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  //
     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 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  

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