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// Copyright (c) 2013 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// This is a simplistic insertion-ordered map. It behaves similarly to an STL
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// map, but only implements a small subset of the map's methods. Internally, we
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// just keep a map and a list going in parallel.
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// This class provides no thread safety guarantees, beyond what you would
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// normally see with std::list.
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// Iterators should be stable in the face of mutations, except for an
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// iterator pointing to an element that was just deleted.
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#ifndef UTIL_GTL_LINKED_HASH_MAP_H_
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#define UTIL_GTL_LINKED_HASH_MAP_H_
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#include "base/containers/hash_tables.h"
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#include "base/logging.h"
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// This holds a list of pair<Key, Value> items. This list is what gets
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// traversed, and it's iterators from this list that we return from
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// We also keep a map<Key, list::iterator> for find. Since std::list is a
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// doubly-linked list, the iterators should remain stable.
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template<class Key, class Value>
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class linked_hash_map {
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typedef std::list<std::pair<Key, Value> > ListType;
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typedef base::hash_map<Key, typename ListType::iterator> MapType;
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typedef typename ListType::iterator iterator;
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typedef typename ListType::reverse_iterator reverse_iterator;
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typedef typename ListType::const_iterator const_iterator;
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typedef typename ListType::const_reverse_iterator const_reverse_iterator;
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typedef typename MapType::key_type key_type;
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typedef typename ListType::value_type value_type;
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typedef typename ListType::size_type size_type;
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linked_hash_map() : map_(), list_() {
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// Returns an iterator to the first (insertion-ordered) element. Like a map,
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// this can be dereferenced to a pair<Key, Value>.
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const_iterator begin() const {
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// Returns an iterator beyond the last element.
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const_iterator end() const {
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// Returns an iterator to the last (insertion-ordered) element. Like a map,
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// this can be dereferenced to a pair<Key, Value>.
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reverse_iterator rbegin() {
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return list_.rbegin();
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const_reverse_iterator rbegin() const {
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return list_.rbegin();
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// Returns an iterator beyond the first element.
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reverse_iterator rend() {
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const_reverse_iterator rend() const {
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// Clears the map of all values.
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// Returns true iff the map is empty.
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// Erases values with the provided key. Returns the number of elements
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// erased. In this implementation, this will be 0 or 1.
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size_type erase(const Key& key) {
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typename MapType::iterator found = map_.find(key);
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if (found == map_.end()) return 0;
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list_.erase(found->second);
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// Erases values with the provided iterator. If the provided iterator is
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// invalid or there is inconsistency between the map and list, a CHECK() error
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void erase(iterator position) {
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typename MapType::iterator found = map_.find(position->first);
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CHECK(found->second == position)
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<< "Inconsisent iterator for map and list, or the iterator is invalid.";
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list_.erase(position);
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// Erases values between first and last, not including last.
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void erase(iterator first, iterator last) {
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while (first != last && first != end()) {
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// Finds the element with the given key. Returns an iterator to the
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// value found, or to end() if the value was not found. Like a map, this
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// iterator points to a pair<Key, Value>.
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iterator find(const Key& key) {
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typename MapType::iterator found = map_.find(key);
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if (found == map_.end()) {
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return found->second;
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const_iterator find(const Key& key) const {
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typename MapType::const_iterator found = map_.find(key);
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if (found == map_.end()) {
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return found->second;
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// Returns the bounds of a range that includes all the elements in the
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// container with a key that compares equal to x.
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std::pair<iterator, iterator> equal_range(const key_type& key) {
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std::pair<typename MapType::iterator, typename MapType::iterator> eq_range =
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map_.equal_range(key);
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return std::make_pair(eq_range.first->second, eq_range.second->second);
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std::pair<const_iterator, const_iterator> equal_range(
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const key_type& key) const {
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std::pair<typename MapType::const_iterator,
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typename MapType::const_iterator> eq_range =
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map_.equal_range(key);
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const const_iterator& start_iter = eq_range.first != map_.end() ?
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eq_range.first->second : end();
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const const_iterator& end_iter = eq_range.second != map_.end() ?
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eq_range.second->second : end();
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return std::make_pair(start_iter, end_iter);
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// Returns the value mapped to key, or an inserted iterator to that position
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Value& operator[](const key_type& key) {
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return (*((this->insert(std::make_pair(key, Value()))).first)).second;
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// Inserts an element into the map
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std::pair<iterator, bool> insert(const std::pair<Key, Value>& pair) {
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// First make sure the map doesn't have a key with this value. If it does,
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// return a pair with an iterator to it, and false indicating that we
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// didn't insert anything.
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typename MapType::iterator found = map_.find(pair.first);
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if (found != map_.end()) return std::make_pair(found->second, false);
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// Otherwise, insert into the list first.
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list_.push_back(pair);
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// Obtain an iterator to the newly added element. We do -- instead of -
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// since list::iterator doesn't implement operator-().
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typename ListType::iterator last = list_.end();
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CHECK(map_.insert(std::make_pair(pair.first, last)).second)
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<< "Map and list are inconsistent";
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return std::make_pair(last, true);
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size_type size() const {
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void swap(linked_hash_map& other) {
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map_.swap(other.map_);
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list_.swap(other.list_);
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// The map component, used for speedy lookups
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// The list component, used for maintaining insertion order
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#endif // UTIL_GTL_LINKED_HASH_MAP_H_