First implementation of flat sets

base/containers/flat_set_unittest.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/containers/flat_set.h"
+
+// Following tests are ported and extended tests from libcpp for std::set.
+// They can be found here:
+// https://github.com/llvm-mirror/libcxx/tree/master/test/std/containers/associative/set
+//
+// Not ported tests:
+// * No tests with PrivateConstructor and std::less<> changed to std::less<T>
+//   These tests have to do with C++14 std::less<>
+//   http://en.cppreference.com/w/cpp/utility/functional/less_void
+//   and add support for templated versions of lookup functions.
+//   Current implementation of flat containers doesn't support it.
+// * No tests with TemplateConstructor.
+//   Library working group issue: LWG #2059
+//   http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2059
+//   There is an ambiguity between erase with an iterator and erase with a key,
+//   if key has a templated constructor. We have to fix this.
+// * No tests for max_size()
+//   Has to do with allocator support.
+// * No tests with DefaultOnly.
+//   Standard containers allocate each element in the separate node on the heap
+//   and then manipulate these nodes. Flat containers store their elements in
+//   contiguous memory and move them around, type is required to be movable.
+// * No tests for N3644.
+//   This proposal suggests that all default constructed iterators compare
+//   equal. Currently we use std::vector iterators and they don't implement
+//   this.
+// * No tests with min_allocator and no tests counting allocations.
+//   Flat sets currently don't support allocators.
+// * No tests for range insertion. Flat sets currently do not support this
+//   functionality.
+
+#include <string>
+#include <vector>
+
+#include "base/macros.h"
+#include "testing/gmock/include/gmock/gmock.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace {
+
+class MoveOnly {
+ public:
+  explicit MoveOnly(int data = 1) : data_(data) {}
+  MoveOnly(MoveOnly&& other) : data_(other.data_) { other.data_ = 0; }
+  MoveOnly& operator=(MoveOnly&& other) {
+    data_ = other.data_;
+    other.data_ = 0;
+    return *this;
+  }
+
+  friend bool operator<(const MoveOnly& lhs, const MoveOnly& rhs) {
+    return lhs.data_ < rhs.data_;
+  }
+
+  int data() const { return data_; }
+
+ private:
+  int data_;
+
+  DISALLOW_COPY_AND_ASSIGN(MoveOnly);
+};
+
+template <class It>
+class InputIterator {
+ public:
+  using iterator_category = std::input_iterator_tag;
+  using value_type = typename std::iterator_traits<It>::value_type;
+  using difference_type = typename std::iterator_traits<It>::difference_type;
+  using pointer = It;
+  using reference = typename std::iterator_traits<It>::reference;
+
+  InputIterator() : it_() {}
+  explicit InputIterator(It it) : it_(it) {}
+
+  reference operator*() const { return *it_; }
+  pointer operator->() const { return it_; }
+
+  InputIterator& operator++() {
+    ++it_;
+    return *this;
+  }
+  InputIterator operator++(int) {
+    InputIterator tmp(*this);
+    ++(*this);
+    return tmp;
+  }
+
+  friend bool operator==(const InputIterator& lhs, const InputIterator& rhs) {
+    return lhs.it_ == rhs.it_;
+  }
+  friend bool operator!=(const InputIterator& lhs, const InputIterator& rhs) {
+    return !(lhs == rhs);
+  }
+
+ private:
+  It it_;
+};
+
+template <typename It>
+InputIterator<It> MakeInputIterator(It it) {
+  return InputIterator<It>(it);
+}
+
+class Emplaceable {
+ public:
+  Emplaceable() : Emplaceable(0, 0.0) {}
+  Emplaceable(int i, double d) : int_(i), double_(d) {}
+  Emplaceable(Emplaceable&& other) : int_(other.int_), double_(other.double_) {
+    other.int_ = 0;
+    other.double_ = 0.0;
+  }
+
+  Emplaceable& operator=(Emplaceable&& other) {
+    int_ = other.int_;
+    other.int_ = 0;
+    double_ = other.double_;
+    other.double_ = 0.0;
+    return *this;
+  }
+
+  friend bool operator==(const Emplaceable& lhs, const Emplaceable& rhs) {
+    return std::tie(lhs.int_, lhs.double_) == std::tie(rhs.int_, rhs.double_);
+  }
+
+  friend bool operator<(const Emplaceable& lhs, const Emplaceable& rhs) {
+    return std::tie(lhs.int_, lhs.double_) < std::tie(rhs.int_, rhs.double_);
+  }
+
+ private:
+  int int_;
+  double double_;
+
+  DISALLOW_COPY_AND_ASSIGN(Emplaceable);
+};
+
+class NonDefaultConstructibleCompare {
+ public:
+  explicit NonDefaultConstructibleCompare(int) {}
+
+  template <typename T>
+  bool operator()(const T& lhs, const T& rhs) {
+    return std::less<T>()(lhs, rhs);
+  }
+};
+
+// Common test sets.
+using IntSet = base::flat_set<int>;
+using MoveOnlySet = base::flat_set<MoveOnly>;
+using EmplaceableSet = base::flat_set<Emplaceable>;
+using ReversedSet = base::flat_set<int, std::greater<int>>;
+
+// TODO(dyaroshev): replace less<int> with less<>, once we have it
+// crbug.com/682254.
+using SetWithLess = base::flat_set<int, std::less<int>>;
+
+using SetWithStrangeCompare =
+    base::flat_set<int, NonDefaultConstructibleCompare>;
+
+using ::testing::ElementsAre;
+
+}  // namespace
+
+// ----------------------------------------------------------------------------
+// Class.
+
+// Check that base::flat_set and its iterators can be instantiated with an
+// incomplete type.
+
+TEST(FlatSet, IncompleteType) {
+  struct A {
+    using Set = base::flat_set<A>;
+    int data;
+    Set set_with_incomplete_type;
+    Set::iterator it;
+    Set::const_iterator cit;
+
+    // We do not declare operator< because clang complains that it's unused.
+  };
+
+  A a;
+}
+
+TEST(FlatSet, Stability) {
+  using Pair = std::pair<int, int>;
+
+  struct LessByFirst {
+    bool operator()(const Pair& lhs, const Pair& rhs) {
+      return lhs.first < rhs.first;
+    }
+  };
+
+  using Set = base::flat_set<Pair, LessByFirst>;
+
+  // Constructors are not stable.
+  Set cont{{0, 0}, {1, 0}, {0, 1}, {2, 0}, {0, 2}, {1, 1}};
+
+  auto NoneOfSecondsAreTwo = [&cont] {
+    return std::none_of(cont.begin(), cont.end(),
+                        [](const Pair& elem) { return elem.second == 2; });
+  };
+
+  // Should not replace existing.
+  cont.insert(Pair(0, 2));
+  cont.insert(Pair(1, 2));
+  cont.insert(Pair(2, 2));
+
+  EXPECT_TRUE(NoneOfSecondsAreTwo())
+      << "insert should be stable with respect to constructor";
+
+  cont.insert(Pair(3, 0));
+  cont.insert(Pair(3, 2));
+
+  EXPECT_TRUE(NoneOfSecondsAreTwo())
+      << "insert should be stable with respect to previous insert";
+}
+
+// ----------------------------------------------------------------------------
+// Types.
+
+// key_type
+// key_compare
+// value_type
+// value_compare
+// pointer
+// const_pointer
+// reference
+// const_reference
+// size_type
+// difference_type
+// iterator
+// const_iterator
+// reverse_iterator
+// const_reverse_iterator
+
+TEST(FlatSet, Types) {
+  // These are guaranteed to be portable.
+  static_assert((std::is_same<int, IntSet::key_type>::value), "");
+  static_assert((std::is_same<int, IntSet::value_type>::value), "");
+  static_assert((std::is_same<std::less<int>, IntSet::key_compare>::value), "");
+  static_assert((std::is_same<std::less<int>, IntSet::value_compare>::value),
+                "");
+  static_assert((std::is_same<int&, IntSet::reference>::value), "");
+  static_assert((std::is_same<const int&, IntSet::const_reference>::value), "");
+  static_assert((std::is_same<int*, IntSet::pointer>::value), "");
+  static_assert((std::is_same<const int*, IntSet::const_pointer>::value), "");
+}
+
+// ----------------------------------------------------------------------------
+// Lifetime.
+
+// flat_set()
+// flat_set(const Compare& comp)
+
+TEST(FlatSet, DefaultConstructor) {
+  {
+    IntSet cont;
+    EXPECT_THAT(cont, ElementsAre());
+  }
+
+  {
+    SetWithStrangeCompare cont(NonDefaultConstructibleCompare(0));
+    EXPECT_THAT(cont, ElementsAre());
+  }
+}
+
+// flat_set(InputIterator first,
+//          InputIterator last,
+//          const Compare& comp = Compare())
+
+TEST(FlatSet, RangeConstructor) {
+  {
+    IntSet::value_type input_vals[] = {1, 1, 1, 2, 2, 2, 3, 3, 3};
+
+    IntSet cont(MakeInputIterator(std::begin(input_vals)),
+                MakeInputIterator(std::end(input_vals)));
+    EXPECT_THAT(cont, ElementsAre(1, 2, 3));
+  }
+  {
+    SetWithStrangeCompare::value_type input_vals[] = {1, 1, 1, 2, 2,
+                                                      2, 3, 3, 3};
+
+    SetWithStrangeCompare cont(MakeInputIterator(std::begin(input_vals)),
+                               MakeInputIterator(std::end(input_vals)),
+                               NonDefaultConstructibleCompare(0));
+    EXPECT_THAT(cont, ElementsAre(1, 2, 3));
+  }
+}
+
+// flat_set(const flat_set& x)
+
+TEST(FlatSet, CopyConstructor) {
+  IntSet original{1, 2, 3, 4};
+  IntSet copied(original);
+
+  EXPECT_THAT(copied, ElementsAre(1, 2, 3, 4));
+
+  EXPECT_THAT(copied, ElementsAre(1, 2, 3, 4));
+  EXPECT_THAT(original, ElementsAre(1, 2, 3, 4));
+  EXPECT_EQ(original, copied);
+}
+
+// flat_set(flat_set&& x)
+
+TEST(FlatSet, MoveConstructor) {
+  int input_range[] = {1, 2, 3, 4};
+
+  MoveOnlySet original(std::begin(input_range), std::end(input_range));
+  MoveOnlySet moved(std::move(original));
+
+  EXPECT_EQ(1U, moved.count(MoveOnly(1)));
+  EXPECT_EQ(1U, moved.count(MoveOnly(2)));
+  EXPECT_EQ(1U, moved.count(MoveOnly(3)));
+  EXPECT_EQ(1U, moved.count(MoveOnly(4)));
+}
+
+//  flat_set(std::initializer_list<value_type> ilist,
+//           const Compare& comp = Compare())
+
+TEST(FlatSet, InitializerListConstructor) {
+  {
+    IntSet cont{1, 2, 3, 4, 5, 6, 10, 8};
+    EXPECT_THAT(cont, ElementsAre(1, 2, 3, 4, 5, 6, 8, 10));
+  }
+  {
+    SetWithStrangeCompare cont({1, 2, 3, 4, 5, 6, 10, 8},
+                               NonDefaultConstructibleCompare(0));
+    EXPECT_THAT(cont, ElementsAre(1, 2, 3, 4, 5, 6, 8, 10));
+  }
+}
+
+// ----------------------------------------------------------------------------
+// Assignments.
+
+// flat_set& operator=(const flat_set&)
+
+TEST(FlatSet, CopyAssignable) {
+  IntSet original{1, 2, 3, 4};
+  IntSet copied;
+  copied = original;
+
+  EXPECT_THAT(copied, ElementsAre(1, 2, 3, 4));
+  EXPECT_THAT(original, ElementsAre(1, 2, 3, 4));
+  EXPECT_EQ(original, copied);
+}
+
+// flat_set& operator=(flat_set&&)
+
+TEST(FlatSet, MoveAssignable) {
+  int input_range[] = {1, 2, 3, 4};
+
+  MoveOnlySet original(std::begin(input_range), std::end(input_range));
+  MoveOnlySet moved;
+  moved = std::move(original);
+
+  EXPECT_EQ(1U, moved.count(MoveOnly(1)));
+  EXPECT_EQ(1U, moved.count(MoveOnly(2)));
+  EXPECT_EQ(1U, moved.count(MoveOnly(3)));
+  EXPECT_EQ(1U, moved.count(MoveOnly(4)));
+}
+
+// flat_set& operator=(std::initializer_list<value_type> ilist)
+
+TEST(FlatSet, InitializerListAssignable) {
+  IntSet cont{0};
+  cont = {1, 2, 3, 4, 5, 6, 10, 8};
+
+  EXPECT_EQ(0U, cont.count(0));
+  EXPECT_THAT(cont, ElementsAre(1, 2, 3, 4, 5, 6, 8, 10));
+}
+
+// --------------------------------------------------------------------------
+// Memory management.
+
+// void reserve(size_type new_capacity)
+
+TEST(FlatSet, Reserve) {
+  IntSet cont{1, 2, 3};
+
+  cont.reserve(5);
+  EXPECT_LE(5U, cont.capacity());
+}
+
+// size_type capacity() const
+
+TEST(FlatSet, Capacity) {
+  IntSet cont{1, 2, 3};
+
+  EXPECT_LE(cont.size(), cont.capacity());
+  cont.reserve(5);
+  EXPECT_LE(cont.size(), cont.capacity());
+}
+
+// void shrink_to_fit()
+
+TEST(FlatSet, ShrinkToFit) {
+  IntSet cont{1, 2, 3};
+
+  IntSet::size_type capacity_before = cont.capacity();
+  cont.shrink_to_fit();
+  EXPECT_GE(capacity_before, cont.capacity());
+}
+
+// ----------------------------------------------------------------------------
+// Size management.
+
+// void clear()
+
+TEST(FlatSet, Clear) {
+  IntSet cont{1, 2, 3, 4, 5, 6, 7, 8};
+  cont.clear();
+  EXPECT_THAT(cont, ElementsAre());
+}
+
+// size_type size() const
+
+TEST(FlatSet, Size) {
+  IntSet cont;
+
+  EXPECT_EQ(0U, cont.size());
+  cont.insert(2);
+  EXPECT_EQ(1U, cont.size());
+  cont.insert(1);
+  EXPECT_EQ(2U, cont.size());
+  cont.insert(3);
+  EXPECT_EQ(3U, cont.size());
+  cont.erase(cont.begin());
+  EXPECT_EQ(2U, cont.size());
+  cont.erase(cont.begin());
+  EXPECT_EQ(1U, cont.size());
+  cont.erase(cont.begin());
+  EXPECT_EQ(0U, cont.size());
+}
+
+// bool empty() const
+
+TEST(FlatSet, Empty) {
+  IntSet cont;
+
+  EXPECT_TRUE(cont.empty());
+  cont.insert(1);
+  EXPECT_FALSE(cont.empty());
+  cont.clear();
+  EXPECT_TRUE(cont.empty());
+}
+
+// ----------------------------------------------------------------------------
+// Iterators.
+
+// iterator begin()
+// const_iterator begin() const
+// iterator end()
+// const_iterator end() const
+//
+// reverse_iterator rbegin()
+// const_reverse_iterator rbegin() const
+// reverse_iterator rend()
+// const_reverse_iterator rend() const
+//
+// const_iterator cbegin() const
+// const_iterator cend() const
+// const_reverse_iterator crbegin() const
+// const_reverse_iterator crend() const
+
+TEST(FlatSet, Iterators) {
+  IntSet cont{1, 2, 3, 4, 5, 6, 7, 8};
+
+  auto size = static_cast<IntSet::difference_type>(cont.size());
+
+  EXPECT_EQ(size, std::distance(cont.begin(), cont.end()));
+  EXPECT_EQ(size, std::distance(cont.cbegin(), cont.cend()));
+  EXPECT_EQ(size, std::distance(cont.rbegin(), cont.rend()));
+  EXPECT_EQ(size, std::distance(cont.crbegin(), cont.crend()));
+
+  {
+    IntSet::iterator it = cont.begin();
+    IntSet::const_iterator c_it = cont.cbegin();
+    EXPECT_EQ(it, c_it);
+    for (int j = 1; it != cont.end(); ++it, ++c_it, ++j) {
+      EXPECT_EQ(j, *it);
+      EXPECT_EQ(j, *c_it);
+    }
+  }
+  {
+    IntSet::reverse_iterator rit = cont.rbegin();
+    IntSet::const_reverse_iterator c_rit = cont.crbegin();
+    EXPECT_EQ(rit, c_rit);
+    for (int j = static_cast<int>(size); rit != cont.rend();
+         ++rit, ++c_rit, --j) {
+      EXPECT_EQ(j, *rit);
+      EXPECT_EQ(j, *c_rit);
+    }
+  }
+}
+
+// ----------------------------------------------------------------------------
+// Insert operations.
+
+// pair<iterator, bool> insert(const value_type& val)
+
+TEST(FlatSet, InsertLValue) {
+  IntSet cont;
+
+  int value = 2;
+  std::pair<IntSet::iterator, bool> result = cont.insert(value);
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(cont.begin(), result.first);
+  EXPECT_EQ(1U, cont.size());
+  EXPECT_EQ(2, *result.first);
+
+  value = 1;
+  result = cont.insert(value);
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(cont.begin(), result.first);
+  EXPECT_EQ(2U, cont.size());
+  EXPECT_EQ(1, *result.first);
+
+  value = 3;
+  result = cont.insert(value);
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(std::prev(cont.end()), result.first);
+  EXPECT_EQ(3U, cont.size());
+  EXPECT_EQ(3, *result.first);
+
+  value = 3;
+  result = cont.insert(value);
+  EXPECT_FALSE(result.second);
+  EXPECT_EQ(std::prev(cont.end()), result.first);
+  EXPECT_EQ(3U, cont.size());
+  EXPECT_EQ(3, *result.first);
+}
+
+// pair<iterator, bool> insert(value_type&& val)
+
+TEST(FlatSet, InsertRValue) {
+  MoveOnlySet cont;
+
+  std::pair<MoveOnlySet::iterator, bool> result = cont.insert(MoveOnly(2));
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(cont.begin(), result.first);
+  EXPECT_EQ(1U, cont.size());
+  EXPECT_EQ(2, result.first->data());
+
+  result = cont.insert(MoveOnly(1));
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(cont.begin(), result.first);
+  EXPECT_EQ(2U, cont.size());
+  EXPECT_EQ(1, result.first->data());
+
+  result = cont.insert(MoveOnly(3));
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(std::prev(cont.end()), result.first);
+  EXPECT_EQ(3U, cont.size());
+  EXPECT_EQ(3, result.first->data());
+
+  result = cont.insert(MoveOnly(3));
+  EXPECT_FALSE(result.second);
+  EXPECT_EQ(std::prev(cont.end()), result.first);
+  EXPECT_EQ(3U, cont.size());
+  EXPECT_EQ(3, result.first->data());
+}
+
+// iterator insert(const_iterator position_hint, const value_type& val)
+
+TEST(FlatSet, InsertPositionLValue) {
+  IntSet cont;
+
+  IntSet::iterator result = cont.insert(cont.cend(), 2);
+  EXPECT_EQ(cont.begin(), result);
+  EXPECT_EQ(1U, cont.size());
+  EXPECT_EQ(2, *result);
+
+  result = cont.insert(cont.cend(), 1);
+  EXPECT_EQ(cont.begin(), result);
+  EXPECT_EQ(2U, cont.size());
+  EXPECT_EQ(1, *result);
+
+  result = cont.insert(cont.cend(), 3);
+  EXPECT_EQ(std::prev(cont.end()), result);
+  EXPECT_EQ(3U, cont.size());
+  EXPECT_EQ(3, *result);
+
+  result = cont.insert(cont.cend(), 3);
+  EXPECT_EQ(std::prev(cont.end()), result);
+  EXPECT_EQ(3U, cont.size());
+  EXPECT_EQ(3, *result);
+}
+
+// iterator insert(const_iterator position_hint, value_type&& val)
+
+TEST(FlatSet, InsertPositionRValue) {
+  MoveOnlySet cont;
+
+  MoveOnlySet::iterator result = cont.insert(cont.cend(), MoveOnly(2));
+  EXPECT_EQ(cont.begin(), result);
+  EXPECT_EQ(1U, cont.size());
+  EXPECT_EQ(2, result->data());
+
+  result = cont.insert(cont.cend(), MoveOnly(1));
+  EXPECT_EQ(cont.begin(), result);
+  EXPECT_EQ(2U, cont.size());
+  EXPECT_EQ(1, result->data());
+
+  result = cont.insert(cont.cend(), MoveOnly(3));
+  EXPECT_EQ(std::prev(cont.end()), result);
+  EXPECT_EQ(3U, cont.size());
+  EXPECT_EQ(3, result->data());
+
+  result = cont.insert(cont.cend(), MoveOnly(3));
+  EXPECT_EQ(std::prev(cont.end()), result);
+  EXPECT_EQ(3U, cont.size());
+  EXPECT_EQ(3, result->data());
+}
+
+// template <class... Args>
+// pair<iterator, bool> emplace(Args&&... args)
+
+TEST(FlatSet, Emplace) {
+  {
+    EmplaceableSet cont;
+
+    std::pair<EmplaceableSet::iterator, bool> result = cont.emplace();
+    EXPECT_TRUE(result.second);
+    EXPECT_EQ(cont.begin(), result.first);
+    EXPECT_EQ(1U, cont.size());
+    EXPECT_EQ(Emplaceable(), *cont.begin());
+
+    result = cont.emplace(2, 3.5);
+    EXPECT_TRUE(result.second);
+    EXPECT_EQ(std::next(cont.begin()), result.first);
+    EXPECT_EQ(2U, cont.size());
+    EXPECT_EQ(Emplaceable(2, 3.5), *result.first);
+
+    result = cont.emplace(2, 3.5);
+    EXPECT_FALSE(result.second);
+    EXPECT_EQ(std::next(cont.begin()), result.first);
+    EXPECT_EQ(2U, cont.size());
+    EXPECT_EQ(Emplaceable(2, 3.5), *result.first);
+  }
+  {
+    IntSet cont;
+
+    std::pair<IntSet::iterator, bool> result = cont.emplace(2);
+    EXPECT_TRUE(result.second);
+    EXPECT_EQ(cont.begin(), result.first);
+    EXPECT_EQ(1U, cont.size());
+    EXPECT_EQ(2, *result.first);
+  }
+}
+
+// template <class... Args>
+// iterator emplace_hint(const_iterator position_hint, Args&&... args)
+
+TEST(FlatSet, EmplacePosition) {
+  {
+    EmplaceableSet cont;
+
+    EmplaceableSet::iterator result = cont.emplace_hint(cont.cend());
+    EXPECT_EQ(cont.begin(), result);
+    EXPECT_EQ(1U, cont.size());
+    EXPECT_EQ(Emplaceable(), *cont.begin());
+
+    result = cont.emplace_hint(cont.cend(), 2, 3.5);
+    EXPECT_EQ(std::next(cont.begin()), result);
+    EXPECT_EQ(2U, cont.size());
+    EXPECT_EQ(Emplaceable(2, 3.5), *result);
+
+    result = cont.emplace_hint(cont.cbegin(), 2, 3.5);
+    EXPECT_EQ(std::next(cont.begin()), result);
+    EXPECT_EQ(2U, cont.size());
+    EXPECT_EQ(Emplaceable(2, 3.5), *result);
+  }
+  {
+    IntSet cont;
+
+    IntSet::iterator result = cont.emplace_hint(cont.cend(), 2);
+    EXPECT_EQ(cont.begin(), result);
+    EXPECT_EQ(1U, cont.size());
+    EXPECT_EQ(2, *result);
+  }
+}
+
+// ----------------------------------------------------------------------------
+// Erase operations.
+
+// iterator erase(const_iterator position_hint)
+
+TEST(FlatSet, ErasePosition) {
+  IntSet cont{1, 2, 3, 4, 5, 6, 7, 8};
+
+  IntSet::iterator it = cont.erase(std::next(cont.cbegin(), 3));
+  EXPECT_EQ(std::next(cont.begin(), 3), it);
+  EXPECT_THAT(cont, ElementsAre(1, 2, 3, 5, 6, 7, 8));
+
+  it = cont.erase(std::next(cont.cbegin(), 0));
+  EXPECT_EQ(cont.begin(), it);
+  EXPECT_THAT(cont, ElementsAre(2, 3, 5, 6, 7, 8));
+
+  it = cont.erase(std::next(cont.cbegin(), 5));
+  EXPECT_EQ(cont.end(), it);
+  EXPECT_THAT(cont, ElementsAre(2, 3, 5, 6, 7));
+
+  it = cont.erase(std::next(cont.cbegin(), 1));
+  EXPECT_EQ(std::next(cont.begin()), it);
+  EXPECT_THAT(cont, ElementsAre(2, 5, 6, 7));
+
+  it = cont.erase(std::next(cont.cbegin(), 2));
+  EXPECT_EQ(std::next(cont.begin(), 2), it);
+  EXPECT_THAT(cont, ElementsAre(2, 5, 7));
+
+  it = cont.erase(std::next(cont.cbegin(), 2));
+  EXPECT_EQ(std::next(cont.begin(), 2), it);
+  EXPECT_THAT(cont, ElementsAre(2, 5));
+
+  it = cont.erase(std::next(cont.cbegin(), 0));
+  EXPECT_EQ(std::next(cont.begin(), 0), it);
+  EXPECT_THAT(cont, ElementsAre(5));
+
+  it = cont.erase(cont.cbegin());
+  EXPECT_EQ(cont.begin(), it);
+  EXPECT_EQ(cont.end(), it);
+}
+
+// iterator erase(const_iterator first, const_iterator last)
+
+TEST(FlatSet, EraseRange) {
+  IntSet cont{1, 2, 3, 4, 5, 6, 7, 8};
+
+  IntSet::iterator it =
+      cont.erase(std::next(cont.cbegin(), 5), std::next(cont.cbegin(), 5));
+  EXPECT_EQ(std::next(cont.begin(), 5), it);
+  EXPECT_THAT(cont, ElementsAre(1, 2, 3, 4, 5, 6, 7, 8));
+
+  it = cont.erase(std::next(cont.cbegin(), 3), std::next(cont.cbegin(), 4));
+  EXPECT_EQ(std::next(cont.begin(), 3), it);
+  EXPECT_THAT(cont, ElementsAre(1, 2, 3, 5, 6, 7, 8));
+
+  it = cont.erase(std::next(cont.cbegin(), 2), std::next(cont.cbegin(), 5));
+  EXPECT_EQ(std::next(cont.begin(), 2), it);
+  EXPECT_THAT(cont, ElementsAre(1, 2, 7, 8));
+
+  it = cont.erase(std::next(cont.cbegin(), 0), std::next(cont.cbegin(), 2));
+  EXPECT_EQ(std::next(cont.begin(), 0), it);
+  EXPECT_THAT(cont, ElementsAre(7, 8));
+
+  it = cont.erase(cont.cbegin(), cont.cend());
+  EXPECT_EQ(cont.begin(), it);
+  EXPECT_EQ(cont.end(), it);
+}
+
+// size_type erase(const key_type& key)
+
+TEST(FlatSet, EraseKey) {
+  IntSet cont{1, 2, 3, 4, 5, 6, 7, 8};
+
+  EXPECT_EQ(0U, cont.erase(9));
+  EXPECT_THAT(cont, ElementsAre(1, 2, 3, 4, 5, 6, 7, 8));
+
+  EXPECT_EQ(1U, cont.erase(4));
+  EXPECT_THAT(cont, ElementsAre(1, 2, 3, 5, 6, 7, 8));
+
+  EXPECT_EQ(1U, cont.erase(1));
+  EXPECT_THAT(cont, ElementsAre(2, 3, 5, 6, 7, 8));
+
+  EXPECT_EQ(1U, cont.erase(8));
+  EXPECT_THAT(cont, ElementsAre(2, 3, 5, 6, 7));
+
+  EXPECT_EQ(1U, cont.erase(3));
+  EXPECT_THAT(cont, ElementsAre(2, 5, 6, 7));
+
+  EXPECT_EQ(1U, cont.erase(6));
+  EXPECT_THAT(cont, ElementsAre(2, 5, 7));
+
+  EXPECT_EQ(1U, cont.erase(7));
+  EXPECT_THAT(cont, ElementsAre(2, 5));
+
+  EXPECT_EQ(1U, cont.erase(2));
+  EXPECT_THAT(cont, ElementsAre(5));
+
+  EXPECT_EQ(1U, cont.erase(5));
+  EXPECT_THAT(cont, ElementsAre());
+}
+
+// ----------------------------------------------------------------------------
+// Comparators.
+
+// key_compare key_comp() const
+
+TEST(FlatSet, KeyComp) {
+  ReversedSet cont{1, 2, 3, 4, 5};
+
+  EXPECT_TRUE(std::is_sorted(cont.begin(), cont.end(), cont.key_comp()));
+  int new_elements[] = {6, 7, 8, 9, 10};
+  std::copy(std::begin(new_elements), std::end(new_elements),
+            std::inserter(cont, cont.end()));
+  EXPECT_TRUE(std::is_sorted(cont.begin(), cont.end(), cont.key_comp()));
+}
+
+// value_compare value_comp() const
+
+TEST(FlatSet, ValueComp) {
+  ReversedSet cont{1, 2, 3, 4, 5};
+
+  EXPECT_TRUE(std::is_sorted(cont.begin(), cont.end(), cont.value_comp()));
+  int new_elements[] = {6, 7, 8, 9, 10};
+  std::copy(std::begin(new_elements), std::end(new_elements),
+            std::inserter(cont, cont.end()));
+  EXPECT_TRUE(std::is_sorted(cont.begin(), cont.end(), cont.value_comp()));
+}
+
+// ----------------------------------------------------------------------------
+// Search operations.
+
+// size_type count(const key_type& key) const
+
+TEST(FlatSet, Count) {
+  {
+    const IntSet cont{5, 6, 7, 8, 9, 10, 11, 12};
+
+    EXPECT_EQ(1U, cont.count(5));
+    EXPECT_EQ(1U, cont.count(6));
+    EXPECT_EQ(1U, cont.count(7));
+    EXPECT_EQ(1U, cont.count(8));
+    EXPECT_EQ(1U, cont.count(9));
+    EXPECT_EQ(1U, cont.count(10));
+    EXPECT_EQ(1U, cont.count(11));
+    EXPECT_EQ(1U, cont.count(12));
+    EXPECT_EQ(0U, cont.count(4));
+  }
+  {
+    const SetWithLess cont{5, 6, 7, 8, 9, 10, 11, 12};
+
+    EXPECT_EQ(1U, cont.count(5));
+    EXPECT_EQ(1U, cont.count(6));
+    EXPECT_EQ(1U, cont.count(7));
+    EXPECT_EQ(1U, cont.count(8));
+    EXPECT_EQ(1U, cont.count(9));
+    EXPECT_EQ(1U, cont.count(10));
+    EXPECT_EQ(1U, cont.count(11));
+    EXPECT_EQ(1U, cont.count(12));
+    EXPECT_EQ(0U, cont.count(4));
+  }
+}
+
+// iterator find(const key_type& key)
+// const_iterator find(const key_type& key) const
+
+TEST(FlatSet, Find) {
+  {
+    IntSet cont{5, 6, 7, 8, 9, 10, 11, 12};
+
+    EXPECT_EQ(cont.begin(), cont.find(5));
+    EXPECT_EQ(std::next(cont.begin()), cont.find(6));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.find(7));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.find(8));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.find(9));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.find(10));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.find(11));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.find(12));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.find(4));
+  }
+  {
+    const IntSet cont{5, 6, 7, 8, 9, 10, 11, 12};
+
+    EXPECT_EQ(cont.begin(), cont.find(5));
+    EXPECT_EQ(std::next(cont.begin()), cont.find(6));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.find(7));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.find(8));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.find(9));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.find(10));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.find(11));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.find(12));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.find(4));
+  }
+  {
+    SetWithLess cont{5, 6, 7, 8, 9, 10, 11, 12};
+
+    EXPECT_EQ(cont.begin(), cont.find(5));
+    EXPECT_EQ(std::next(cont.begin()), cont.find(6));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.find(7));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.find(8));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.find(9));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.find(10));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.find(11));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.find(12));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.find(4));
+  }
+}
+
+// pair<iterator, iterator> equal_range(const key_type& key)
+// pair<const_iterator, const_iterator> equal_range(const key_type& key) const
+
+TEST(FlatSet, EqualRange) {
+  {
+    IntSet cont{5, 7, 9, 11, 13, 15, 17, 19};
+
+    std::pair<IntSet::iterator, IntSet::iterator> result = cont.equal_range(5);
+    EXPECT_EQ(std::next(cont.begin(), 0), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.second);
+    result = cont.equal_range(7);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.second);
+    result = cont.equal_range(9);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.second);
+    result = cont.equal_range(11);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.second);
+    result = cont.equal_range(13);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.second);
+    result = cont.equal_range(15);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.second);
+    result = cont.equal_range(17);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.second);
+    result = cont.equal_range(19);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 8), result.second);
+    result = cont.equal_range(4);
+    EXPECT_EQ(std::next(cont.begin(), 0), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 0), result.second);
+    result = cont.equal_range(6);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.second);
+    result = cont.equal_range(8);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.second);
+    result = cont.equal_range(10);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.second);
+    result = cont.equal_range(12);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.second);
+    result = cont.equal_range(14);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.second);
+    result = cont.equal_range(16);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.second);
+    result = cont.equal_range(18);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.second);
+    result = cont.equal_range(20);
+    EXPECT_EQ(std::next(cont.begin(), 8), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 8), result.second);
+  }
+  {
+    const IntSet cont{5, 7, 9, 11, 13, 15, 17, 19};
+
+    std::pair<IntSet::const_iterator, IntSet::const_iterator> result =
+        cont.equal_range(5);
+    EXPECT_EQ(std::next(cont.begin(), 0), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.second);
+    result = cont.equal_range(7);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.second);
+    result = cont.equal_range(9);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.second);
+    result = cont.equal_range(11);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.second);
+    result = cont.equal_range(13);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.second);
+    result = cont.equal_range(15);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.second);
+    result = cont.equal_range(17);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.second);
+    result = cont.equal_range(19);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 8), result.second);
+    result = cont.equal_range(4);
+    EXPECT_EQ(std::next(cont.begin(), 0), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 0), result.second);
+    result = cont.equal_range(6);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.second);
+    result = cont.equal_range(8);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.second);
+    result = cont.equal_range(10);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.second);
+    result = cont.equal_range(12);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.second);
+    result = cont.equal_range(14);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.second);
+    result = cont.equal_range(16);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.second);
+    result = cont.equal_range(18);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.second);
+    result = cont.equal_range(20);
+    EXPECT_EQ(std::next(cont.begin(), 8), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 8), result.second);
+  }
+  {
+    SetWithLess cont{5, 7, 9, 11, 13, 15, 17, 19};
+
+    std::pair<SetWithLess::iterator, SetWithLess::iterator> result =
+        cont.equal_range(5);
+    EXPECT_EQ(std::next(cont.begin(), 0), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.second);
+    result = cont.equal_range(7);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.second);
+    result = cont.equal_range(9);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.second);
+    result = cont.equal_range(11);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.second);
+    result = cont.equal_range(13);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.second);
+    result = cont.equal_range(15);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.second);
+    result = cont.equal_range(17);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.second);
+    result = cont.equal_range(19);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 8), result.second);
+    result = cont.equal_range(4);
+    EXPECT_EQ(std::next(cont.begin(), 0), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 0), result.second);
+    result = cont.equal_range(6);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 1), result.second);
+    result = cont.equal_range(8);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 2), result.second);
+    result = cont.equal_range(10);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 3), result.second);
+    result = cont.equal_range(12);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 4), result.second);
+    result = cont.equal_range(14);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 5), result.second);
+    result = cont.equal_range(16);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 6), result.second);
+    result = cont.equal_range(18);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 7), result.second);
+    result = cont.equal_range(20);
+    EXPECT_EQ(std::next(cont.begin(), 8), result.first);
+    EXPECT_EQ(std::next(cont.begin(), 8), result.second);
+  }
+}
+
+//       iterator lower_bound(const key_type& key);
+// const_iterator lower_bound(const key_type& key) const;
+
+TEST(FlatSet, LowerBound) {
+  {
+    IntSet cont{5, 7, 9, 11, 13, 15, 17, 19};
+
+    EXPECT_EQ(cont.begin(), cont.lower_bound(5));
+    EXPECT_EQ(std::next(cont.begin()), cont.lower_bound(7));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.lower_bound(9));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.lower_bound(11));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.lower_bound(13));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.lower_bound(15));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.lower_bound(17));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.lower_bound(19));
+    EXPECT_EQ(std::next(cont.begin(), 0), cont.lower_bound(4));
+    EXPECT_EQ(std::next(cont.begin(), 1), cont.lower_bound(6));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.lower_bound(8));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.lower_bound(10));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.lower_bound(12));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.lower_bound(14));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.lower_bound(16));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.lower_bound(18));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.lower_bound(20));
+  }
+  {
+    const IntSet cont{5, 7, 9, 11, 13, 15, 17, 19};
+
+    EXPECT_EQ(cont.begin(), cont.lower_bound(5));
+    EXPECT_EQ(std::next(cont.begin()), cont.lower_bound(7));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.lower_bound(9));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.lower_bound(11));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.lower_bound(13));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.lower_bound(15));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.lower_bound(17));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.lower_bound(19));
+    EXPECT_EQ(std::next(cont.begin(), 0), cont.lower_bound(4));
+    EXPECT_EQ(std::next(cont.begin(), 1), cont.lower_bound(6));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.lower_bound(8));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.lower_bound(10));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.lower_bound(12));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.lower_bound(14));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.lower_bound(16));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.lower_bound(18));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.lower_bound(20));
+  }
+  {
+    SetWithLess cont{5, 7, 9, 11, 13, 15, 17, 19};
+
+    EXPECT_EQ(cont.begin(), cont.lower_bound(5));
+    EXPECT_EQ(std::next(cont.begin()), cont.lower_bound(7));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.lower_bound(9));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.lower_bound(11));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.lower_bound(13));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.lower_bound(15));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.lower_bound(17));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.lower_bound(19));
+    EXPECT_EQ(std::next(cont.begin(), 0), cont.lower_bound(4));
+    EXPECT_EQ(std::next(cont.begin(), 1), cont.lower_bound(6));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.lower_bound(8));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.lower_bound(10));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.lower_bound(12));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.lower_bound(14));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.lower_bound(16));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.lower_bound(18));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.lower_bound(20));
+  }
+}
+
+// iterator upper_bound(const key_type& key)
+// const_iterator upper_bound(const key_type& key) const
+
+TEST(FlatSet, UpperBound) {
+  {
+    IntSet cont{5, 7, 9, 11, 13, 15, 17, 19};
+
+    EXPECT_EQ(std::next(cont.begin(), 1), cont.upper_bound(5));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.upper_bound(7));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.upper_bound(9));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.upper_bound(11));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.upper_bound(13));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.upper_bound(15));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.upper_bound(17));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.upper_bound(19));
+    EXPECT_EQ(std::next(cont.begin(), 0), cont.upper_bound(4));
+    EXPECT_EQ(std::next(cont.begin(), 1), cont.upper_bound(6));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.upper_bound(8));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.upper_bound(10));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.upper_bound(12));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.upper_bound(14));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.upper_bound(16));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.upper_bound(18));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.upper_bound(20));
+  }
+  {
+    const IntSet cont{5, 7, 9, 11, 13, 15, 17, 19};
+
+    EXPECT_EQ(std::next(cont.begin(), 1), cont.upper_bound(5));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.upper_bound(7));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.upper_bound(9));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.upper_bound(11));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.upper_bound(13));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.upper_bound(15));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.upper_bound(17));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.upper_bound(19));
+    EXPECT_EQ(std::next(cont.begin(), 0), cont.upper_bound(4));
+    EXPECT_EQ(std::next(cont.begin(), 1), cont.upper_bound(6));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.upper_bound(8));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.upper_bound(10));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.upper_bound(12));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.upper_bound(14));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.upper_bound(16));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.upper_bound(18));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.upper_bound(20));
+  }
+  {
+    SetWithLess cont{5, 7, 9, 11, 13, 15, 17, 19};
+
+    EXPECT_EQ(std::next(cont.begin(), 1), cont.upper_bound(5));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.upper_bound(7));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.upper_bound(9));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.upper_bound(11));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.upper_bound(13));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.upper_bound(15));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.upper_bound(17));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.upper_bound(19));
+    EXPECT_EQ(std::next(cont.begin(), 0), cont.upper_bound(4));
+    EXPECT_EQ(std::next(cont.begin(), 1), cont.upper_bound(6));
+    EXPECT_EQ(std::next(cont.begin(), 2), cont.upper_bound(8));
+    EXPECT_EQ(std::next(cont.begin(), 3), cont.upper_bound(10));
+    EXPECT_EQ(std::next(cont.begin(), 4), cont.upper_bound(12));
+    EXPECT_EQ(std::next(cont.begin(), 5), cont.upper_bound(14));
+    EXPECT_EQ(std::next(cont.begin(), 6), cont.upper_bound(16));
+    EXPECT_EQ(std::next(cont.begin(), 7), cont.upper_bound(18));
+    EXPECT_EQ(std::next(cont.begin(), 8), cont.upper_bound(20));
+  }
+}
+
+// ----------------------------------------------------------------------------
+// General operations.
+
+// void swap(flat_set& other)
+// void swap(flat_set& lhs, flat_set& rhs)
+
+TEST(FlatSetOurs, Swap) {
+  IntSet x{1, 2, 3};
+  IntSet y{4};
+  swap(x, y);
+  EXPECT_THAT(x, ElementsAre(4));
+  EXPECT_THAT(y, ElementsAre(1, 2, 3));
+
+  y.swap(x);
+  EXPECT_THAT(x, ElementsAre(1, 2, 3));
+  EXPECT_THAT(y, ElementsAre(4));
+}
+
+// bool operator==(const flat_set& lhs, const flat_set& rhs)
+// bool operator!=(const flat_set& lhs, const flat_set& rhs)
+// bool operator<(const flat_set& lhs, const flat_set& rhs)
+// bool operator>(const flat_set& lhs, const flat_set& rhs)
+// bool operator<=(const flat_set& lhs, const flat_set& rhs)
+// bool operator>=(const flat_set& lhs, const flat_set& rhs)
+
+TEST(FlatSet, Comparison) {
+  // Provided comparator does not participate in comparison.
+  ReversedSet biggest{3};
+  ReversedSet smallest{1};
+  ReversedSet middle{1, 2};
+
+  EXPECT_EQ(biggest, biggest);
+  EXPECT_NE(biggest, smallest);
+  EXPECT_LT(smallest, middle);
+  EXPECT_LE(smallest, middle);
+  EXPECT_LE(middle, middle);
+  EXPECT_GT(biggest, middle);
+  EXPECT_GE(biggest, middle);
+  EXPECT_GE(biggest, biggest);
+}