[libc++] Unspecified behavior randomization in libc++
This effort is dedicated to deflake the tests of the users which depend
on the unspecified behavior of algorithms and containers. This also
might help updating the sorting algorithm in libcxx which has the
quadratic worst case in the future or at least create a new one under
flag.
For detailed design, please see the design doc I provide in the patch.
Differential Revision: https://reviews.llvm.org/D96946
GitOrigin-RevId: a45d2287adf7942a26be02193731701e61eda7b4
diff --git a/docs/DesignDocs/DebugMode.rst b/docs/DesignDocs/DebugMode.rst
index abcd5e5..faf7b8e 100644
--- a/docs/DesignDocs/DebugMode.rst
+++ b/docs/DesignDocs/DebugMode.rst
@@ -58,6 +58,15 @@
The remaining containers do not currently support iterator debugging.
Patches welcome.
+Randomizing Unspecified Behavior (``_LIBCPP_DEBUG == 1``)
+---------------------------------------------------------
+This also enables the randomization of unspecified behavior, for
+example, for equal elements in ``std::sort`` or randomizing both parts of
+the partition after ``std::nth_element`` call. This effort helps you to migrate
+to potential future faster versions of these algorithms and deflake your tests
+which depend on such behavior. To fix the seed, use
+``_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY_SEED=seed`` definition.
+
Handling Assertion Failures
===========================
When a debug assertion fails the assertion handler is called via the
diff --git a/docs/DesignDocs/UnspecifiedBehaviorRandomization.rst b/docs/DesignDocs/UnspecifiedBehaviorRandomization.rst
new file mode 100644
index 0000000..783950b
--- /dev/null
+++ b/docs/DesignDocs/UnspecifiedBehaviorRandomization.rst
@@ -0,0 +1,86 @@
+==================================
+Unspecified Behavior Randomization
+==================================
+
+Background
+==========
+
+Consider the follow snippet which steadily happens in tests:
+
+
+.. code-block:: cpp
+
+ std::vector<std::pair<int, int>> v(SomeData());
+ std::sort(v.begin(), v.end(), [](const auto& lhs, const auto& rhs) {
+ return lhs.first < rhs.first;
+ });
+
+Under this assumption all elements in the vector whose first elements are equal
+do not guarantee any order. Unfortunately, this prevents libcxx introducing
+other implementatiosn because tests might silently fail and the users might
+heavily depend on the stability of implementations.
+
+Goal
+===================
+
+Provide functionality for randomizing the unspecified behavior so that the users
+can test and migrate their components and libcxx can introduce new sorting
+algorithms and optimizations to the containers.
+
+For example, as of LLVM version 13, libcxx sorting algorithm takes
+`O(n^2) worst case <https://llvm.org/PR20837>`_ but according
+to the standard its worst case should be `O(n log n)`. This effort helps users
+to gradually fix their tests while updating to new faster algorithms.
+
+Design
+======
+
+* Introduce new macro `_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY` which should
+ be a part of the libcxx config.
+* This macro randomizes the unspecified behavior of algorithms and containers.
+ For example, for sorting algorithm the input range is shuffled and then
+ sorted.
+* This macro is off by default because users should enable it only for testing
+ purposes and/or migrations if they happen to libcxx.
+* This feature is only available for C++11 and further because of
+ `std::shuffle` availability.
+* We may use `ASLR <https://en.wikipedia.org/wiki/Address_space_layout_randomization>`_ or
+ static `std::random_device` for seeding the random number generator. This
+ guarantees the same stability guarantee within a run but not through different
+ runs, for example, for tests become flaky and eventually be seen as broken.
+ For platforms which do not support ASLR, the seed is fixed during build.
+* The users can fix the seed of the random number generator by providing
+ `_LIBCPP_RANDOMIZE_UNSPECIFIED_STABILITY_SEED=seed` definition.
+
+This comes with some side effects if any of the flags is on:
+
+* Computation penalty, we think users are OK with that if they use this feature.
+* Non reproducible results if they don't use the fixed seed.
+
+
+Impact
+------------------
+
+Google has measured couple of thousands of tests to be dependent on the
+stability of sorting and selection algorithms. As we also plan on updating
+(or least, providing under flag more) sorting algorithms, this effort helps
+doing it gradually and sustainably. This is also bad for users to depend on the
+unspecified behavior in their tests, this effort helps to turn this flag in
+debug mode.
+
+Potential breakages
+-------------------
+
+None if the flag is off. If the flag is on, it may lead to some non-reproducible
+results, for example, for caching.
+
+Currently supported randomization
+---------------------------------
+
+* `std::sort`, there is no guarantee on the order of equal elements
+* `std::partial_sort`, there is no guarantee on the order of equal elements and
+ on the order of the remaining part
+* `std::nth_element`, there is no guarantee on the order from both sides of the
+ partition
+
+Patches welcome.
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 62e5a0c..11c7798 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -51,6 +51,10 @@
added. This is useful for building libc++ in an embedded setting, and it adds itself to the various
freestanding-friendly options provided by libc++.
+- ``_LIBCPP_DEBUG`` equals to ``1`` enables the randomization of unspecified
+ behavior of standard algorithms (e.g. equal elements in ``std::sort`` or
+ randomization of both sides of partition for ``std::nth_element``)
+
API Changes
-----------
diff --git a/docs/index.rst b/docs/index.rst
index c171656..a150232 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -177,6 +177,7 @@
DesignDocs/NoexceptPolicy
DesignDocs/ThreadingSupportAPI
DesignDocs/UniquePtrTrivialAbi
+ DesignDocs/UnspecifiedBehaviorRandomization
DesignDocs/VisibilityMacros
diff --git a/include/__algorithm/comp_ref_type.h b/include/__algorithm/comp_ref_type.h
index 6d14c55..6cc6405 100644
--- a/include/__algorithm/comp_ref_type.h
+++ b/include/__algorithm/comp_ref_type.h
@@ -11,6 +11,11 @@
#include <__config>
+#ifdef _LIBCPP_DEBUG
+# include <__debug>
+# include <__utility/declval.h>
+#endif
+
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif
diff --git a/include/__algorithm/nth_element.h b/include/__algorithm/nth_element.h
index bec6355..63feba1 100644
--- a/include/__algorithm/nth_element.h
+++ b/include/__algorithm/nth_element.h
@@ -16,6 +16,10 @@
#include <__iterator/iterator_traits.h>
#include <__utility/swap.h>
+#if defined(_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY)
+# include <__algorithm/shuffle.h>
+#endif
+
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif
@@ -222,8 +226,13 @@
void
nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
{
- typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
- _VSTD::__nth_element<_Comp_ref>(__first, __nth, __last, __comp);
+ _LIBCPP_DEBUG_RANDOMIZE_RANGE(__first, __last);
+ typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
+ _VSTD::__nth_element<_Comp_ref>(__first, __nth, __last, __comp);
+ _LIBCPP_DEBUG_RANDOMIZE_RANGE(__first, __nth);
+ if (__nth != __last) {
+ _LIBCPP_DEBUG_RANDOMIZE_RANGE(++__nth, __last);
+ }
}
template <class _RandomAccessIterator>
diff --git a/include/__algorithm/partial_sort.h b/include/__algorithm/partial_sort.h
index b1d2e1a..622624e 100644
--- a/include/__algorithm/partial_sort.h
+++ b/include/__algorithm/partial_sort.h
@@ -18,6 +18,10 @@
#include <__iterator/iterator_traits.h>
#include <__utility/swap.h>
+#if defined(_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY)
+# include <__algorithm/shuffle.h>
+#endif
+
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif
@@ -48,8 +52,10 @@
partial_sort(_RandomAccessIterator __first, _RandomAccessIterator __middle, _RandomAccessIterator __last,
_Compare __comp)
{
- typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
- _VSTD::__partial_sort<_Comp_ref>(__first, __middle, __last, __comp);
+ _LIBCPP_DEBUG_RANDOMIZE_RANGE(__first, __last);
+ typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
+ _VSTD::__partial_sort<_Comp_ref>(__first, __middle, __last, __comp);
+ _LIBCPP_DEBUG_RANDOMIZE_RANGE(__middle, __last);
}
template <class _RandomAccessIterator>
diff --git a/include/__algorithm/shuffle.h b/include/__algorithm/shuffle.h
index 292e60d..7f6ad50 100644
--- a/include/__algorithm/shuffle.h
+++ b/include/__algorithm/shuffle.h
@@ -25,6 +25,40 @@
_LIBCPP_BEGIN_NAMESPACE_STD
+class _LIBCPP_TYPE_VIS __libcpp_debug_randomizer {
+public:
+ __libcpp_debug_randomizer() {
+ __state = __seed();
+ __inc = __state + 0xda3e39cb94b95bdbULL;
+ __inc = (__inc << 1) | 1;
+ }
+ typedef uint_fast32_t result_type;
+
+ static const result_type _Min = 0;
+ static const result_type _Max = 0xFFFFFFFF;
+
+ _LIBCPP_HIDE_FROM_ABI result_type operator()() {
+ uint_fast64_t __oldstate = __state;
+ __state = __oldstate * 6364136223846793005ULL + __inc;
+ return __oldstate >> 32;
+ }
+
+ static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR result_type min() { return _Min; }
+ static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR result_type max() { return _Max; }
+
+private:
+ uint_fast64_t __state;
+ uint_fast64_t __inc;
+ _LIBCPP_HIDE_FROM_ABI static uint_fast64_t __seed() {
+#ifdef _LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY_SEED
+ return _LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY_SEED;
+#else
+ static char __x;
+ return reinterpret_cast<uintptr_t>(&__x);
+#endif
+ }
+};
+
#if _LIBCPP_STD_VER <= 14 || defined(_LIBCPP_ENABLE_CXX17_REMOVED_RANDOM_SHUFFLE) \
|| defined(_LIBCPP_BUILDING_LIBRARY)
class _LIBCPP_TYPE_VIS __rs_default;
diff --git a/include/__algorithm/sort.h b/include/__algorithm/sort.h
index c0b602b..24d0a49 100644
--- a/include/__algorithm/sort.h
+++ b/include/__algorithm/sort.h
@@ -18,6 +18,10 @@
#include <__utility/swap.h>
#include <memory>
+#if defined(_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY)
+# include <__algorithm/shuffle.h>
+#endif
+
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif
@@ -529,12 +533,13 @@
void
sort(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
{
- typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
- if (__libcpp_is_constant_evaluated()) {
- _VSTD::__partial_sort<_Comp_ref>(__first, __last, __last, _Comp_ref(__comp));
- } else {
- _VSTD::__sort<_Comp_ref>(_VSTD::__unwrap_iter(__first), _VSTD::__unwrap_iter(__last), _Comp_ref(__comp));
- }
+ _LIBCPP_DEBUG_RANDOMIZE_RANGE(__first, __last);
+ typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
+ if (__libcpp_is_constant_evaluated()) {
+ _VSTD::__partial_sort<_Comp_ref>(__first, __last, __last, _Comp_ref(__comp));
+ } else {
+ _VSTD::__sort<_Comp_ref>(_VSTD::__unwrap_iter(__first), _VSTD::__unwrap_iter(__last), _Comp_ref(__comp));
+ }
}
template <class _RandomAccessIterator>
diff --git a/include/__config b/include/__config
index e537fe7..90c0631 100644
--- a/include/__config
+++ b/include/__config
@@ -858,16 +858,35 @@
// _LIBCPP_DEBUG potential values:
// - undefined: No assertions. This is the default.
// - 0: Basic assertions
-// - 1: Basic assertions + iterator validity checks.
-#if !defined(_LIBCPP_DEBUG)
-# define _LIBCPP_DEBUG_LEVEL 0
-#elif _LIBCPP_DEBUG == 0
-# define _LIBCPP_DEBUG_LEVEL 1
-#elif _LIBCPP_DEBUG == 1
-# define _LIBCPP_DEBUG_LEVEL 2
-#else
-# error Supported values for _LIBCPP_DEBUG are 0 and 1
-#endif
+// - 1: Basic assertions + iterator validity checks + unspecified behavior randomization.
+# if !defined(_LIBCPP_DEBUG)
+# define _LIBCPP_DEBUG_LEVEL 0
+# elif _LIBCPP_DEBUG == 0
+# define _LIBCPP_DEBUG_LEVEL 1
+# elif _LIBCPP_DEBUG == 1
+# define _LIBCPP_DEBUG_LEVEL 2
+# else
+# error Supported values for _LIBCPP_DEBUG are 0 and 1
+# endif
+
+# if _LIBCPP_DEBUG_LEVEL >= 2 && !defined(_LIBCPP_CXX03_LANG)
+# define _LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY
+# endif
+
+# if defined(_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY)
+# if defined(_LIBCPP_CXX03_LANG)
+# error Support for unspecified stability is only for C++11 and higher
+# endif
+# define _LIBCPP_DEBUG_RANDOMIZE_RANGE(__first, __last) \
+ do { \
+ if (!__builtin_is_constant_evaluated()) \
+ _VSTD::shuffle(__first, __last, __libcpp_debug_randomizer()); \
+ } while (false)
+# else
+# define _LIBCPP_DEBUG_RANDOMIZE_RANGE(__first, __last) \
+ do { \
+ } while (false)
+# endif
// Libc++ allows disabling extern template instantiation declarations by
// means of users defining _LIBCPP_DISABLE_EXTERN_TEMPLATE.
diff --git a/test/libcxx/algorithms/nth_element_stability.pass.cpp b/test/libcxx/algorithms/nth_element_stability.pass.cpp
new file mode 100644
index 0000000..b680b87
--- /dev/null
+++ b/test/libcxx/algorithms/nth_element_stability.pass.cpp
@@ -0,0 +1,103 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <algorithm>
+
+// Test std::nth_element stability randomization
+
+// UNSUPPORTED: libcxx-no-debug-mode
+// UNSUPPORTED: c++03
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=1
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <functional>
+#include <iterator>
+#include <vector>
+
+#include "test_macros.h"
+
+struct MyType {
+ int value = 0;
+ constexpr bool operator<(const MyType& other) const { return value < other.value; }
+};
+
+std::vector<MyType> deterministic() {
+ static constexpr int kSize = 100;
+ std::vector<MyType> v;
+ v.resize(kSize);
+ for (int i = 0; i < kSize; ++i) {
+ v[i].value = (i % 2 ? i : kSize / 2 + i);
+ }
+ std::__nth_element(v.begin(), v.begin() + kSize / 2, v.end(), std::less<MyType>());
+ return v;
+}
+
+void test_randomization() {
+ static constexpr int kSize = 100;
+ std::vector<MyType> v;
+ v.resize(kSize);
+ for (int i = 0; i < kSize; ++i) {
+ v[i].value = (i % 2 ? i : kSize / 2 + i);
+ }
+ auto deterministic_v = deterministic();
+ std::nth_element(v.begin(), v.begin() + kSize / 2, v.end());
+ bool all_equal = true;
+ for (int i = 0; i < kSize; ++i) {
+ if (v[i].value != deterministic_v[i].value) {
+ all_equal = false;
+ }
+ }
+ assert(!all_equal);
+}
+
+void test_same() {
+ static constexpr int kSize = 100;
+ std::vector<MyType> v;
+ v.resize(kSize);
+ for (int i = 0; i < kSize; ++i) {
+ v[i].value = (i % 2 ? i : kSize / 2 + i);
+ }
+ auto snapshot_v = v;
+ auto snapshot_custom_v = v;
+ std::nth_element(v.begin(), v.begin() + kSize / 2, v.end());
+ std::nth_element(snapshot_v.begin(), snapshot_v.begin() + kSize / 2, snapshot_v.end());
+ std::nth_element(snapshot_custom_v.begin(), snapshot_custom_v.begin() + kSize / 2, snapshot_custom_v.end(), std::less<MyType>());
+ bool all_equal = true;
+ for (int i = 0; i < kSize; ++i) {
+ if (v[i].value != snapshot_v[i].value || v[i].value != snapshot_custom_v[i].value) {
+ all_equal = false;
+ }
+ if (i < kSize / 2) {
+ assert(v[i].value <= v[kSize / 2].value);
+ }
+ }
+ assert(all_equal);
+}
+
+#if TEST_STD_VER > 17
+constexpr bool test_constexpr() {
+ std::array<MyType, 10> v;
+ for (int i = 9; i >= 0; --i) {
+ v[9 - i].value = i;
+ }
+ std::nth_element(v.begin(), v.begin() + 5, v.end());
+ return std::is_partitioned(v.begin(), v.end(), [&](const MyType& m) { return m.value <= v[5].value; });
+}
+#endif
+
+int main(int, char**) {
+ test_randomization();
+ test_same();
+#if TEST_STD_VER > 17
+ static_assert(test_constexpr(), "");
+#endif
+ return 0;
+}
diff --git a/test/libcxx/algorithms/partial_sort_stability.pass.cpp b/test/libcxx/algorithms/partial_sort_stability.pass.cpp
new file mode 100644
index 0000000..0fdd41f
--- /dev/null
+++ b/test/libcxx/algorithms/partial_sort_stability.pass.cpp
@@ -0,0 +1,103 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <algorithm>
+
+// Test std::partial_sort stability randomization
+
+// UNSUPPORTED: libcxx-no-debug-mode
+// UNSUPPORTED: c++03
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=1
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <functional>
+#include <iterator>
+#include <vector>
+
+#include "test_macros.h"
+
+struct MyType {
+ int value = 0;
+ constexpr bool operator<(const MyType& other) const { return value < other.value; }
+};
+
+std::vector<MyType> deterministic() {
+ static constexpr int kSize = 100;
+ std::vector<MyType> v;
+ v.resize(kSize);
+ for (int i = 0; i < kSize; ++i) {
+ v[i].value = (i % 2 ? 1 : kSize / 2 + i);
+ }
+ std::__partial_sort(v.begin(), v.begin() + kSize / 2, v.end(), std::less<MyType>());
+ return v;
+}
+
+void test_randomization() {
+ static constexpr int kSize = 100;
+ std::vector<MyType> v;
+ v.resize(kSize);
+ for (int i = 0; i < kSize; ++i) {
+ v[i].value = (i % 2 ? 1 : kSize / 2 + i);
+ }
+ auto deterministic_v = deterministic();
+ std::partial_sort(v.begin(), v.begin() + kSize / 2, v.end());
+ bool all_equal = true;
+ for (int i = 0; i < kSize; ++i) {
+ if (v[i].value != deterministic_v[i].value) {
+ all_equal = false;
+ }
+ }
+ assert(!all_equal);
+}
+
+void test_same() {
+ static constexpr int kSize = 100;
+ std::vector<MyType> v;
+ v.resize(kSize);
+ for (int i = 0; i < kSize; ++i) {
+ v[i].value = (i % 2 ? 1 : kSize / 2 + i);
+ }
+ auto snapshot_v = v;
+ auto snapshot_custom_v = v;
+ std::partial_sort(v.begin(), v.begin() + kSize / 2, v.end());
+ std::partial_sort(snapshot_v.begin(), snapshot_v.begin() + kSize / 2, snapshot_v.end());
+ std::partial_sort(snapshot_custom_v.begin(), snapshot_custom_v.begin() + kSize / 2, snapshot_custom_v.end(), std::less<MyType>());
+ bool all_equal = true;
+ for (int i = 0; i < kSize; ++i) {
+ if (v[i].value != snapshot_v[i].value || v[i].value != snapshot_custom_v[i].value) {
+ all_equal = false;
+ }
+ if (i < kSize / 2) {
+ assert(v[i].value == 1);
+ }
+ }
+ assert(all_equal);
+}
+
+#if TEST_STD_VER > 17
+constexpr bool test_constexpr() {
+ std::array<MyType, 10> v;
+ for (int i = 9; i >= 0; --i) {
+ v[9 - i].value = i;
+ }
+ std::partial_sort(v.begin(), v.begin() + 5, v.end());
+ return std::is_sorted(v.begin(), v.begin() + 5);
+}
+#endif
+
+int main(int, char**) {
+ test_randomization();
+ test_same();
+#if TEST_STD_VER > 17
+ static_assert(test_constexpr(), "");
+#endif
+ return 0;
+}
diff --git a/test/libcxx/algorithms/sort_stability.pass.cpp b/test/libcxx/algorithms/sort_stability.pass.cpp
new file mode 100644
index 0000000..451eeef
--- /dev/null
+++ b/test/libcxx/algorithms/sort_stability.pass.cpp
@@ -0,0 +1,100 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <algorithm>
+
+// Test std::sort stability randomization
+
+// UNSUPPORTED: libcxx-no-debug-mode
+// UNSUPPORTED: c++03
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=1
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <iterator>
+#include <vector>
+
+#include "test_macros.h"
+
+struct EqualType {
+ int value = 0;
+ constexpr bool operator<(const EqualType&) const { return false; }
+};
+
+std::vector<EqualType> deterministic() {
+ static constexpr int kSize = 100;
+ std::vector<EqualType> v;
+ v.resize(kSize);
+ for (int i = 0; i < kSize; ++i) {
+ v[i].value = kSize / 2 - i * (i % 2 ? -1 : 1);
+ }
+ std::__sort(v.begin(), v.end(), std::less<EqualType>());
+ return v;
+}
+
+void test_randomization() {
+ static constexpr int kSize = 100;
+ std::vector<EqualType> v;
+ v.resize(kSize);
+ for (int i = 0; i < kSize; ++i) {
+ v[i].value = kSize / 2 - i * (i % 2 ? -1 : 1);
+ }
+ auto deterministic_v = deterministic();
+ std::sort(v.begin(), v.end());
+ bool all_equal = true;
+ for (int i = 0; i < kSize; ++i) {
+ if (v[i].value != deterministic_v[i].value) {
+ all_equal = false;
+ }
+ }
+ assert(!all_equal);
+}
+
+void test_same() {
+ static constexpr int kSize = 100;
+ std::vector<EqualType> v;
+ v.resize(kSize);
+ for (int i = 0; i < kSize; ++i) {
+ v[i].value = kSize / 2 - i * (i % 2 ? -1 : 1);
+ }
+ auto snapshot_v = v;
+ auto snapshot_custom_v = v;
+ std::sort(v.begin(), v.end());
+ std::sort(snapshot_v.begin(), snapshot_v.end());
+ std::sort(snapshot_custom_v.begin(), snapshot_custom_v.end(),
+ [](const EqualType&, const EqualType&) { return false; });
+ bool all_equal = true;
+ for (int i = 0; i < kSize; ++i) {
+ if (v[i].value != snapshot_v[i].value || v[i].value != snapshot_custom_v[i].value) {
+ all_equal = false;
+ }
+ }
+ assert(all_equal);
+}
+
+#if TEST_STD_VER > 17
+constexpr bool test_constexpr() {
+ std::array<EqualType, 10> v;
+ for (int i = 9; i >= 0; --i) {
+ v[9 - i].value = i;
+ }
+ std::sort(v.begin(), v.end());
+ return std::is_sorted(v.begin(), v.end());
+}
+#endif
+
+int main(int, char**) {
+ test_randomization();
+ test_same();
+#if TEST_STD_VER > 17
+ static_assert(test_constexpr(), "");
+#endif
+ return 0;
+}
