[clang-tidy] Add a note about modernize-replace-random-shuffle

Summary:
This adds a note warning the users that the way the suggested fix seeds the
random number generator is poor.

Reviewers: hokein

Reviewed By: hokein

Subscribers: cfe-commits, xazax.hun

Differential Revision: https://reviews.llvm.org/D39787

llvm-svn: 317689

diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
index 353f35a..50674d4 100644 (file)
--- a/clang-tools-extra/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
@@ -26,3 +26,16 @@ Both of these examples will be replaced with:
 The second example will also receive a warning that ``randomFunc`` is no longer supported in the same way as before so if the user wants the same functionality, the user will need to change the implementation of the ``randomFunc``.
 
 One thing to be aware of here is that ``std::random_device`` is quite expensive to initialize. So if you are using the code in a performance critical place, you probably want to initialize it elsewhere. 
+Another thing is that the seeding quality of the suggested fix is quite poor: ``std::mt19937`` has an internal state of 624 32-bit integers, but is only seeded with a single integer. So if you require
+higher quality randomness, you should consider seeding better, for example:
+
+.. code-block:: c++
+
+  std::shuffle(v.begin(), v.end(), []() {
+    std::mt19937::result_type seeds[std::mt19937::state_size];
+    std::random_device device;
+    std::uniform_int_distribution<typename std::mt19937::result_type> dist;
+    std::generate(std::begin(seeds), std::end(seeds), [&] { return dist(device); });
+    std::seed_seq seq(std::begin(seeds), std::end(seeds));
+    return std::mt19937(seq);
+  }());