This revision was automatically updated to reflect the committed changes.
Closed by commit rL317689: [clang-tidy] Add a note about
modernize-replace-random-shuffle (authored by krasimir).
Repository:
rL LLVM
https://reviews.llvm.org/D39787
Files:
clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
Index:
clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
===================================================================
---
clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
+++
clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
@@ -26,3 +26,16 @@
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);
+ }());
Index: clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
===================================================================
--- clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
+++ clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst
@@ -26,3 +26,16 @@
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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