Add a flat_map container

base/containers/flat_map.h

Index: base/containers/flat_map.h
diff --git a/base/containers/flat_map.h b/base/containers/flat_map.h
new file mode 100644
index 0000000000000000000000000000000000000000..d034e0a9abddfa8bf9bbbc7f5dbcb33b39211fc7
--- /dev/null
+++ b/base/containers/flat_map.h
@@ -0,0 +1,212 @@
+// 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_MAP_H_
+#define BASE_CONTAINERS_FLAT_MAP_H_
+
+#include <utility>
+
+#include "base/containers/flat_tree.h"
+#include "base/logging.h"
+
+namespace base {
+
+namespace internal {
+
+// An implementation of the flat_set GetKeyFromValue template parameter that
+// extracts the key as the first() element of a pair.
+template <class Key, class Mapped>
+struct GetKeyFromValuePairFirst {
+  const Key& operator()(const std::pair<Key, Mapped>& p) const {
+    return p.first;
+  }
+};
+
+}  // namespace internal
+
+// Overview:
+// This file implements flat_map container. It is an alternative to standard
+// sorted containers that stores its elements in contiguous memory (a vector>
+//
+// Additional documentation and usage advice is in flat_set.h.
+//
+// Most of the core functionality is inherited from flat_tree.
+template <class Key, class Mapped, class Compare = std::less<Key>>
+// Meets the requirements of Container, AssociativeContainer,
+// ReversibleContainer.
+// Requires: Key is Movable, Compare is a StrictWeakOrdering on Key.
+class flat_map : public ::base::internal::flat_tree<
+                     Key,
+                     std::pair<Key, Mapped>,
+                     ::base::internal::GetKeyFromValuePairFirst<Key, Mapped>,
+                     Compare> {
+ private:
+  using tree = typename ::base::internal::flat_tree<
+      Key,
+      std::pair<Key, Mapped>,
+      ::base::internal::GetKeyFromValuePairFirst<Key, Mapped>,
+      Compare>;
+
+ public:
+  using mapped_type = Mapped;
+  using value_type = typename tree::value_type;
+
+  // --------------------------------------------------------------------------
+  // Lifetime.
+  //
+  // Constructors that take range guarantee O(N * log(N)) + O(N) complexity
+  // (N is a range length). Thr range constructors are NOT stable. If there are
+  // duplicates an arbitrary one will be chosen.
+  //
+  // Assume that move constructors invalidate iterators and references.
+
+  flat_map();
+  explicit flat_map(const Compare& comp);
+
+  template <class InputIterator>
+  flat_map(InputIterator first,
+           InputIterator last,
+           const Compare& comp = Compare());
+
+  flat_map(const flat_map&);
+  flat_map(flat_map&&) noexcept;
+
+  flat_map(std::initializer_list<value_type> ilist,
+           const Compare& comp = Compare());

[dyaroshev, 2017/02/28 18:40:53]

It would be nice to inherit all of these constructors.
There are some issues with inheriting constructors on Visual Studio - but can
you check if you bump into them?

You already have a test where you use initilizer list, I would also add one for
first, last pair - because templates can expose some weird compiler bugs.

[dyaroshev, 2017/03/06 17:41:55]

I've checked - it doesn't work - compiler doesn't do lasy initialization - so it
tries to instantiate a copy constructor for MoveOnly types.

[brettw, 2017/03/20 22:42:19]

Thanks for checking!

+
+  ~flat_map();
+
+  // --------------------------------------------------------------------------
+  // Assignments.
+  //
+  // Assume that move assignment invalidates iterators and references.
+
+  flat_map& operator=(const flat_map&);
+  flat_map& operator=(flat_map&&);
+  // Not stable in the presence of duplicates in the initializer list.
+  flat_map& operator=(std::initializer_list<value_type> ilist);
+
+  // --------------------------------------------------------------------------
+  // Map-specific insert operations.
+  //
+  // Normal insert() functions are inherited from flat_tree.
+  //
+  // Assume that every operation invalidates iterators and references.
+  // Insertion of one element can take O(size).
+
+  Mapped& operator[](const Key& key);
+  Mapped& operator[](Key&& key);
+
+  // CHECK()s if the element does not exist.
+  value_type& at(const Key& key);
+  const value_type& at(const Key& key) const;
+
+  // --------------------------------------------------------------------------
+  // General operations.
+  //
+  // Assume that swap invalidates iterators and references.
+
+  void swap(flat_map& other) noexcept;
+
+  friend void swap(flat_map& lhs, flat_map& rhs) noexcept { lhs.swap(rhs); }
+};
+
+// ----------------------------------------------------------------------------
+// Lifetime.
+
+template <class Key, class Mapped, class Compare>
+flat_map<Key, Mapped, Compare>::flat_map() = default;
+
+template <class Key, class Mapped, class Compare>
+flat_map<Key, Mapped, Compare>::flat_map(const Compare& comp) : tree(comp) {}
+
+template <class Key, class Mapped, class Compare>
+template <class InputIterator>
+flat_map<Key, Mapped, Compare>::flat_map(InputIterator first,
+                                         InputIterator last,
+                                         const Compare& comp)
+    : tree(first, last, comp) {}
+
+template <class Key, class Mapped, class Compare>
+flat_map<Key, Mapped, Compare>::flat_map(const flat_map&) = default;
+
+template <class Key, class Mapped, class Compare>
+flat_map<Key, Mapped, Compare>::flat_map(flat_map&&) = default;
+
+template <class Key, class Mapped, class Compare>
+flat_map<Key, Mapped, Compare>::flat_map(
+    std::initializer_list<value_type> ilist,
+    const Compare& comp)
+    : flat_map(std::begin(ilist), std::end(ilist), comp) {}
+
+template <class Key, class Mapped, class Compare>
+flat_map<Key, Mapped, Compare>::~flat_map() = default;
+
+// ----------------------------------------------------------------------------
+// Assignments.
+
+template <class Key, class Mapped, class Compare>
+auto flat_map<Key, Mapped, Compare>::operator=(const flat_map&)
+    -> flat_map& = default;
+
+template <class Key, class Mapped, class Compare>
+auto flat_map<Key, Mapped, Compare>::operator=(flat_map &&)
+    -> flat_map& = default;
+
+template <class Key, class Mapped, class Compare>
+auto flat_map<Key, Mapped, Compare>::operator=(
+    std::initializer_list<value_type> ilist) -> flat_map& {
+  tree::operator=(ilist);
+  return *this;
+}
+
+// ----------------------------------------------------------------------------
+// Insert operations.
+
+template <class Key, class Mapped, class Compare>
+auto flat_map<Key, Mapped, Compare>::operator[](const Key& key) -> Mapped& {
+  typename tree::iterator found = tree::lower_bound(key);
+  if (found == tree::end() || tree::key_comp()(key, found->first))
+    found = unsafe_insert_new_at(found, value_type(key, Mapped()));
+  return found->second;
+}
+
+template <class Key, class Mapped, class Compare>
+auto flat_map<Key, Mapped, Compare>::operator[](Key&& key)
+    -> Mapped& {
+  const Key& key_ref = key;
+  typename tree::iterator found = tree::lower_bound(key_ref);
+  if (found == tree::end() || tree::key_comp()(key, found->first)) {
+    found = tree::unsafe_insert_new_at(found,
+                                       value_type(std::move(key), Mapped()));
+  }
+  return found->second;
+}
+
+template <class Key, class Mapped, class Compare>
+auto flat_map<Key, Mapped, Compare>::at(const Key& key) -> value_type& {
+  typename tree::const_iterator found = tree::lower_bound(key);
+  CHECK(found == tree::end() || tree::key_comp()(key, found->first));

[dyaroshev, 2017/02/28 18:40:53]

This should probably just be tree::find.

+  return found->second;
+}
+
+template <class Key, class Mapped, class Compare>
+auto flat_map<Key, Mapped, Compare>::at(const Key& key) const
+    -> const value_type& {
+  typename tree::iterator found = tree::lower_bound(key);
+  CHECK(found == tree::end() || tree::key_comp()(key, found->first));
+  return found->second;
+}
+
+// ----------------------------------------------------------------------------
+// General operations.
+
+template <class Key, class Mapped, class Compare>
+void flat_map<Key, Mapped, Compare>::swap(flat_map& other) {
+  tree::swap(other);
+}
+
+}  // namespace base
+
+#endif  // BASE_CONTAINERS_FLAT_MAP_H_