[clang-tools-extra] r317689 - [clang-tidy] Add a note about modernize-replace-random-shuffle

Krasimir Georgiev via cfe-commits cfe-commits at lists.llvm.org
Wed Nov 8 05:28:53 PST 2017

Author: krasimir
Date: Wed Nov  8 05:28:53 2017
New Revision: 317689

URL: http://llvm.org/viewvc/llvm-project?rev=317689&view=rev
[clang-tidy] Add a note about modernize-replace-random-shuffle

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


Modified: clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
URL: http://llvm.org/viewvc/llvm-project/clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst?rev=317689&r1=317688&r2=317689&view=diff
--- clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst (original)
+++ clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst Wed Nov  8 05:28:53 2017
@@ -26,3 +26,16 @@ Both of these examples will be replaced
 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);
+  }());

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