[PATCH] Fix Bug 83237 - Values returned by std::poisson_distribution are not distributed correctly

[Michele Pezzutti <mpezz at tiscali dot it>]

Hi.

This patch intends to fix Bug 83237 - Values returned by 
std::poisson_distribution are not distributed correctly.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83237for issue 
description and tests.


    * include/bits/random.tcc (poisson_distribution<_IntType>::operator()):
Value of the comparison function shall be set to 1/78 for __x = 1


diff --git a/random.tcc b/random.tcc
index 95bcf0a..f36587e 100644
--- a/random.tcc
+++ b/random.tcc
@@ -1301,6 +1301,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         const double __c2 = __param._M_c2b + __c1;
         const double __c3 = __c2 + 1;
         const double __c4 = __c3 + 1;
+        // 1 / 78
+        const double __178 = 0.0128205128205128205128205128205128L;
         // e^(1 / 78)
         const double __e178 = 1.0129030479320018583185514777512983L;
         const double __c5 = __c4 + __e178;
@@ -1340,7 +1342,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         else if (__u <= __c4)
           __x = 0;
         else if (__u <= __c5)
+          {
           __x = 1;
+          __w = __178;
+          }
         else
           {
             const double __v = -std::log(1.0 - __aurng());