[PATCH] libstdc++: add uniform on sphere distribution

[Ulrich Drepper]

Jonathan Wakely <jwakely@redhat.com> writes:

> On 23/07/14 11:58 +0200, Marc Glisse wrote:
> As an aside, we already have divide-by-zero bugs in <ext/random>, it
> would be nice if someone could look at that.
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60037

Sorry, it took a while to get back to tihs and now the referenced bug is
already fixed.  Good.  Now also for a fix of the sphere distribution.
Unless someone objects I'll check in the patch below.


2014-08-08  Ulrich Drepper  <drepper@gmail.com>

	* include/ext/random.tcc
	(uniform_on_sphere_distribution::__generate_impl): Reject
	vectors with norm zero.


diff --git a/libstdc++-v3/include/ext/random.tcc b/libstdc++-v3/include/ext/random.tcc
index 05361d8..d1f0b9c 100644
--- a/libstdc++-v3/include/ext/random.tcc
+++ b/libstdc++-v3/include/ext/random.tcc
@@ -1548,13 +1548,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		 const param_type& __p)
       {
 	result_type __ret;
-	_RealType __sum = _RealType(0);
+	_RealType __norm;
+
+	do
+	  {
+	    _RealType __sum = _RealType(0);
+
+	    std::generate(__ret.begin(), __ret.end(),
+		          [&__urng, &__sum, this](){
+			    _RealType __t = _M_nd(__urng);
+			    __sum += __t * __t;
+			    return __t; });
+	    __norm = std::sqrt(__sum);
+	  }
+	while (__norm == _RealType(0));
 
-	std::generate(__ret.begin(), __ret.end(),
-		      [&__urng, &__sum, this](){ _RealType __t = _M_nd(__urng);
-						 __sum += __t * __t;
-						 return __t; });
-	auto __norm = std::sqrt(__sum);
 	std::transform(__ret.begin(), __ret.end(), __ret.begin(),
 		       [__norm](_RealType __val){ return __val / __norm; });