[Bug libstdc++/48114] <random> binomial_distribution incorrect for p > .5

["aaz at althenia dot net" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48114

--- Comment #3 from Andrey Zholos <aaz at althenia dot net> 2011-03-14 15:23:25 UTC ---
I'll add this here since it's in the same place and the test will be
similar.

For geometric_distribution(.25) the first 10 probabilities are:

        sample  correct
p_0     0.000   0.250
p_1     0.750   0.188
p_2     0.187   0.141
p_3     0.046   0.105
p_4     0.012   0.079
p_5     0.003   0.059
p_6     0.001   0.044
p_7     0.000   0.033
p_8     0.000   0.025
p_9     0.000   0.019

The smallest value returned is 1, but should be 0 (and d.min() is
correctly 0); and p is used as 1-p.

Fix:

--- include/c++/bits/random.h
+++ include/c++/bits/random.h
@@ -3628,8 +3628,8 @@
        void
        _M_initialize()
-       { _M_log_p = std::log(_M_p); }
+       { _M_log_1_p = std::log(1 - _M_p); }

        double _M_p;

-       double _M_log_p;
+       double _M_log_1_p;
       };

--- include/c++/bits/random.tcc
+++ include/c++/bits/random.tcc
@@ -1027,3 +1027,3 @@
        do
-         __cand = std::ceil(std::log(__aurng()) / __param._M_log_p);
+         __cand = std::floor(std::log(__aurng()) / __param._M_log_1_p);
        while (__cand >= __thr);

Also, there's a line in random.h

          _GLIBCXX_DEBUG_ASSERT((_M_p >= 0.0)
                             && (_M_p <= 1.0));

but the standard only requires 0 < p < 1.