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// Copyright (c) 2009 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_LINKED_LIST_H_
#define BASE_CONTAINERS_LINKED_LIST_H_
#include "base/macros.h"
// Simple LinkedList type. (See the Q&A section to understand how this
// differs from std::list).
//
// To use, start by declaring the class which will be contained in the linked
// list, as extending LinkNode (this gives it next/previous pointers).
//
// class MyNodeType : public LinkNode<MyNodeType> {
// ...
// };
//
// Next, to keep track of the list's head/tail, use a LinkedList instance:
//
// LinkedList<MyNodeType> list;
//
// To add elements to the list, use any of LinkedList::Append,
// LinkNode::InsertBefore, or LinkNode::InsertAfter:
//
// LinkNode<MyNodeType>* n1 = ...;
// LinkNode<MyNodeType>* n2 = ...;
// LinkNode<MyNodeType>* n3 = ...;
//
// list.Append(n1);
// list.Append(n3);
// n3->InsertBefore(n3);
//
// Lastly, to iterate through the linked list forwards:
//
// for (LinkNode<MyNodeType>* node = list.head();
// node != list.end();
// node = node->next()) {
// MyNodeType* value = node->value();
// ...
// }
//
// Or to iterate the linked list backwards:
//
// for (LinkNode<MyNodeType>* node = list.tail();
// node != list.end();
// node = node->previous()) {
// MyNodeType* value = node->value();
// ...
// }
//
// Questions and Answers:
//
// Q. Should I use std::list or base::LinkedList?
//
// A. The main reason to use base::LinkedList over std::list is
// performance. If you don't care about the performance differences
// then use an STL container, as it makes for better code readability.
//
// Comparing the performance of base::LinkedList<T> to std::list<T*>:
//
// * Erasing an element of type T* from base::LinkedList<T> is
// an O(1) operation. Whereas for std::list<T*> it is O(n).
// That is because with std::list<T*> you must obtain an
// iterator to the T* element before you can call erase(iterator).
//
// * Insertion operations with base::LinkedList<T> never require
// heap allocations.
//
// Q. How does base::LinkedList implementation differ from std::list?
//
// A. Doubly-linked lists are made up of nodes that contain "next" and
// "previous" pointers that reference other nodes in the list.
//
// With base::LinkedList<T>, the type being inserted already reserves
// space for the "next" and "previous" pointers (base::LinkNode<T>*).
// Whereas with std::list<T> the type can be anything, so the implementation
// needs to glue on the "next" and "previous" pointers using
// some internal node type.
namespace base {
template <typename T>
class LinkNode {
public:
LinkNode() : previous_(nullptr), next_(nullptr) {}
LinkNode(LinkNode<T>* previous, LinkNode<T>* next)
: previous_(previous), next_(next) {}
LinkNode(LinkNode<T>&& rhs) {
next_ = rhs.next_;
rhs.next_ = nullptr;
previous_ = rhs.previous_;
rhs.previous_ = nullptr;
// If the node belongs to a list, next_ and previous_ are both non-null.
// Otherwise, they are both null.
if (next_) {
next_->previous_ = this;
previous_->next_ = this;
}
}
// Insert |this| into the linked list, before |e|.
void InsertBefore(LinkNode<T>* e) {
this->next_ = e;
this->previous_ = e->previous_;
e->previous_->next_ = this;
e->previous_ = this;
}
// Insert |this| into the linked list, after |e|.
void InsertAfter(LinkNode<T>* e) {
this->next_ = e->next_;
this->previous_ = e;
e->next_->previous_ = this;
e->next_ = this;
}
// Remove |this| from the linked list.
void RemoveFromList() {
this->previous_->next_ = this->next_;
this->next_->previous_ = this->previous_;
// next() and previous() return non-null if and only this node is not in any
// list.
this->next_ = nullptr;
this->previous_ = nullptr;
}
LinkNode<T>* previous() const {
return previous_;
}
LinkNode<T>* next() const {
return next_;
}
// Cast from the node-type to the value type.
const T* value() const {
return static_cast<const T*>(this);
}
T* value() {
return static_cast<T*>(this);
}
private:
LinkNode<T>* previous_;
LinkNode<T>* next_;
DISALLOW_COPY_AND_ASSIGN(LinkNode);
};
template <typename T>
class LinkedList {
public:
// The "root" node is self-referential, and forms the basis of a circular
// list (root_.next() will point back to the start of the list,
// and root_->previous() wraps around to the end of the list).
LinkedList() : root_(&root_, &root_) {}
// Appends |e| to the end of the linked list.
void Append(LinkNode<T>* e) {
e->InsertBefore(&root_);
}
LinkNode<T>* head() const {
return root_.next();
}
LinkNode<T>* tail() const {
return root_.previous();
}
const LinkNode<T>* end() const {
return &root_;
}
bool empty() const { return head() == end(); }
private:
LinkNode<T> root_;
DISALLOW_COPY_AND_ASSIGN(LinkedList);
};
} // namespace base
#endif // BASE_CONTAINERS_LINKED_LIST_H_
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