GO語言list剖析
本節內容
- 使用方法
- list提供的方法
- 源碼剖析
1. 使用方法
在GO語言的標准庫中，提供了一個container包，這個包中提供了三種數據類型，就是heap,list和ring，本節要講的是list的使用以及源碼剖析。
要使用GO提供的list鏈表，則首先需要導入list包，如下所示：
package main
import(
"container/list"
)
導入包之后，需要了解list中定義了兩種數據類型，Element和List，定義如下：
// 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{}
}
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
}
Element里面定義了兩個Element類型的指針next, prev以及List類型的指針list, Value用來存儲值，List里面定義了一個Element作為鏈表的Root，len作為鏈表的長度。
import之后，就可以使用鏈表了：
func main() {
list_test:=list.New() // 創建list對象
list_test.PushBack("123") // 往List隊列尾部插入數據
list_test.PushBack("456")
list_test.PushBack("789")
fmt.Println(list_test.Len()) // 輸出list長度
fmt.Println(list_test.Front()) // 輸出list第一個元素
fmt.Println(list_test.Front().Next()) // 輸出list第一個元素的下一個元素
fmt.Println(list_test.Front().Next().Next()) // 輸出list第三個元素
}
2. list提供的方法
list提供的方法如下：
type Element
func (e *Element) Next() *Element
func (e *Element) Prev() *Element
type List
func New() *List
func (l *List) Back() *Element // 返回最后一個元素
func (l *List) Front() *Element // 返回第一個元素
func (l *List) Init() *List // 鏈表初始化
func (l *List) InsertAfter(v interface{}, mark *Element) *Element // 在某個元素前插入
func (l *List) InsertBefore(v interface{}, mark *Element) *Element // 在某個元素后插入
func (l *List) Len() int // 返回鏈表長度
func (l *List) MoveAfter(e, mark *Element) // 把e元素移動到mark之后
func (l *List) MoveBefore(e, mark *Element) // 把e元素移動到mark之前
func (l *List) MoveToBack(e *Element) // 把e元素移動到隊列最后
func (l *List) MoveToFront(e *Element) // 把e元素移動到隊列最頭部
func (l *List) PushBack(v interface{}) *Element // 在隊列最后插入元素
func (l *List) PushBackList(other *List) // 在隊列最后插入接上新隊列
func (l *List) PushFront(v interface{}) *Element // 在隊列頭部插入元素
func (l *List) PushFrontList(other *List) // 在隊列頭部插入接上新隊列
func (l *List) Remove(e *Element) interface{} // 刪除某個元素
3. 源碼剖析
首先，使用list.New()方法，返回的是一個List對象的指針，源碼func New() *List { return new(List).Init() }並執行了List對象的Init()方法對list進行初始化，初始化root的prev和next指針以及list的長度。
之后調用list_test.PushBack("123")在隊列尾部插入元素123，源碼如下：
func (l *List) PushBack(v interface{}) *Element {
l.lazyInit()
return l.insertValue(v, l.root.prev)
}
調用lazyInit(),如果鏈表沒有初始化，則先初始化一遍，之后，調用list的insertValue方法，insertValue方法初始化節點之后，調用insert方法進行插入鏈表。
func (l *List) insertValue(v interface{}, at *Element) *Element {
return l.insert(&Element{Value: v}, at)
}
整篇文章最精髓的地方就在insert方法中了，源碼如下：
func (l *List) insert(e, at *Element) *Element {
n := at.next // 用中間變量n保存at節點的next指針
at.next = e // at節點的next指向要插入的節點
e.prev = at // 要插入的節點e的prev指向at節點
e.next = n // e的next節點指向中間變量n保存的指針
n.prev = e // at節點的下一個節點的prev指向e節點
e.list = l // e節點的list指向鏈表的root節點
l.len++ // 鏈表的長度加一
return e // 返回剛插入節點的指針
}
這里的鏈表結構是雙向鏈表，並且在root節點的prev指針指向了鏈表的結尾，鏈表結尾的next指針也指向了root節點，這樣，其實形成了一個環形結構，如果是向鏈表的尾部插入新數據，則將root.prev傳遞給insert方法的at參數，如果是向頭部插入，則將root傳遞給insert方法的at參數。
這樣做的好處是顯而易見的，那就是從鏈表的尾部插入數據，將不需要遍歷一遍鏈表，而只需要將root節點的prev傳遞給insert方法中就可以了，大大節省了從尾部插入節點的時間。這段代碼我看了很久，覺得這個包中最精髓的地方也就在這了，這也是這篇文章誕生的原因。
源碼如下：
// 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)
}
}