flat containers prototype

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
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+// Copyright (c) 2016 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 <algorithm>
+#include <functional>
+#include <map>
+#include <utility>
+#include <vector>
+
+#include "base/containers/flat_sorted_container_base.h"
+#include "base/logging.h"
+
+namespace base {
+
+namespace internal {
+
+template <typename Key, typename T, class Compare>
+struct map_compare : private Compare {
+  using key_type = Key;
+  using value_type = std::pair<key_type, T>;
+
+  using Compare::operator();
+
+  bool operator()(const value_type& lhs, const key_type& rhs) {
+    return operator()(lhs.first, rhs);
+  }
+
+  bool operator()(const key_type& lhs, const value_type& rhs) {
+    return operator()(lhs, rhs.first);
+  }
+
+  bool operator()(const value_type& lhs, const value_type& rhs) {
+    return operator()(lhs.first, rhs.first);
+  }
+
+  template <typename Lhs, typename Rhs>
+  bool equal(const Lhs& lhs, const Rhs& rhs) {
+    return !operator()(lhs, rhs) && !operator()(rhs, lhs);
+  }
+
+  const key_type& key_from_value(const value_type& value) {
+    return value.first;
+  }
+
+  key_type& key_from_value(value_type& value) {  // NOLINT
+    return value.first;
+  }
+};
+
+}  // namespace internal
+
+// std::vector is not particulary friendly with const value type,
+// so, unlike std::map, we use non const Key
+template <typename Key,
+          typename T,
+          class Compare = std::less<Key>,
+          class UnderlyingType = std::vector<std::pair<Key, T>>>
+class flat_map : public internal::flat_sorted_container_base<
+                     internal::map_compare<Key, T, Compare>,
+                     UnderlyingType> {
+  using base_type = internal::flat_sorted_container_base<
+      internal::map_compare<Key, T, Compare>,
+      UnderlyingType>;
+
+ public:
+  // typedefs------------------------------------------------------------------
+
+  // ours
+  using std_map = std::map<Key, T, Compare>;
+  using mapped_type = T;
+  using key_type = typename base_type::key_type;
+
+  // ctors---------------------------------------------------------------------
+  using base_type::base_type;
+
+  // methods-------------------------------------------------------------------
+
+  mapped_type& at(const key_type& key) {
+    return const_cast<T&>(static_cast<const flat_map&>(*this).at(key));
+  }
+
+  const mapped_type& at(const key_type& key) const {
+    auto pos = this->find(key);
+    CHECK(pos != this->end());
+    return pos->second;
+  }
+
+  mapped_type& operator[](key_type key) {
+    auto pos = this->lower_bound(key);
+    if (pos != this->end() && this->key_value_comp().equal(*pos, key)) {
+      return pos->second;
+    }
+    auto guard = this->unsafe_access();
+    T& res = guard->emplace(pos, std::move(key), T())->second;
+    guard.release();
+    return res;
+  }
+};
+
+}  // namespace base
+
+#endif  // BASE_CONTAINERS_FLAT_MAP_H_