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author | Krasimir Georgiev <krasimir@google.com> | 2017-11-08 13:28:53 +0000 |
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committer | Krasimir Georgiev <krasimir@google.com> | 2017-11-08 13:28:53 +0000 |
commit | e02f32ba0e6d794e6196aca69f14f8130d0bd455 (patch) | |
tree | ccf3e31369a3c283a51c3d77cf9f8f2f4fa82e54 | |
parent | fc8e981f5daea5ccb75ef1dbe9052bc3ca2c0ec0 (diff) |
[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
git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@317689 91177308-0d34-0410-b5e6-96231b3b80d8
-rw-r--r-- | docs/clang-tidy/checks/modernize-replace-random-shuffle.rst | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst b/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst index 353f35a8..50674d42 100644 --- a/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst +++ b/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); + }()); |