1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
4 5 // Package list implements a doubly linked list.
6 //
7 // To iterate over a list (where l is a *List):
8 //
9 // for e := l.Front(); e != nil; e = e.Next() {
10 // // do something with e.Value
11 // }
12 package list
13 14 // Element is an element of a linked list.
15 type Element struct {
16 // Next and previous pointers in the doubly-linked list of elements.
17 // To simplify the implementation, internally a list l is implemented
18 // as a ring, such that &l.root is both the next element of the last
19 // list element (l.Back()) and the previous element of the first list
20 // element (l.Front()).
21 next, prev *Element
22 23 // The list to which this element belongs.
24 list *List
25 26 // The value stored with this element.
27 Value any
28 }
29 30 // Next returns the next list element or nil.
31 func (e *Element) Next() *Element {
32 if p := e.next; e.list != nil && p != &e.list.root {
33 return p
34 }
35 return nil
36 }
37 38 // Prev returns the previous list element or nil.
39 func (e *Element) Prev() *Element {
40 if p := e.prev; e.list != nil && p != &e.list.root {
41 return p
42 }
43 return nil
44 }
45 46 // List represents a doubly linked list.
47 // The zero value for List is an empty list ready to use.
48 type List struct {
49 root Element // sentinel list element, only &root, root.prev, and root.next are used
50 len int // current list length excluding (this) sentinel element
51 }
52 53 // Init initializes or clears list l.
54 func (l *List) Init() *List {
55 l.root.next = &l.root
56 l.root.prev = &l.root
57 l.len = 0
58 return l
59 }
60 61 // New returns an initialized list.
62 func New() *List { return (&List{}).Init() }
63 64 // Len returns the number of elements of list l.
65 // The complexity is O(1).
66 func (l *List) Len() int { return l.len }
67 68 // Front returns the first element of list l or nil if the list is empty.
69 func (l *List) Front() *Element {
70 if l.len == 0 {
71 return nil
72 }
73 return l.root.next
74 }
75 76 // Back returns the last element of list l or nil if the list is empty.
77 func (l *List) Back() *Element {
78 if l.len == 0 {
79 return nil
80 }
81 return l.root.prev
82 }
83 84 // lazyInit lazily initializes a zero List value.
85 func (l *List) lazyInit() {
86 if l.root.next == nil {
87 l.Init()
88 }
89 }
90 91 // insert inserts e after at, increments l.len, and returns e.
92 func (l *List) insert(e, at *Element) *Element {
93 e.prev = at
94 e.next = at.next
95 e.prev.next = e
96 e.next.prev = e
97 e.list = l
98 l.len++
99 return e
100 }
101 102 // insertValue is a convenience wrapper for insert(&Element{Value: v}, at).
103 func (l *List) insertValue(v any, at *Element) *Element {
104 return l.insert(&Element{Value: v}, at)
105 }
106 107 // remove removes e from its list, decrements l.len
108 func (l *List) remove(e *Element) {
109 e.prev.next = e.next
110 e.next.prev = e.prev
111 e.next = nil // avoid memory leaks
112 e.prev = nil // avoid memory leaks
113 e.list = nil
114 l.len--
115 }
116 117 // move moves e to next to at.
118 func (l *List) move(e, at *Element) {
119 if e == at {
120 return
121 }
122 e.prev.next = e.next
123 e.next.prev = e.prev
124 125 e.prev = at
126 e.next = at.next
127 e.prev.next = e
128 e.next.prev = e
129 }
130 131 // Remove removes e from l if e is an element of list l.
132 // It returns the element value e.Value.
133 // The element must not be nil.
134 func (l *List) Remove(e *Element) any {
135 if e.list == l {
136 // if e.list == l, l must have been initialized when e was inserted
137 // in l or l == nil (e is a zero Element) and l.remove will crash
138 l.remove(e)
139 }
140 return e.Value
141 }
142 143 // PushFront inserts a new element e with value v at the front of list l and returns e.
144 func (l *List) PushFront(v any) *Element {
145 l.lazyInit()
146 return l.insertValue(v, &l.root)
147 }
148 149 // PushBack inserts a new element e with value v at the back of list l and returns e.
150 func (l *List) PushBack(v any) *Element {
151 l.lazyInit()
152 return l.insertValue(v, l.root.prev)
153 }
154 155 // InsertBefore inserts a new element e with value v immediately before mark and returns e.
156 // If mark is not an element of l, the list is not modified.
157 // The mark must not be nil.
158 func (l *List) InsertBefore(v any, mark *Element) *Element {
159 if mark.list != l {
160 return nil
161 }
162 // see comment in List.Remove about initialization of l
163 return l.insertValue(v, mark.prev)
164 }
165 166 // InsertAfter inserts a new element e with value v immediately after mark and returns e.
167 // If mark is not an element of l, the list is not modified.
168 // The mark must not be nil.
169 func (l *List) InsertAfter(v any, mark *Element) *Element {
170 if mark.list != l {
171 return nil
172 }
173 // see comment in List.Remove about initialization of l
174 return l.insertValue(v, mark)
175 }
176 177 // MoveToFront moves element e to the front of list l.
178 // If e is not an element of l, the list is not modified.
179 // The element must not be nil.
180 func (l *List) MoveToFront(e *Element) {
181 if e.list != l || l.root.next == e {
182 return
183 }
184 // see comment in List.Remove about initialization of l
185 l.move(e, &l.root)
186 }
187 188 // MoveToBack moves element e to the back of list l.
189 // If e is not an element of l, the list is not modified.
190 // The element must not be nil.
191 func (l *List) MoveToBack(e *Element) {
192 if e.list != l || l.root.prev == e {
193 return
194 }
195 // see comment in List.Remove about initialization of l
196 l.move(e, l.root.prev)
197 }
198 199 // MoveBefore moves element e to its new position before mark.
200 // If e or mark is not an element of l, or e == mark, the list is not modified.
201 // The element and mark must not be nil.
202 func (l *List) MoveBefore(e, mark *Element) {
203 if e.list != l || e == mark || mark.list != l {
204 return
205 }
206 l.move(e, mark.prev)
207 }
208 209 // MoveAfter moves element e to its new position after mark.
210 // If e or mark is not an element of l, or e == mark, the list is not modified.
211 // The element and mark must not be nil.
212 func (l *List) MoveAfter(e, mark *Element) {
213 if e.list != l || e == mark || mark.list != l {
214 return
215 }
216 l.move(e, mark)
217 }
218 219 // PushBackList inserts a copy of another list at the back of list l.
220 // The lists l and other may be the same. They must not be nil.
221 func (l *List) PushBackList(other *List) {
222 l.lazyInit()
223 for i, e := other.Len(), other.Front(); i > 0; i, e = i-1, e.Next() {
224 l.insertValue(e.Value, l.root.prev)
225 }
226 }
227 228 // PushFrontList inserts a copy of another list at the front of list l.
229 // The lists l and other may be the same. They must not be nil.
230 func (l *List) PushFrontList(other *List) {
231 l.lazyInit()
232 for i, e := other.Len(), other.Back(); i > 0; i, e = i-1, e.Prev() {
233 l.insertValue(e.Value, &l.root)
234 }
235 }
236