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Source file src/container/list/list.go

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // Package list implements a doubly linked list.
  //
  // To iterate over a list (where l is a *List):
  //	for e := l.Front(); e != nil; e = e.Next() {
  //		// do something with e.Value
  //	}
  //
  package list
  
  // Element is an element of a linked list.
  type Element struct {
  	// Next and previous pointers in the doubly-linked list of elements.
  	// To simplify the implementation, internally a list l is implemented
  	// as a ring, such that &l.root is both the next element of the last
  	// list element (l.Back()) and the previous element of the first list
  	// element (l.Front()).
  	next, prev *Element
  
  	// The list to which this element belongs.
  	list *List
  
  	// The value stored with this element.
  	Value interface{}
  }
  
  // Next returns the next list element or nil.
  func (e *Element) Next() *Element {
  	if p := e.next; e.list != nil && p != &e.list.root {
  		return p
  	}
  	return nil
  }
  
  // Prev returns the previous list element or nil.
  func (e *Element) Prev() *Element {
  	if p := e.prev; e.list != nil && p != &e.list.root {
  		return p
  	}
  	return nil
  }
  
  // List represents a doubly linked list.
  // The zero value for List is an empty list ready to use.
  type List struct {
  	root Element // sentinel list element, only &root, root.prev, and root.next are used
  	len  int     // current list length excluding (this) sentinel element
  }
  
  // Init initializes or clears list l.
  func (l *List) Init() *List {
  	l.root.next = &l.root
  	l.root.prev = &l.root
  	l.len = 0
  	return l
  }
  
  // New returns an initialized list.
  func New() *List { return new(List).Init() }
  
  // Len returns the number of elements of list l.
  // The complexity is O(1).
  func (l *List) Len() int { return l.len }
  
  // Front returns the first element of list l or nil.
  func (l *List) Front() *Element {
  	if l.len == 0 {
  		return nil
  	}
  	return l.root.next
  }
  
  // Back returns the last element of list l or nil.
  func (l *List) Back() *Element {
  	if l.len == 0 {
  		return nil
  	}
  	return l.root.prev
  }
  
  // lazyInit lazily initializes a zero List value.
  func (l *List) lazyInit() {
  	if l.root.next == nil {
  		l.Init()
  	}
  }
  
  // insert inserts e after at, increments l.len, and returns e.
  func (l *List) insert(e, at *Element) *Element {
  	n := at.next
  	at.next = e
  	e.prev = at
  	e.next = n
  	n.prev = e
  	e.list = l
  	l.len++
  	return e
  }
  
  // insertValue is a convenience wrapper for insert(&Element{Value: v}, at).
  func (l *List) insertValue(v interface{}, at *Element) *Element {
  	return l.insert(&Element{Value: v}, at)
  }
  
  // remove removes e from its list, decrements l.len, and returns e.
  func (l *List) remove(e *Element) *Element {
  	e.prev.next = e.next
  	e.next.prev = e.prev
  	e.next = nil // avoid memory leaks
  	e.prev = nil // avoid memory leaks
  	e.list = nil
  	l.len--
  	return e
  }
  
  // Remove removes e from l if e is an element of list l.
  // It returns the element value e.Value.
  func (l *List) Remove(e *Element) interface{} {
  	if e.list == l {
  		// if e.list == l, l must have been initialized when e was inserted
  		// in l or l == nil (e is a zero Element) and l.remove will crash
  		l.remove(e)
  	}
  	return e.Value
  }
  
  // PushFront inserts a new element e with value v at the front of list l and returns e.
  func (l *List) PushFront(v interface{}) *Element {
  	l.lazyInit()
  	return l.insertValue(v, &l.root)
  }
  
  // PushBack inserts a new element e with value v at the back of list l and returns e.
  func (l *List) PushBack(v interface{}) *Element {
  	l.lazyInit()
  	return l.insertValue(v, l.root.prev)
  }
  
  // InsertBefore inserts a new element e with value v immediately before mark and returns e.
  // If mark is not an element of l, the list is not modified.
  func (l *List) InsertBefore(v interface{}, mark *Element) *Element {
  	if mark.list != l {
  		return nil
  	}
  	// see comment in List.Remove about initialization of l
  	return l.insertValue(v, mark.prev)
  }
  
  // InsertAfter inserts a new element e with value v immediately after mark and returns e.
  // If mark is not an element of l, the list is not modified.
  func (l *List) InsertAfter(v interface{}, mark *Element) *Element {
  	if mark.list != l {
  		return nil
  	}
  	// see comment in List.Remove about initialization of l
  	return l.insertValue(v, mark)
  }
  
  // MoveToFront moves element e to the front of list l.
  // If e is not an element of l, the list is not modified.
  func (l *List) MoveToFront(e *Element) {
  	if e.list != l || l.root.next == e {
  		return
  	}
  	// see comment in List.Remove about initialization of l
  	l.insert(l.remove(e), &l.root)
  }
  
  // MoveToBack moves element e to the back of list l.
  // If e is not an element of l, the list is not modified.
  func (l *List) MoveToBack(e *Element) {
  	if e.list != l || l.root.prev == e {
  		return
  	}
  	// see comment in List.Remove about initialization of l
  	l.insert(l.remove(e), l.root.prev)
  }
  
  // MoveBefore moves element e to its new position before mark.
  // If e or mark is not an element of l, or e == mark, the list is not modified.
  func (l *List) MoveBefore(e, mark *Element) {
  	if e.list != l || e == mark || mark.list != l {
  		return
  	}
  	l.insert(l.remove(e), mark.prev)
  }
  
  // MoveAfter moves element e to its new position after mark.
  // If e or mark is not an element of l, or e == mark, the list is not modified.
  func (l *List) MoveAfter(e, mark *Element) {
  	if e.list != l || e == mark || mark.list != l {
  		return
  	}
  	l.insert(l.remove(e), mark)
  }
  
  // PushBackList inserts a copy of an other list at the back of list l.
  // The lists l and other may be the same.
  func (l *List) PushBackList(other *List) {
  	l.lazyInit()
  	for i, e := other.Len(), other.Front(); i > 0; i, e = i-1, e.Next() {
  		l.insertValue(e.Value, l.root.prev)
  	}
  }
  
  // PushFrontList inserts a copy of an other list at the front of list l.
  // The lists l and other may be the same.
  func (l *List) PushFrontList(other *List) {
  	l.lazyInit()
  	for i, e := other.Len(), other.Back(); i > 0; i, e = i-1, e.Prev() {
  		l.insertValue(e.Value, &l.root)
  	}
  }
  

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