Add a flat_map container

base/containers/flat_tree.h

Index: base/containers/flat_tree.h
diff --git a/base/containers/flat_tree.h b/base/containers/flat_tree.h
new file mode 100644
index 0000000000000000000000000000000000000000..57335d8882b0268fb6d607595b4bec9579f27c55
--- /dev/null
+++ b/base/containers/flat_tree.h
@@ -0,0 +1,717 @@
+// 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.
+
+#ifndef BASE_CONTAINERS_FLAT_TREE_H_
+#define BASE_CONTAINERS_FLAT_TREE_H_
+
+#include <algorithm>
+#include <vector>
+
+namespace base {
+namespace internal {
+
+// Implementation of a sorted vector for backing flat_set and flat_map. Do not
+// use directly.
+//
+// The use of "value" in this is like std::map uses, meaning it's the thing
+// contained (in the case of map it's a <Kay, Mapped> pair). The Key is how
+// things are looked up. In the case of a set, Key == Value. In the case of
+// a map, the Key is a component of a Value.
+//
+// The helper class GetKeyFromValue provides the means to extract a key from a
+// value for comparison purposes. It should implement:
+//   const Key& operator()(const Value&).
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+class flat_tree {
+ public:
+ private:
+  using underlying_type = std::vector<Value>;
+
+ public:
+  // --------------------------------------------------------------------------
+  // Types.
+  //
+  using key_type = Key;
+  using key_compare = KeyCompare;
+  using value_type = Value;
+
+  // Wraps the templated key comparison to compare values.
+  class value_compare : public key_compare {
+   public:
+    value_compare() = default;
+
+    template <class Cmp>
+    explicit value_compare(Cmp&& compare_arg)
+        : KeyCompare(std::forward<Cmp>(compare_arg)) {}
+
+    bool operator()(const value_type& left, const value_type& right) const {
+      GetKeyFromValue extractor;
+      return key_compare::operator()(extractor(left), extractor(right));
+    }
+  };
+
+  using pointer = typename underlying_type::pointer;
+  using const_pointer = typename underlying_type::const_pointer;
+  using reference = typename underlying_type::reference;
+  using const_reference = typename underlying_type::const_reference;
+  using size_type = typename underlying_type::size_type;
+  using difference_type = typename underlying_type::difference_type;
+  using iterator = typename underlying_type::iterator;
+  using const_iterator = typename underlying_type::const_iterator;
+  using reverse_iterator = typename underlying_type::reverse_iterator;
+  using const_reverse_iterator =
+      typename underlying_type::const_reverse_iterator;
+
+  // --------------------------------------------------------------------------
+  // Lifetime.
+  //
+  // Constructors that take range guarantee O(N * log^2(N)) + O(N) complexity
+  // and take O(N * log(N)) + O(N) if extra memory is available (N is a range
+  // length).
+  //
+  // Assume that move constructors invalidate iterators and references.
+
+  flat_tree();
+  explicit flat_tree(const key_compare& comp);
+
+  // Not stable in the presence of duplicates in the initializer list.
+  template <class InputIterator>
+  flat_tree(InputIterator first,
+            InputIterator last,
+            const key_compare& comp = key_compare());
+
+  flat_tree(const flat_tree&);
+  flat_tree(flat_tree&&);
+
+  // Not stable in the presence of duplicates in the initializer list.
+  flat_tree(std::initializer_list<value_type> ilist,
+            const key_compare& comp = key_compare());
+
+  ~flat_tree();
+
+  // --------------------------------------------------------------------------
+  // Assignments.
+  //
+  // Assume that move assignment invalidates iterators and references.
+
+  flat_tree& operator=(const flat_tree&);
+  flat_tree& operator=(flat_tree&&);
+  // Not stable in the presence of duplicates in the initializer list.
+  flat_tree& operator=(std::initializer_list<value_type> ilist);
+
+  // --------------------------------------------------------------------------
+  // Memory management.
+  //
+  // Beware that shrink_to_fit() simply forwards the request to the
+  // underlying_type and its implementation is free to optimize otherwise and
+  // leave capacity() to be greater that its size.
+  //
+  // reserve() and shrink_to_fit() invalidate iterators and references.
+
+  void reserve(size_type new_capacity);
+  size_type capacity() const;
+  void shrink_to_fit();
+
+  // --------------------------------------------------------------------------
+  // Size management.
+  //
+  // clear() leaves the capacity() of the flat_tree unchanged.
+
+  void clear();
+
+  size_type size() const;
+  size_type max_size() const;
+  bool empty() const;
+
+  // --------------------------------------------------------------------------
+  // Iterators.
+
+  iterator begin();
+  const_iterator begin() const;
+  const_iterator cbegin() const;
+
+  iterator end();
+  const_iterator end() const;
+  const_iterator cend() const;
+
+  reverse_iterator rbegin();
+  const_reverse_iterator rbegin() const;
+  const_reverse_iterator crbegin() const;
+
+  reverse_iterator rend();
+  const_reverse_iterator rend() const;
+  const_reverse_iterator crend() const;
+
+  // --------------------------------------------------------------------------
+  // Insert operations.
+  //
+  // Assume that every operation invalidates iterators and references.
+  // Insertion of one element can take O(size). See the Notes section in the
+  // class comments on why we do not currently implement range insertion.
+  // Capacity of flat_tree grows in an implementation-defined manner.
+  //
+  // NOTE: Prefer to build a new flat_tree from a std::vector (or similar)
+  // instead of calling insert() repeatedly.
+
+  std::pair<iterator, bool> insert(const value_type& val);
+  std::pair<iterator, bool> insert(value_type&& val);
+
+  iterator insert(const_iterator position_hint, const value_type& x);
+  iterator insert(const_iterator position_hint, value_type&& x);
+
+  template <class... Args>
+  std::pair<iterator, bool> emplace(Args&&... args);
+
+  template <class... Args>
+  iterator emplace_hint(const_iterator position_hint, Args&&... args);
+
+  // --------------------------------------------------------------------------
+  // Erase operations.
+  //
+  // Assume that every operation invalidates iterators and references.
+  //
+  // erase(position), erase(first, last) can take O(size).
+  // erase(key) may take O(size) + O(log(size)).
+  //
+  // Prefer base::EraseIf() or some other variation on erase(remove(), end())
+  // idiom when deleting multiple non-consecutive elements.
+
+  iterator erase(const_iterator position);
+  iterator erase(const_iterator first, const_iterator last);
+  size_type erase(const key_type& key);
+
+  // --------------------------------------------------------------------------
+  // Comparators.
+
+  key_compare key_comp() const;
+  value_compare value_comp() const;
+
+  // --------------------------------------------------------------------------
+  // Search operations.
+  //
+  // Search operations have O(log(size)) complexity.
+
+  size_type count(const key_type& key) const;
+
+  iterator find(const key_type& key);
+  const_iterator find(const key_type& key) const;
+
+  std::pair<iterator, iterator> equal_range(const key_type& ket);
+  std::pair<const_iterator, const_iterator> equal_range(
+      const key_type& key) const;
+
+  iterator lower_bound(const key_type& key);
+  const_iterator lower_bound(const key_type& key) const;
+
+  iterator upper_bound(const key_type& key);
+  const_iterator upper_bound(const key_type& key) const;
+
+  // --------------------------------------------------------------------------
+  // General operations.
+  //
+  // Assume that swap invalidates iterators and references.
+  //
+  // Implementation note: currently we use operator==() and operator<() on
+  // std::vector, because they have the same contract we need, so we use them
+  // directly for brevity and in case it is more optimal than calling equal()
+  // and lexicograhpical_compare(). If the underlying container type is changed,
+  // this code may need to be modified.
+
+  void swap(flat_tree& other);
+
+  friend bool operator==(const flat_tree& lhs, const flat_tree& rhs) {
+    return lhs.impl_.body_ == rhs.impl_.body_;
+  }
+
+  friend bool operator!=(const flat_tree& lhs, const flat_tree& rhs) {
+    return !(lhs == rhs);
+  }
+
+  friend bool operator<(const flat_tree& lhs, const flat_tree& rhs) {
+    return lhs.impl_.body_ < rhs.impl_.body_;
+  }
+
+  friend bool operator>(const flat_tree& lhs, const flat_tree& rhs) {
+    return rhs < lhs;
+  }
+
+  friend bool operator>=(const flat_tree& lhs, const flat_tree& rhs) {
+    return !(lhs < rhs);
+  }
+
+  friend bool operator<=(const flat_tree& lhs, const flat_tree& rhs) {
+    return !(lhs > rhs);
+  }
+
+  friend void swap(flat_tree& lhs, flat_tree& rhs) { lhs.swap(rhs); }
+
+ protected:
+  // Emplaces a new item into the tree that is known not to be in it. This
+  // is for implementing map [] and at().
+  template <class... Args>
+  iterator unsafe_emplace(const_iterator position, Args&&... args);
+
+ private:
+  // Helper class for e.g. lower_bound that can compare a value on the left
+  // to a key on the right.
+  struct KeyValueCompare {
+    // The key comparison object must outlive this class.
+    explicit KeyValueCompare(const key_compare& key_comp)
+        : key_comp_(key_comp) {}
+
+    bool operator()(const value_type& left, const key_type& right) const {
+      GetKeyFromValue extractor;
+      return key_comp_(extractor(left), right);
+    }
+
+   private:
+    const key_compare& key_comp_;
+  };
+
+  const flat_tree& as_const() { return *this; }
+
+  iterator const_cast_it(const_iterator c_it) {
+    auto distance = std::distance(cbegin(), c_it);
+    return std::next(begin(), distance);
+  }
+
+  void sort_and_unique() {
+    // std::set sorts elements preserving stability because it doesn't have any
+    // performance wins in not doing that. We do, so we use an unstable sort.
+    std::sort(begin(), end(), impl_.get_value_comp());
+    erase(std::unique(begin(), end(),
+                      [this](const value_type& lhs, const value_type& rhs) {
+                        // lhs is already <= rhs due to sort, therefore
+                        // !(lhs < rhs) <=> lhs == rhs.
+                        return !impl_.get_value_comp()(lhs, rhs);
+                      }),
+          end());
+  }
+
+  // To support comparators that may not be possible to default-construct, we
+  // have to store an instance of Compare. Using this to store all internal
+  // state of flat_tree and using private inheritance to store compare lets us
+  // take advantage of an empty base class optimization to avoid extra space in
+  // the common case when Compare has no state.
+  struct Impl : private value_compare {
+    Impl() = default;
+
+    template <class Cmp, class... Body>
+    explicit Impl(Cmp&& compare_arg, Body&&... underlying_type_args)
+        : value_compare(std::forward<Cmp>(compare_arg)),
+          body_(std::forward<Body>(underlying_type_args)...) {}
+
+    const value_compare& get_value_comp() const { return *this; }
+    const key_compare& get_key_comp() const { return *this; }
+
+    underlying_type body_;
+  } impl_;
+};
+
+// ----------------------------------------------------------------------------
+// Lifetime.
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::flat_tree() = default;
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::flat_tree(
+    const KeyCompare& comp)
+    : impl_(comp) {}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+template <class InputIterator>
+flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::flat_tree(
+    InputIterator first,
+    InputIterator last,
+    const KeyCompare& comp)
+    : impl_(comp, first, last) {
+  sort_and_unique();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::flat_tree(
+    const flat_tree&) = default;
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::flat_tree(flat_tree&&) =
+    default;
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::flat_tree(
+    std::initializer_list<value_type> ilist,
+    const KeyCompare& comp)
+    : flat_tree(std::begin(ilist), std::end(ilist), comp) {}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::~flat_tree() = default;
+
+// ----------------------------------------------------------------------------
+// Assignments.
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::operator=(
+    const flat_tree&) -> flat_tree& = default;
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::operator=(flat_tree &&)
+    -> flat_tree& = default;
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::operator=(
+    std::initializer_list<value_type> ilist) -> flat_tree& {
+  impl_.body_ = ilist;
+  sort_and_unique();
+  return *this;
+}
+
+// ----------------------------------------------------------------------------
+// Memory management.
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+void flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::reserve(
+    size_type new_capacity) {
+  impl_.body_.reserve(new_capacity);
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::capacity() const
+    -> size_type {
+  return impl_.body_.capacity();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+void flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::shrink_to_fit() {
+  impl_.body_.shrink_to_fit();
+}
+
+// ----------------------------------------------------------------------------
+// Size management.
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+void flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::clear() {
+  impl_.body_.clear();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::size() const
+    -> size_type {
+  return impl_.body_.size();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::max_size() const
+    -> size_type {
+  return impl_.body_.max_size();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+bool flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::empty() const {
+  return impl_.body_.empty();
+}
+
+// ----------------------------------------------------------------------------
+// Iterators.
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::begin() -> iterator {
+  return impl_.body_.begin();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::begin() const
+    -> const_iterator {
+  return impl_.body_.begin();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::cbegin() const
+    -> const_iterator {
+  return impl_.body_.cbegin();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::end() -> iterator {
+  return impl_.body_.end();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::end() const
+    -> const_iterator {
+  return impl_.body_.end();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::cend() const
+    -> const_iterator {
+  return impl_.body_.cend();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::rbegin()
+    -> reverse_iterator {
+  return impl_.body_.rbegin();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::rbegin() const
+    -> const_reverse_iterator {
+  return impl_.body_.rbegin();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::crbegin() const
+    -> const_reverse_iterator {
+  return impl_.body_.crbegin();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::rend()
+    -> reverse_iterator {
+  return impl_.body_.rend();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::rend() const
+    -> const_reverse_iterator {
+  return impl_.body_.rend();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::crend() const
+    -> const_reverse_iterator {
+  return impl_.body_.crend();
+}
+
+// ----------------------------------------------------------------------------
+// Insert operations.
+//
+// Currently we use position_hint the same way as eastl or boost:
+// https://github.com/electronicarts/EASTL/blob/master/include/EASTL/vector_set.h#L493
+//
+// We duplicate code between copy and move version so that we can avoid
+// creating a temporary value.
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::insert(
+    const value_type& val) -> std::pair<iterator, bool> {
+  auto position = lower_bound(val);
+
+  if (position == end() || impl_.get_value_comp()(val, *position))
+    return {impl_.body_.insert(position, val), true};
+
+  return {position, false};
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::insert(
+    value_type&& val) -> std::pair<iterator, bool> {
+  GetKeyFromValue extractor;
+  auto position = lower_bound(extractor(val));
+
+  if (position == end() || impl_.get_value_comp()(val, *position))
+    return {impl_.body_.insert(position, std::move(val)), true};
+
+  return {position, false};
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::insert(
+    const_iterator position_hint,
+    const value_type& val) -> iterator {
+  if (position_hint == end() || impl_.get_value_comp()(val, *position_hint)) {
+    if (position_hint == begin() ||
+        impl_.get_value_comp()(*(position_hint - 1), val))
+      // We have to cast away const because of crbug.com/677044.
+      return impl_.body_.insert(const_cast_it(position_hint), val);
+  }
+  return insert(val).first;
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::insert(
+    const_iterator position_hint,
+    value_type&& val) -> iterator {
+  if (position_hint == end() || impl_.get_value_comp()(val, *position_hint)) {
+    if (position_hint == begin() ||
+        impl_.get_value_comp()(*(position_hint - 1), val))
+      // We have to cast away const because of crbug.com/677044.
+      return impl_.body_.insert(const_cast_it(position_hint), std::move(val));
+  }
+  return insert(std::move(val)).first;
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+template <class... Args>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::emplace(Args&&... args)
+    -> std::pair<iterator, bool> {
+  return insert(value_type(std::forward<Args>(args)...));
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+template <class... Args>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::emplace_hint(
+    const_iterator position_hint,
+    Args&&... args) -> iterator {
+  return insert(position_hint, value_type(std::forward<Args>(args)...));
+}
+
+// ----------------------------------------------------------------------------
+// Erase operations.
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::erase(
+    const_iterator position) -> iterator {
+  // We have to cast away const because of crbug.com/677044.
+  return impl_.body_.erase(const_cast_it(position));
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::erase(
+    const key_type& val) -> size_type {
+  auto eq_range = equal_range(val);
+  auto res = std::distance(eq_range.first, eq_range.second);
+  // We have to cast away const because of crbug.com/677044.
+  erase(const_cast_it(eq_range.first), const_cast_it(eq_range.second));
+  return res;
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::erase(
+    const_iterator first,
+    const_iterator last) -> iterator {
+  // We have to cast away const because of crbug.com/677044.
+  return impl_.body_.erase(const_cast_it(first), const_cast_it(last));
+}
+
+// ----------------------------------------------------------------------------
+// Comparators.
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::key_comp() const
+    -> key_compare {
+  return impl_.get_key_comp();
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::value_comp() const
+    -> value_compare {
+  return impl_.get_value_comp();
+}
+
+// ----------------------------------------------------------------------------
+// Search operations.
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::count(
+    const key_type& key) const -> size_type {
+  auto eq_range = equal_range(key);
+  return std::distance(eq_range.first, eq_range.second);
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::find(
+    const key_type& key) -> iterator {
+  return const_cast_it(as_const().find(key));
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::find(
+    const key_type& key) const -> const_iterator {
+  auto eq_range = equal_range(key);
+  return (eq_range.first == eq_range.second) ? end() : eq_range.first;
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::equal_range(
+    const key_type& key) -> std::pair<iterator, iterator> {
+  auto res = as_const().equal_range(key);
+  return {const_cast_it(res.first), const_cast_it(res.second)};
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::equal_range(
+    const key_type& key) const -> std::pair<const_iterator, const_iterator> {
+  auto lower = lower_bound(key);
+
+  GetKeyFromValue extractor;
+  if (lower == end() || impl_.get_key_comp()(key, extractor(*lower)))
+    return {lower, lower};
+
+  return {lower, std::next(lower)};
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::lower_bound(
+    const key_type& key) -> iterator {
+  return const_cast_it(as_const().lower_bound(key));
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::lower_bound(
+    const key_type& key) const -> const_iterator {
+  KeyValueCompare key_value(impl_.get_key_comp());
+  return std::lower_bound(begin(), end(), key, key_value);
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::upper_bound(
+    const key_type& key) -> iterator {
+  return const_cast_it(as_const().upper_bound(key));
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::upper_bound(
+    const key_type& key) const -> const_iterator {
+  KeyValueCompare key_value(impl_.get_key_comp());
+  return std::upper_bound(begin(), end(), key, key_value);
+}
+
+// ----------------------------------------------------------------------------
+// General operations.
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+void flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::swap(
+    flat_tree& other) {
+  std::swap(impl_, other.impl_);
+}
+
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+template <class... Args>
+auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::unsafe_emplace(
+    const_iterator position,
+    Args&&... args) -> iterator {
+  // We have to cast away const because of crbug.com/677044.
+  return impl_.body_.insert(const_cast_it(position),
+                            value_type(std::forward<Args>(args)...));
+}
+
+// For containers like sets, the key is the same as the value. This implements
+// the GetKeyFromValue template parameter to flat_tree for this case.
+template <class Key>
+struct GetKeyFromValueIdentity {
+  const Key& operator()(const Key& k) const { return k; }
+};
+
+}  // namespace internal
+
+// ----------------------------------------------------------------------------
+// Free functions.
+
+// Erases all elements that match predicate. It has O(size) complexity.
+template <class Key,
+          class Value,
+          class GetKeyFromValue,
+          class KeyCompare,
+          typename Predicate>
+void EraseIf(base::internal::flat_tree<Key, Value, GetKeyFromValue, KeyCompare>&
+                 container,
+             Predicate pred) {
+  container.erase(std::remove_if(container.begin(), container.end(), pred),
+                  container.end());
+}
+
+}  // namespace base
+
+#endif  // BASE_CONTAINERS_FLAT_TREE_H_