base: Add bulk insert to flat_tree.

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
index c3a234fa784c4f0e0e151d9711d0265b756473fd..98b817cf02f941ffb32270a100123f805dbfa55a 100644
--- a/base/containers/flat_tree.h
+++ b/base/containers/flat_tree.h
@@ -184,8 +184,7 @@ class flat_tree {
   // 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.
+  // Insertion of one element can take O(size).
   // Capacity of flat_tree grows in an implementation-defined manner.
   //
   // NOTE: Prefer to build a new flat_tree from a std::vector (or similar)
@@ -197,6 +196,9 @@ class flat_tree {
   iterator insert(const_iterator position_hint, const value_type& x);
   iterator insert(const_iterator position_hint, value_type&& x);
 
+  template <class InputIterator>
+  void insert(InputIterator first, InputIterator last);
+
   template <class... Args>
   std::pair<iterator, bool> emplace(Args&&... args);
 
@@ -600,6 +602,30 @@ auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::insert(
 }
 
 template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+template <class InputIterator>
+void flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::insert(
+    InputIterator first,
+    InputIterator last) {
+  // Insert the values at the end.
+  auto size = impl_.body_.size();
+  impl_.body_.insert(end(), first, last);
+  auto mid = begin() + size;
+
+  // Sort the newly inserted elements.
+  std::stable_sort(mid, end(), impl_.get_value_comp());
+
+  // Merge the two sublists and unique them.
+  std::inplace_merge(begin(), mid, end(), impl_.get_value_comp());
+  auto comparator = [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);
+  };
+  auto erase_after = std::unique(begin(), end(), comparator);
+  erase(erase_after, end());
+}
+
+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> {