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// for e := l.Front(); e != nil; e = e.Next() {
9// // do something with e.Value
10// }
11//
12package list
13
14// Element is an element of a linked list.
15type Element[T any] 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[T]
22
23 // The list to which this element belongs.
24 list *List[T]
25
26 // The value stored with this element.
27 Value T
28}
29
30// Next returns the next list element or nil.
31func (e *Element[T]) Next() *Element[T] {
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.
39func (e *Element[T]) Prev() *Element[T] {
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.
48type List[T any] struct {
49 root Element[T] // 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.
54func (l *List[T]) Init() *List[T] {
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.
62func New[T any]() *List[T] { return new(List[T]).Init() }
63
64// Len returns the number of elements of list l.
65// The complexity is O(1).
66func (l *List[T]) Len() int { return l.len }
67
68// Front returns the first element of list l or nil if the list is empty.
69func (l *List[T]) Front() *Element[T] {
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.
77func (l *List[T]) Back() *Element[T] {
78 if l.len == 0 {
79 return nil
80 }
81 return l.root.prev
82}
83
84// lazyInit lazily initializes a zero List value.
85func (l *List[T]) 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.
92func (l *List[T]) insert(e, at *Element[T]) *Element[T] {
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).
103func (l *List[T]) insertValue(v T, at *Element[T]) *Element[T] {
104 return l.insert(&Element[T]{Value: v}, at)
105}
106
107// remove removes e from its list, decrements l.len
108func (l *List[T]) remove(e *Element[T]) {
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.
118func (l *List[T]) move(e, at *Element[T]) {
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.
134func (l *List[T]) Remove(e *Element[T]) T {
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.
144func (l *List[T]) PushFront(v T) *Element[T] {
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.
150func (l *List[T]) PushBack(v T) *Element[T] {
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.
158func (l *List[T]) InsertBefore(v T, mark *Element[T]) *Element[T] {
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.
169func (l *List[T]) InsertAfter(v T, mark *Element[T]) *Element[T] {
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.
180func (l *List[T]) MoveToFront(e *Element[T]) {
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.
191func (l *List[T]) MoveToBack(e *Element[T]) {
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.
202func (l *List[T]) MoveBefore(e, mark *Element[T]) {
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.
212func (l *List[T]) MoveAfter(e, mark *Element[T]) {
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.
221func (l *List[T]) PushBackList(other *List[T]) {
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.
230func (l *List[T]) PushFrontList(other *List[T]) {
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}