Source file src/container/list/list.go
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 // 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 new(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