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 interface{} 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 n := at.next 94 at.next = e 95 e.prev = at 96 e.next = n 97 n.prev = e 98 e.list = l 99 l.len++ 100 return e 101 } 102 103 // insertValue is a convenience wrapper for insert(&Element{Value: v}, at). 104 func (l *List) insertValue(v interface{}, at *Element) *Element { 105 return l.insert(&Element{Value: v}, at) 106 } 107 108 // remove removes e from its list, decrements l.len, and returns e. 109 func (l *List) remove(e *Element) *Element { 110 e.prev.next = e.next 111 e.next.prev = e.prev 112 e.next = nil // avoid memory leaks 113 e.prev = nil // avoid memory leaks 114 e.list = nil 115 l.len-- 116 return e 117 } 118 119 // move moves e to next to at and returns e. 120 func (l *List) move(e, at *Element) *Element { 121 if e == at { 122 return e 123 } 124 e.prev.next = e.next 125 e.next.prev = e.prev 126 127 n := at.next 128 at.next = e 129 e.prev = at 130 e.next = n 131 n.prev = e 132 133 return e 134 } 135 136 // Remove removes e from l if e is an element of list l. 137 // It returns the element value e.Value. 138 // The element must not be nil. 139 func (l *List) Remove(e *Element) interface{} { 140 if e.list == l { 141 // if e.list == l, l must have been initialized when e was inserted 142 // in l or l == nil (e is a zero Element) and l.remove will crash 143 l.remove(e) 144 } 145 return e.Value 146 } 147 148 // PushFront inserts a new element e with value v at the front of list l and returns e. 149 func (l *List) PushFront(v interface{}) *Element { 150 l.lazyInit() 151 return l.insertValue(v, &l.root) 152 } 153 154 // PushBack inserts a new element e with value v at the back of list l and returns e. 155 func (l *List) PushBack(v interface{}) *Element { 156 l.lazyInit() 157 return l.insertValue(v, l.root.prev) 158 } 159 160 // InsertBefore inserts a new element e with value v immediately before mark and returns e. 161 // If mark is not an element of l, the list is not modified. 162 // The mark must not be nil. 163 func (l *List) InsertBefore(v interface{}, mark *Element) *Element { 164 if mark.list != l { 165 return nil 166 } 167 // see comment in List.Remove about initialization of l 168 return l.insertValue(v, mark.prev) 169 } 170 171 // InsertAfter inserts a new element e with value v immediately after mark and returns e. 172 // If mark is not an element of l, the list is not modified. 173 // The mark must not be nil. 174 func (l *List) InsertAfter(v interface{}, mark *Element) *Element { 175 if mark.list != l { 176 return nil 177 } 178 // see comment in List.Remove about initialization of l 179 return l.insertValue(v, mark) 180 } 181 182 // MoveToFront moves element e to the front of list l. 183 // If e is not an element of l, the list is not modified. 184 // The element must not be nil. 185 func (l *List) MoveToFront(e *Element) { 186 if e.list != l || l.root.next == e { 187 return 188 } 189 // see comment in List.Remove about initialization of l 190 l.move(e, &l.root) 191 } 192 193 // MoveToBack moves element e to the back of list l. 194 // If e is not an element of l, the list is not modified. 195 // The element must not be nil. 196 func (l *List) MoveToBack(e *Element) { 197 if e.list != l || l.root.prev == e { 198 return 199 } 200 // see comment in List.Remove about initialization of l 201 l.move(e, l.root.prev) 202 } 203 204 // MoveBefore moves element e to its new position before mark. 205 // If e or mark is not an element of l, or e == mark, the list is not modified. 206 // The element and mark must not be nil. 207 func (l *List) MoveBefore(e, mark *Element) { 208 if e.list != l || e == mark || mark.list != l { 209 return 210 } 211 l.move(e, mark.prev) 212 } 213 214 // MoveAfter moves element e to its new position after mark. 215 // If e or mark is not an element of l, or e == mark, the list is not modified. 216 // The element and mark must not be nil. 217 func (l *List) MoveAfter(e, mark *Element) { 218 if e.list != l || e == mark || mark.list != l { 219 return 220 } 221 l.move(e, mark) 222 } 223 224 // PushBackList inserts a copy of an other list at the back of list l. 225 // The lists l and other may be the same. They must not be nil. 226 func (l *List) PushBackList(other *List) { 227 l.lazyInit() 228 for i, e := other.Len(), other.Front(); i > 0; i, e = i-1, e.Next() { 229 l.insertValue(e.Value, l.root.prev) 230 } 231 } 232 233 // PushFrontList inserts a copy of an other list at the front of list l. 234 // The lists l and other may be the same. They must not be nil. 235 func (l *List) PushFrontList(other *List) { 236 l.lazyInit() 237 for i, e := other.Len(), other.Back(); i > 0; i, e = i-1, e.Prev() { 238 l.insertValue(e.Value, &l.root) 239 } 240 } 241