
-							 / __p.p() );
+	std::poisson_distribution<result_type> __poisson(__y);
 	return __poisson(__urng);
       }