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Source file src/container/list/list.go

Documentation: container/list

     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  

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