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// Copyright (c) 2013 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.
// This is a simplistic insertion-ordered map. It behaves similarly to an STL
// map, but only implements a small subset of the map's methods. Internally, we
// just keep a map and a list going in parallel.
// This class provides no thread safety guarantees, beyond what you would
// normally see with std::list.
// Iterators should be stable in the face of mutations, except for an
// iterator pointing to an element that was just deleted.
#include <stddef.h>
#include <list>
#include <unordered_map>
#include <utility>
#include "base/logging.h"
#include "base/macros.h"
namespace net {
// This holds a list of pair<Key, Value> items. This list is what gets
// traversed, and it's iterators from this list that we return from
// begin/end/find.
// We also keep a map<Key, list::iterator> for find. Since std::list is a
// doubly-linked list, the iterators should remain stable.
template <class Key, class Value, class Hash = std::hash<Key>>
class linked_hash_map {
typedef std::list<std::pair<Key, Value> > ListType;
typedef std::unordered_map<Key, typename ListType::iterator, Hash> MapType;
typedef typename ListType::iterator iterator;
typedef typename ListType::reverse_iterator reverse_iterator;
typedef typename ListType::const_iterator const_iterator;
typedef typename ListType::const_reverse_iterator const_reverse_iterator;
typedef typename MapType::key_type key_type;
typedef typename ListType::value_type value_type;
typedef typename ListType::size_type size_type;
linked_hash_map() : map_(), list_() {
// Returns an iterator to the first (insertion-ordered) element. Like a map,
// this can be dereferenced to a pair<Key, Value>.
iterator begin() {
return list_.begin();
const_iterator begin() const {
return list_.begin();
// Returns an iterator beyond the last element.
iterator end() {
return list_.end();
const_iterator end() const {
return list_.end();
// Returns an iterator to the last (insertion-ordered) element. Like a map,
// this can be dereferenced to a pair<Key, Value>.
reverse_iterator rbegin() {
return list_.rbegin();
const_reverse_iterator rbegin() const {
return list_.rbegin();
// Returns an iterator beyond the first element.
reverse_iterator rend() {
return list_.rend();
const_reverse_iterator rend() const {
return list_.rend();
// Front and back accessors common to many stl containers.
// Returns the earliest-inserted element
const value_type& front() const {
return list_.front();
// Returns the earliest-inserted element.
value_type& front() {
return list_.front();
// Returns the most-recently-inserted element.
const value_type& back() const {
return list_.back();
// Returns the most-recently-inserted element.
value_type& back() {
return list_.back();
// Clears the map of all values.
void clear() {
// Returns true iff the map is empty.
bool empty() const {
return list_.empty();
// Removes the first element from the list.
void pop_front() { erase(begin()); }
// Erases values with the provided key. Returns the number of elements
// erased. In this implementation, this will be 0 or 1.
size_type erase(const Key& key) {
typename MapType::iterator found = map_.find(key);
if (found == map_.end()) return 0;
return 1;
// Erases the item that 'position' points to. Returns an iterator that points
// to the item that comes immediately after the deleted item in the list, or
// end().
// If the provided iterator is invalid or there is inconsistency between the
// map and list, a CHECK() error will occur.
iterator erase(iterator position) {
typename MapType::iterator found = map_.find(position->first);
CHECK(found->second == position)
<< "Inconsisent iterator for map and list, or the iterator is invalid.";
return list_.erase(position);
// Erases all the items in the range [first, last). Returns an iterator that
// points to the item that comes immediately after the last deleted item in
// the list, or end().
iterator erase(iterator first, iterator last) {
while (first != last && first != end()) {
first = erase(first);
return first;
// Finds the element with the given key. Returns an iterator to the
// value found, or to end() if the value was not found. Like a map, this
// iterator points to a pair<Key, Value>.
iterator find(const Key& key) {
typename MapType::iterator found = map_.find(key);
if (found == map_.end()) {
return end();
return found->second;
const_iterator find(const Key& key) const {
typename MapType::const_iterator found = map_.find(key);
if (found == map_.end()) {
return end();
return found->second;
// Returns the bounds of a range that includes all the elements in the
// container with a key that compares equal to x.
std::pair<iterator, iterator> equal_range(const key_type& key) {
std::pair<typename MapType::iterator, typename MapType::iterator> eq_range =
return std::make_pair(eq_range.first->second, eq_range.second->second);
std::pair<const_iterator, const_iterator> equal_range(
const key_type& key) const {
std::pair<typename MapType::const_iterator,
typename MapType::const_iterator> eq_range =
const const_iterator& start_iter = eq_range.first != map_.end() ?
eq_range.first->second : end();
const const_iterator& end_iter = eq_range.second != map_.end() ?
eq_range.second->second : end();
return std::make_pair(start_iter, end_iter);
// Returns the value mapped to key, or an inserted iterator to that position
// in the map.
Value& operator[](const key_type& key) {
return (*((this->insert(std::make_pair(key, Value()))).first)).second;
// Inserts an element into the map
std::pair<iterator, bool> insert(const std::pair<Key, Value>& pair) {
// First make sure the map doesn't have a key with this value. If it does,
// return a pair with an iterator to it, and false indicating that we
// didn't insert anything.
typename MapType::iterator found = map_.find(pair.first);
if (found != map_.end()) return std::make_pair(found->second, false);
// Otherwise, insert into the list first.
// Obtain an iterator to the newly added element. We do -- instead of -
// since list::iterator doesn't implement operator-().
typename ListType::iterator last = list_.end();
CHECK(map_.insert(std::make_pair(pair.first, last)).second)
<< "Map and list are inconsistent";
return std::make_pair(last, true);
size_type size() const {
return list_.size();
template <typename... Args>
std::pair<iterator, bool> emplace(Args&&... args) {
ListType node_donor;
auto node_pos =
node_donor.emplace(node_donor.end(), std::forward<Args>(args)...);
const auto& k = node_pos->first;
auto ins = map_.insert({k, node_pos});
if (!ins.second)
return {ins.first->second, false};
list_.splice(list_.end(), node_donor, node_pos);
return {ins.first->second, true};
void swap(linked_hash_map& other) {
// The map component, used for speedy lookups
MapType map_;
// The list component, used for maintaining insertion order
ListType list_;
// |map_| contains iterators to |list_|, therefore a default copy constructor
// or copy assignment operator would result in an inconsistent state.
} // namespace net