mirror of
https://github.com/golang/go
synced 2024-09-25 15:10:11 -06:00
container/list: Correctly maintain internal invariants
The previous implementation was a mess with invariants maintained inconsistently. Essentially reimplemented the package: - used a circular list as internal representation for significantly simpler implementation with fewer special cases while maintaining the illusion of a nil-terminated doubly linked list externally - more precise documentation - cleaned up and simplified tests, added test case for issue 4103. No changes to the API or documented semantics. All this said, I would be in favor of removing this package eventually. container/ring provides a faster implementation and a simpler and more powerful API. Fixes #4103. R=r CC=golang-dev https://golang.org/cl/6569072
This commit is contained in:
Robert Griesemer
parent
6ed800c01d
commit
0e9daef2d1
@ -11,201 +11,181 @@
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//
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package list
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// Element is an element in the linked list.
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// Element is an element of a linked list.
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type Element struct {
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// Next and previous pointers in the doubly-linked list of elements.
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// The front of the list has prev = nil, and the back has next = nil.
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// To simplify the implementation, internally a list l is implemented
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// as a ring, such that &l.root is both the next element of the last
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// list element (l.Back()) and the previous element of the first list
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// element (l.Front()).
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next, prev *Element
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// The list to which this element belongs.
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list *List
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// The contents of this list element.
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// The value stored with this element.
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Value interface{}
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}
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// Next returns the next list element or nil.
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func (e *Element) Next() *Element { return e.next }
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func (e *Element) Next() *Element {
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if p := e.next; p != &e.list.root {
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return p
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}
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return nil
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}
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// Prev returns the previous list element or nil.
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func (e *Element) Prev() *Element { return e.prev }
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func (e *Element) Prev() *Element {
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if p := e.prev; p != &e.list.root {
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return p
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}
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return nil
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}
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// List represents a doubly linked list.
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// The zero value for List is an empty list ready to use.
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type List struct {
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front, back *Element
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len int
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root Element // sentinel list element, only &root, root.prev, and root.next are used
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len int // current list length excluding (this) sentinel element
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}
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// Init initializes or clears a List.
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// Init initializes or clears list l.
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func (l *List) Init() *List {
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l.front = nil
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l.back = nil
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l.root.next = &l.root
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l.root.prev = &l.root
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l.len = 0
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return l
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}
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// New returns an initialized list.
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func New() *List { return new(List) }
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func New() *List { return new(List).Init() }
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// Front returns the first element in the list.
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func (l *List) Front() *Element { return l.front }
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// Len returns the number of elements of list l.
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func (l *List) Len() int { return l.len }
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// Back returns the last element in the list.
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func (l *List) Back() *Element { return l.back }
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// Front returns the first element of list l or nil
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func (l *List) Front() *Element {
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if l.len == 0 {
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return nil
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}
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return l.root.next
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}
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// Remove removes the element from the list
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// and returns its Value.
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// Back returns the last element of list l or nil.
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func (l *List) Back() *Element {
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if l.len == 0 {
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return nil
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}
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return l.root.prev
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}
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// lazyInit lazily initializes a zero List value.
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func (l *List) lazyInit() {
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if l.root.next == nil {
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l.root.next = &l.root
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l.root.prev = &l.root
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}
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}
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// insert inserts e after at, increments l.len, and returns e.
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func (l *List) insert(e, at *Element) *Element {
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n := at.next
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at.next = e
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e.prev = at
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e.next = n
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n.prev = e
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e.list = l
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l.len++
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return e
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}
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// remove removes e from its list, decrements l.len, and returns e.
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func (l *List) remove(e *Element) *Element {
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e.prev.next = e.next
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e.next.prev = e.prev
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e.list = nil
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l.len--
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return e
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}
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// Remove removes e from l if e is an element of list l.
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// It returns the element value e.Value.
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func (l *List) Remove(e *Element) interface{} {
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l.remove(e)
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e.list = nil // do what remove does not
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if e.list == l {
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// if e.list == l, l must have been initialized when e was inserted
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// in l or l == nil (e is a zero Element) and l.remove will crash
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l.remove(e)
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}
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return e.Value
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}
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// remove the element from the list, but do not clear the Element's list field.
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// This is so that other List methods may use remove when relocating Elements
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// without needing to restore the list field.
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func (l *List) remove(e *Element) {
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if e.list != l {
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return
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}
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if e.prev == nil {
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l.front = e.next
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} else {
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e.prev.next = e.next
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}
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if e.next == nil {
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l.back = e.prev
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} else {
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e.next.prev = e.prev
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}
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e.prev = nil
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e.next = nil
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l.len--
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// Pushfront inserts a new element e with value v at the front of list l and returns e.
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func (l *List) PushFront(v interface{}) *Element {
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l.lazyInit()
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return l.insert(&Element{Value: v}, &l.root)
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}
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func (l *List) insertBefore(e *Element, mark *Element) {
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if mark.prev == nil {
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// new front of the list
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l.front = e
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} else {
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mark.prev.next = e
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}
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e.prev = mark.prev
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mark.prev = e
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e.next = mark
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l.len++
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// PushBack inserts a new element e with value v at the back of list l and returns e.
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func (l *List) PushBack(v interface{}) *Element {
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l.lazyInit()
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return l.insert(&Element{Value: v}, l.root.prev)
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}
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func (l *List) insertAfter(e *Element, mark *Element) {
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if mark.next == nil {
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// new back of the list
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l.back = e
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} else {
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mark.next.prev = e
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}
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e.next = mark.next
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mark.next = e
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e.prev = mark
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l.len++
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}
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func (l *List) insertFront(e *Element) {
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if l.front == nil {
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// empty list
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l.front, l.back = e, e
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e.prev, e.next = nil, nil
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l.len = 1
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return
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}
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l.insertBefore(e, l.front)
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}
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func (l *List) insertBack(e *Element) {
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if l.back == nil {
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// empty list
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l.front, l.back = e, e
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e.prev, e.next = nil, nil
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l.len = 1
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return
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}
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l.insertAfter(e, l.back)
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}
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// PushFront inserts the value at the front of the list and returns a new Element containing the value.
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func (l *List) PushFront(value interface{}) *Element {
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e := &Element{nil, nil, l, value}
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l.insertFront(e)
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return e
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}
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// PushBack inserts the value at the back of the list and returns a new Element containing the value.
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func (l *List) PushBack(value interface{}) *Element {
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e := &Element{nil, nil, l, value}
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l.insertBack(e)
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return e
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}
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// InsertBefore inserts the value immediately before mark and returns a new Element containing the value.
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func (l *List) InsertBefore(value interface{}, mark *Element) *Element {
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// InsertBefore inserts a new element e with value v immediately before mark and returns e.
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// If mark is not an element of l, the list is not modified.
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func (l *List) InsertBefore(v interface{}, mark *Element) *Element {
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if mark.list != l {
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return nil
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}
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e := &Element{nil, nil, l, value}
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l.insertBefore(e, mark)
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return e
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// see comment in List.Remove about initialization of l
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return l.insert(&Element{Value: v}, mark.prev)
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}
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// InsertAfter inserts the value immediately after mark and returns a new Element containing the value.
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// InsertAfter inserts a new element e with value v immediately after mark and returns e.
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// If mark is not an element of l, the list is not modified.
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func (l *List) InsertAfter(value interface{}, mark *Element) *Element {
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if mark.list != l {
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return nil
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}
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e := &Element{nil, nil, l, value}
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l.insertAfter(e, mark)
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return e
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// see comment in List.Remove about initialization of l
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return l.insert(&Element{Value: value}, mark)
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}
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// MoveToFront moves the element to the front of the list.
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// MoveToFront moves element e to the front of list l.
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// If e is not an element of l, the list is not modified.
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func (l *List) MoveToFront(e *Element) {
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if e.list != l || l.front == e {
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if e.list != l || l.root.next == e {
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return
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}
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l.remove(e)
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l.insertFront(e)
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// see comment in List.Remove about initialization of l
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l.insert(l.remove(e), &l.root)
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}
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// MoveToBack moves the element to the back of the list.
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// MoveToBack moves element e to the back of list l.
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// If e is not an element of l, the list is not modified.
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func (l *List) MoveToBack(e *Element) {
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if e.list != l || l.back == e {
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if e.list != l || l.root.prev == e {
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return
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}
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l.remove(e)
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l.insertBack(e)
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// see comment in List.Remove about initialization of l
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l.insert(l.remove(e), l.root.prev)
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}
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// Len returns the number of elements in the list.
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func (l *List) Len() int { return l.len }
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// PushBackList inserts each element of ol at the back of the list.
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func (l *List) PushBackList(ol *List) {
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last := ol.Back()
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for e := ol.Front(); e != nil; e = e.Next() {
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l.PushBack(e.Value)
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if e == last {
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break
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}
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// PuchBackList inserts a copy of an other list at the back of list l.
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// The lists l and other may be the same.
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func (l *List) PushBackList(other *List) {
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l.lazyInit()
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for i, e := other.Len(), other.Front(); i > 0; i, e = i-1, e.Next() {
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l.insert(&Element{Value: e.Value}, l.root.prev)
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}
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}
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// PushFrontList inserts each element of ol at the front of the list. The ordering of the passed list is preserved.
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func (l *List) PushFrontList(ol *List) {
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first := ol.Front()
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for e := ol.Back(); e != nil; e = e.Prev() {
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l.PushFront(e.Value)
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if e == first {
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break
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}
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// PushFrontList inserts a copy of an other list at the front of list l.
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// The lists l and other may be the same.
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func (l *List) PushFrontList(other *List) {
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l.lazyInit()
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for i, e := other.Len(), other.Back(); i > 0; i, e = i-1, e.Prev() {
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l.insert(&Element{Value: e.Value}, &l.root)
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}
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}
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@ -4,65 +4,75 @@
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package list
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import (
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"testing"
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)
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import "testing"
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func checkListLen(t *testing.T, l *List, len int) bool {
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if n := l.Len(); n != len {
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t.Errorf("l.Len() = %d, want %d", n, len)
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return false
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}
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return true
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}
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func checkListPointers(t *testing.T, l *List, es []*Element) {
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if len(es) == 0 {
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if l.front != nil || l.back != nil {
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t.Errorf("l.front/l.back = %v/%v should be nil/nil", l.front, l.back)
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}
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root := &l.root
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if !checkListLen(t, l, len(es)) {
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return
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}
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if l.front != es[0] {
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t.Errorf("l.front = %v, want %v", l.front, es[0])
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}
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if last := es[len(es)-1]; l.back != last {
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t.Errorf("l.back = %v, want %v", l.back, last)
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// zero length lists must be the zero value or properly initialized (sentinel circle)
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if len(es) == 0 {
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if l.root.next != nil && l.root.next != root || l.root.prev != nil && l.root.prev != root {
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t.Errorf("l.root.next = %p, l.root.prev = %p; both should both be nil or %p", l.root.next, l.root.prev, root)
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}
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return
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}
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// len(es) > 0
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// check internal and external prev/next connections
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for i, e := range es {
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var e_prev, e_next *Element = nil, nil
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prev := root
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Prev := (*Element)(nil)
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if i > 0 {
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e_prev = es[i-1]
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prev = es[i-1]
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Prev = prev
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}
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if i < len(es)-1 {
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e_next = es[i+1]
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if p := e.prev; p != prev {
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t.Errorf("elt[%d](%p).prev = %p, want %p", i, e, p, prev)
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}
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if e.prev != e_prev {
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t.Errorf("elt #%d (%v) has prev=%v, want %v", i, e, e.prev, e_prev)
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if p := e.Prev(); p != Prev {
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t.Errorf("elt[%d](%p).Prev() = %p, want %p", i, e, p, Prev)
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}
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if e.next != e_next {
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t.Errorf("elt #%d (%v) has next=%v, want %v", i, e, e.next, e_next)
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}
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}
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}
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func checkListLen(t *testing.T, l *List, n int) {
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if an := l.Len(); an != n {
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t.Errorf("l.Len() = %d, want %d", an, n)
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next := root
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Next := (*Element)(nil)
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if i < len(es)-1 {
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next = es[i+1]
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Next = next
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}
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if n := e.next; n != next {
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t.Errorf("elt[%d](%p).next = %p, want %p", i, e, n, next)
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}
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if n := e.Next(); n != Next {
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t.Errorf("elt[%d](%p).Next() = %p, want %p", i, e, n, Next)
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}
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}
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}
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func TestList(t *testing.T) {
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l := New()
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checkListPointers(t, l, []*Element{})
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checkListLen(t, l, 0)
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// Single element list
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e := l.PushFront("a")
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checkListLen(t, l, 1)
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checkListPointers(t, l, []*Element{e})
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l.MoveToFront(e)
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checkListPointers(t, l, []*Element{e})
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l.MoveToBack(e)
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checkListPointers(t, l, []*Element{e})
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checkListLen(t, l, 1)
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l.Remove(e)
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checkListPointers(t, l, []*Element{})
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checkListLen(t, l, 0)
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// Bigger list
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e2 := l.PushFront(2)
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@ -70,11 +80,9 @@ func TestList(t *testing.T) {
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e3 := l.PushBack(3)
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e4 := l.PushBack("banana")
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checkListPointers(t, l, []*Element{e1, e2, e3, e4})
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checkListLen(t, l, 4)
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l.Remove(e2)
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checkListPointers(t, l, []*Element{e1, e3, e4})
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checkListLen(t, l, 3)
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l.MoveToFront(e3) // move from middle
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checkListPointers(t, l, []*Element{e3, e1, e4})
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@ -121,7 +129,7 @@ func TestList(t *testing.T) {
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}
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}
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if sum != 4 {
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t.Errorf("sum over l.Iter() = %d, want 4", sum)
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t.Errorf("sum over l = %d, want 4", sum)
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}
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// Clear all elements by iterating
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@ -131,19 +139,18 @@ func TestList(t *testing.T) {
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l.Remove(e)
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}
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checkListPointers(t, l, []*Element{})
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checkListLen(t, l, 0)
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}
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func checkList(t *testing.T, l *List, es []interface{}) {
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if l.Len() != len(es) {
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t.Errorf("list has len=%v, want %v", l.Len(), len(es))
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if !checkListLen(t, l, len(es)) {
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return
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}
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i := 0
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for e := l.Front(); e != nil; e = e.Next() {
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le := e.Value.(int)
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if le != es[i] {
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t.Errorf("elt #%d has value=%v, want %v", i, le, es[i])
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t.Errorf("elt[%d].Value = %v, want %v", i, le, es[i])
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||||
}
|
||||
i++
|
||||
}
|
||||
@ -202,8 +209,27 @@ func TestRemove(t *testing.T) {
|
||||
e := l.Front()
|
||||
l.Remove(e)
|
||||
checkListPointers(t, l, []*Element{e2})
|
||||
checkListLen(t, l, 1)
|
||||
l.Remove(e)
|
||||
checkListPointers(t, l, []*Element{e2})
|
||||
checkListLen(t, l, 1)
|
||||
}
|
||||
|
||||
func TestIssue4103(t *testing.T) {
|
||||
l1 := New()
|
||||
l1.PushBack(1)
|
||||
l1.PushBack(2)
|
||||
|
||||
l2 := New()
|
||||
l2.PushBack(3)
|
||||
l2.PushBack(4)
|
||||
|
||||
e := l1.Front()
|
||||
l2.Remove(e) // l2 should not change because e is not an element of l2
|
||||
if n := l2.Len(); n != 2 {
|
||||
t.Errorf("l2.Len() = %d, want 2", n)
|
||||
}
|
||||
|
||||
l1.InsertBefore(8, e)
|
||||
if n := l1.Len(); n != 3 {
|
||||
t.Errorf("l1.Len() = %d, want 3", n)
|
||||
}
|
||||
}
|
||||
|
||||
Loading…
Reference in New Issue
Block a user