From 7dfabdf2d29f4c177a7dffb4539b763bac6f8a3b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ion=20Gazta=C3=B1aga?= <igaztanaga@gmail.com>
Date: Mon, 31 Dec 2018 00:55:28 +0100
Subject: [PATCH] Improve randomness of random_shuffle

---
 test/random_shuffle.hpp | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/test/random_shuffle.hpp b/test/random_shuffle.hpp
index 5041d96..907d195 100644
--- a/test/random_shuffle.hpp
+++ b/test/random_shuffle.hpp
@@ -6,13 +6,27 @@
 #include <boost/move/detail/iterator_traits.hpp>
 #include <stdlib.h>
 
+inline unsigned long long rand_15_bit()
+{
+   //Many rand implementation only use 15 bits
+   //so make sure we have only 15 bits
+   return (unsigned long long)((std::rand()) & 0x7fffu);
+}
+
+inline unsigned long long ullrand()
+{
+   return  (rand_15_bit() << 54u) ^ (rand_15_bit() << 39u) 
+         ^ (rand_15_bit() << 26u) ^ (rand_15_bit() << 13u)
+         ^ rand_15_bit();
+}
+
 template< class RandomIt >
 void random_shuffle( RandomIt first, RandomIt last )
 {
    typedef typename boost::movelib::iterator_traits<RandomIt>::difference_type difference_type;
    difference_type n = last - first;
    for (difference_type i = n-1; i > 0; --i) {
-      difference_type j = std::rand() % (i+1);
+      difference_type j = ullrand() % (i+1);
       if(j != i) {
          boost::adl_move_swap(first[i], first[j]);
       }