1// Copyright (c) 2009 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef BASE_CONTAINERS_LINKED_LIST_H_
6#define BASE_CONTAINERS_LINKED_LIST_H_
7
8#include "base/macros.h"
9
10// Simple LinkedList type. (See the Q&A section to understand how this
11// differs from std::list).
12//
13// To use, start by declaring the class which will be contained in the linked
14// list, as extending LinkNode (this gives it next/previous pointers).
15//
16//   class MyNodeType : public LinkNode<MyNodeType> {
17//     ...
18//   };
19//
20// Next, to keep track of the list's head/tail, use a LinkedList instance:
21//
22//   LinkedList<MyNodeType> list;
23//
24// To add elements to the list, use any of LinkedList::Append,
25// LinkNode::InsertBefore, or LinkNode::InsertAfter:
26//
27//   LinkNode<MyNodeType>* n1 = ...;
28//   LinkNode<MyNodeType>* n2 = ...;
29//   LinkNode<MyNodeType>* n3 = ...;
30//
31//   list.Append(n1);
32//   list.Append(n3);
33//   n3->InsertBefore(n3);
34//
35// Lastly, to iterate through the linked list forwards:
36//
37//   for (LinkNode<MyNodeType>* node = list.head();
38//        node != list.end();
39//        node = node->next()) {
40//     MyNodeType* value = node->value();
41//     ...
42//   }
43//
44// Or to iterate the linked list backwards:
45//
46//   for (LinkNode<MyNodeType>* node = list.tail();
47//        node != list.end();
48//        node = node->previous()) {
49//     MyNodeType* value = node->value();
50//     ...
51//   }
52//
53// Questions and Answers:
54//
55// Q. Should I use std::list or base::LinkedList?
56//
57// A. The main reason to use base::LinkedList over std::list is
58//    performance. If you don't care about the performance differences
59//    then use an STL container, as it makes for better code readability.
60//
61//    Comparing the performance of base::LinkedList<T> to std::list<T*>:
62//
63//    * Erasing an element of type T* from base::LinkedList<T> is
64//      an O(1) operation. Whereas for std::list<T*> it is O(n).
65//      That is because with std::list<T*> you must obtain an
66//      iterator to the T* element before you can call erase(iterator).
67//
68//    * Insertion operations with base::LinkedList<T> never require
69//      heap allocations.
70//
71// Q. How does base::LinkedList implementation differ from std::list?
72//
73// A. Doubly-linked lists are made up of nodes that contain "next" and
74//    "previous" pointers that reference other nodes in the list.
75//
76//    With base::LinkedList<T>, the type being inserted already reserves
77//    space for the "next" and "previous" pointers (base::LinkNode<T>*).
78//    Whereas with std::list<T> the type can be anything, so the implementation
79//    needs to glue on the "next" and "previous" pointers using
80//    some internal node type.
81
82namespace base {
83
84template <typename T>
85class LinkNode {
86 public:
87  LinkNode() : previous_(NULL), next_(NULL) {}
88  LinkNode(LinkNode<T>* previous, LinkNode<T>* next)
89      : previous_(previous), next_(next) {}
90
91  // Insert |this| into the linked list, before |e|.
92  void InsertBefore(LinkNode<T>* e) {
93    this->next_ = e;
94    this->previous_ = e->previous_;
95    e->previous_->next_ = this;
96    e->previous_ = this;
97  }
98
99  // Insert |this| into the linked list, after |e|.
100  void InsertAfter(LinkNode<T>* e) {
101    this->next_ = e->next_;
102    this->previous_ = e;
103    e->next_->previous_ = this;
104    e->next_ = this;
105  }
106
107  // Remove |this| from the linked list.
108  void RemoveFromList() {
109    this->previous_->next_ = this->next_;
110    this->next_->previous_ = this->previous_;
111    // next() and previous() return non-NULL if and only this node is not in any
112    // list.
113    this->next_ = NULL;
114    this->previous_ = NULL;
115  }
116
117  LinkNode<T>* previous() const {
118    return previous_;
119  }
120
121  LinkNode<T>* next() const {
122    return next_;
123  }
124
125  // Cast from the node-type to the value type.
126  const T* value() const {
127    return static_cast<const T*>(this);
128  }
129
130  T* value() {
131    return static_cast<T*>(this);
132  }
133
134 private:
135  LinkNode<T>* previous_;
136  LinkNode<T>* next_;
137
138  DISALLOW_COPY_AND_ASSIGN(LinkNode);
139};
140
141template <typename T>
142class LinkedList {
143 public:
144  // The "root" node is self-referential, and forms the basis of a circular
145  // list (root_.next() will point back to the start of the list,
146  // and root_->previous() wraps around to the end of the list).
147  LinkedList() : root_(&root_, &root_) {}
148
149  // Appends |e| to the end of the linked list.
150  void Append(LinkNode<T>* e) {
151    e->InsertBefore(&root_);
152  }
153
154  LinkNode<T>* head() const {
155    return root_.next();
156  }
157
158  LinkNode<T>* tail() const {
159    return root_.previous();
160  }
161
162  const LinkNode<T>* end() const {
163    return &root_;
164  }
165
166  bool empty() const { return head() == end(); }
167
168 private:
169  LinkNode<T> root_;
170
171  DISALLOW_COPY_AND_ASSIGN(LinkedList);
172};
173
174}  // namespace base
175
176#endif  // BASE_CONTAINERS_LINKED_LIST_H_
177