mirror of
https://github.com/golang/go
synced 2024-11-22 14:24:45 -07:00
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 @@
|
||||
//
|
||||
package list
|
||||
|
||||
// Element is an element in the linked list.
|
||||
// Element is an element of a linked list.
|
||||
type Element struct {
|
||||
// Next and previous pointers in the doubly-linked list of elements.
|
||||
// The front of the list has prev = nil, and the back has next = nil.
|
||||
// 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 contents of this list element.
|
||||
// The value stored with this element.
|
||||
Value interface{}
|
||||
}
|
||||
|
||||
// Next returns the next list element or nil.
|
||||
func (e *Element) Next() *Element { return e.next }
|
||||
func (e *Element) Next() *Element {
|
||||
if p := e.next; p != &e.list.root {
|
||||
return p
|
||||
}
|
||||
return nil
|
||||
}
|
||||
|
||||
// Prev returns the previous list element or nil.
|
||||
func (e *Element) Prev() *Element { return e.prev }
|
||||
func (e *Element) Prev() *Element {
|
||||
if p := e.prev; 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 {
|
||||
front, back *Element
|
||||
len int
|
||||
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 a List.
|
||||
// Init initializes or clears list l.
|
||||
func (l *List) Init() *List {
|
||||
l.front = nil
|
||||
l.back = nil
|
||||
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) }
|
||||
func New() *List { return new(List).Init() }
|
||||
|
||||
// Front returns the first element in the list.
|
||||
func (l *List) Front() *Element { return l.front }
|
||||
// Len returns the number of elements of list l.
|
||||
func (l *List) Len() int { return l.len }
|
||||
|
||||
// Back returns the last element in the list.
|
||||
func (l *List) Back() *Element { return l.back }
|
||||
// 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
|
||||
}
|
||||
|
||||
// Remove removes the element from the list
|
||||
// and returns its Value.
|
||||
// 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.root.next = &l.root
|
||||
l.root.prev = &l.root
|
||||
}
|
||||
}
|
||||
|
||||
// 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
|
||||
}
|
||||
|
||||
// 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.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{} {
|
||||
l.remove(e)
|
||||
e.list = nil // do what remove does not
|
||||
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
|
||||
}
|
||||
|
||||
// remove the element from the list, but do not clear the Element's list field.
|
||||
// This is so that other List methods may use remove when relocating Elements
|
||||
// without needing to restore the list field.
|
||||
func (l *List) remove(e *Element) {
|
||||
if e.list != l {
|
||||
return
|
||||
}
|
||||
if e.prev == nil {
|
||||
l.front = e.next
|
||||
} else {
|
||||
e.prev.next = e.next
|
||||
}
|
||||
if e.next == nil {
|
||||
l.back = e.prev
|
||||
} else {
|
||||
e.next.prev = e.prev
|
||||
}
|
||||
|
||||
e.prev = nil
|
||||
e.next = nil
|
||||
l.len--
|
||||
// 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.insert(&Element{Value: v}, &l.root)
|
||||
}
|
||||
|
||||
func (l *List) insertBefore(e *Element, mark *Element) {
|
||||
if mark.prev == nil {
|
||||
// new front of the list
|
||||
l.front = e
|
||||
} else {
|
||||
mark.prev.next = e
|
||||
}
|
||||
e.prev = mark.prev
|
||||
mark.prev = e
|
||||
e.next = mark
|
||||
l.len++
|
||||
// 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.insert(&Element{Value: v}, l.root.prev)
|
||||
}
|
||||
|
||||
func (l *List) insertAfter(e *Element, mark *Element) {
|
||||
if mark.next == nil {
|
||||
// new back of the list
|
||||
l.back = e
|
||||
} else {
|
||||
mark.next.prev = e
|
||||
}
|
||||
e.next = mark.next
|
||||
mark.next = e
|
||||
e.prev = mark
|
||||
l.len++
|
||||
}
|
||||
|
||||
func (l *List) insertFront(e *Element) {
|
||||
if l.front == nil {
|
||||
// empty list
|
||||
l.front, l.back = e, e
|
||||
e.prev, e.next = nil, nil
|
||||
l.len = 1
|
||||
return
|
||||
}
|
||||
l.insertBefore(e, l.front)
|
||||
}
|
||||
|
||||
func (l *List) insertBack(e *Element) {
|
||||
if l.back == nil {
|
||||
// empty list
|
||||
l.front, l.back = e, e
|
||||
e.prev, e.next = nil, nil
|
||||
l.len = 1
|
||||
return
|
||||
}
|
||||
l.insertAfter(e, l.back)
|
||||
}
|
||||
|
||||
// PushFront inserts the value at the front of the list and returns a new Element containing the value.
|
||||
func (l *List) PushFront(value interface{}) *Element {
|
||||
e := &Element{nil, nil, l, value}
|
||||
l.insertFront(e)
|
||||
return e
|
||||
}
|
||||
|
||||
// PushBack inserts the value at the back of the list and returns a new Element containing the value.
|
||||
func (l *List) PushBack(value interface{}) *Element {
|
||||
e := &Element{nil, nil, l, value}
|
||||
l.insertBack(e)
|
||||
return e
|
||||
}
|
||||
|
||||
// InsertBefore inserts the value immediately before mark and returns a new Element containing the value.
|
||||
func (l *List) InsertBefore(value interface{}, mark *Element) *Element {
|
||||
// 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
|
||||
}
|
||||
e := &Element{nil, nil, l, value}
|
||||
l.insertBefore(e, mark)
|
||||
return e
|
||||
// see comment in List.Remove about initialization of l
|
||||
return l.insert(&Element{Value: v}, mark.prev)
|
||||
}
|
||||
|
||||
// InsertAfter inserts the value immediately after mark and returns a new Element containing the value.
|
||||
// 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(value interface{}, mark *Element) *Element {
|
||||
if mark.list != l {
|
||||
return nil
|
||||
}
|
||||
e := &Element{nil, nil, l, value}
|
||||
l.insertAfter(e, mark)
|
||||
return e
|
||||
// see comment in List.Remove about initialization of l
|
||||
return l.insert(&Element{Value: value}, mark)
|
||||
}
|
||||
|
||||
// MoveToFront moves the element to the front of the list.
|
||||
// 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.front == e {
|
||||
if e.list != l || l.root.next == e {
|
||||
return
|
||||
}
|
||||
l.remove(e)
|
||||
l.insertFront(e)
|
||||
// see comment in List.Remove about initialization of l
|
||||
l.insert(l.remove(e), &l.root)
|
||||
}
|
||||
|
||||
// MoveToBack moves the element to the back of the list.
|
||||
// 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.back == e {
|
||||
if e.list != l || l.root.prev == e {
|
||||
return
|
||||
}
|
||||
l.remove(e)
|
||||
l.insertBack(e)
|
||||
// see comment in List.Remove about initialization of l
|
||||
l.insert(l.remove(e), l.root.prev)
|
||||
}
|
||||
|
||||
// Len returns the number of elements in the list.
|
||||
func (l *List) Len() int { return l.len }
|
||||
|
||||
// PushBackList inserts each element of ol at the back of the list.
|
||||
func (l *List) PushBackList(ol *List) {
|
||||
last := ol.Back()
|
||||
for e := ol.Front(); e != nil; e = e.Next() {
|
||||
l.PushBack(e.Value)
|
||||
if e == last {
|
||||
break
|
||||
}
|
||||
// PuchBackList 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.insert(&Element{Value: e.Value}, l.root.prev)
|
||||
}
|
||||
}
|
||||
|
||||
// PushFrontList inserts each element of ol at the front of the list. The ordering of the passed list is preserved.
|
||||
func (l *List) PushFrontList(ol *List) {
|
||||
first := ol.Front()
|
||||
for e := ol.Back(); e != nil; e = e.Prev() {
|
||||
l.PushFront(e.Value)
|
||||
if e == first {
|
||||
break
|
||||
}
|
||||
// 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.insert(&Element{Value: e.Value}, &l.root)
|
||||
}
|
||||
}
|
||||
|
||||
@ -4,65 +4,75 @@
|
||||
|
||||
package list
|
||||
|
||||
import (
|
||||
"testing"
|
||||
)
|
||||
import "testing"
|
||||
|
||||
func checkListLen(t *testing.T, l *List, len int) bool {
|
||||
if n := l.Len(); n != len {
|
||||
t.Errorf("l.Len() = %d, want %d", n, len)
|
||||
return false
|
||||
}
|
||||
return true
|
||||
}
|
||||
|
||||
func checkListPointers(t *testing.T, l *List, es []*Element) {
|
||||
if len(es) == 0 {
|
||||
if l.front != nil || l.back != nil {
|
||||
t.Errorf("l.front/l.back = %v/%v should be nil/nil", l.front, l.back)
|
||||
}
|
||||
root := &l.root
|
||||
|
||||
if !checkListLen(t, l, len(es)) {
|
||||
return
|
||||
}
|
||||
|
||||
if l.front != es[0] {
|
||||
t.Errorf("l.front = %v, want %v", l.front, es[0])
|
||||
}
|
||||
if last := es[len(es)-1]; l.back != last {
|
||||
t.Errorf("l.back = %v, want %v", l.back, last)
|
||||
// zero length lists must be the zero value or properly initialized (sentinel circle)
|
||||
if len(es) == 0 {
|
||||
if l.root.next != nil && l.root.next != root || l.root.prev != nil && l.root.prev != root {
|
||||
t.Errorf("l.root.next = %p, l.root.prev = %p; both should both be nil or %p", l.root.next, l.root.prev, root)
|
||||
}
|
||||
return
|
||||
}
|
||||
// len(es) > 0
|
||||
|
||||
// check internal and external prev/next connections
|
||||
for i, e := range es {
|
||||
var e_prev, e_next *Element = nil, nil
|
||||
prev := root
|
||||
Prev := (*Element)(nil)
|
||||
if i > 0 {
|
||||
e_prev = es[i-1]
|
||||
prev = es[i-1]
|
||||
Prev = prev
|
||||
}
|
||||
if i < len(es)-1 {
|
||||
e_next = es[i+1]
|
||||
if p := e.prev; p != prev {
|
||||
t.Errorf("elt[%d](%p).prev = %p, want %p", i, e, p, prev)
|
||||
}
|
||||
if e.prev != e_prev {
|
||||
t.Errorf("elt #%d (%v) has prev=%v, want %v", i, e, e.prev, e_prev)
|
||||
if p := e.Prev(); p != Prev {
|
||||
t.Errorf("elt[%d](%p).Prev() = %p, want %p", i, e, p, Prev)
|
||||
}
|
||||
if e.next != e_next {
|
||||
t.Errorf("elt #%d (%v) has next=%v, want %v", i, e, e.next, e_next)
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
func checkListLen(t *testing.T, l *List, n int) {
|
||||
if an := l.Len(); an != n {
|
||||
t.Errorf("l.Len() = %d, want %d", an, n)
|
||||
next := root
|
||||
Next := (*Element)(nil)
|
||||
if i < len(es)-1 {
|
||||
next = es[i+1]
|
||||
Next = next
|
||||
}
|
||||
if n := e.next; n != next {
|
||||
t.Errorf("elt[%d](%p).next = %p, want %p", i, e, n, next)
|
||||
}
|
||||
if n := e.Next(); n != Next {
|
||||
t.Errorf("elt[%d](%p).Next() = %p, want %p", i, e, n, Next)
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
func TestList(t *testing.T) {
|
||||
l := New()
|
||||
checkListPointers(t, l, []*Element{})
|
||||
checkListLen(t, l, 0)
|
||||
|
||||
// Single element list
|
||||
e := l.PushFront("a")
|
||||
checkListLen(t, l, 1)
|
||||
checkListPointers(t, l, []*Element{e})
|
||||
l.MoveToFront(e)
|
||||
checkListPointers(t, l, []*Element{e})
|
||||
l.MoveToBack(e)
|
||||
checkListPointers(t, l, []*Element{e})
|
||||
checkListLen(t, l, 1)
|
||||
l.Remove(e)
|
||||
checkListPointers(t, l, []*Element{})
|
||||
checkListLen(t, l, 0)
|
||||
|
||||
// Bigger list
|
||||
e2 := l.PushFront(2)
|
||||
@ -70,11 +80,9 @@ func TestList(t *testing.T) {
|
||||
e3 := l.PushBack(3)
|
||||
e4 := l.PushBack("banana")
|
||||
checkListPointers(t, l, []*Element{e1, e2, e3, e4})
|
||||
checkListLen(t, l, 4)
|
||||
|
||||
l.Remove(e2)
|
||||
checkListPointers(t, l, []*Element{e1, e3, e4})
|
||||
checkListLen(t, l, 3)
|
||||
|
||||
l.MoveToFront(e3) // move from middle
|
||||
checkListPointers(t, l, []*Element{e3, e1, e4})
|
||||
@ -121,7 +129,7 @@ func TestList(t *testing.T) {
|
||||
}
|
||||
}
|
||||
if sum != 4 {
|
||||
t.Errorf("sum over l.Iter() = %d, want 4", sum)
|
||||
t.Errorf("sum over l = %d, want 4", sum)
|
||||
}
|
||||
|
||||
// Clear all elements by iterating
|
||||
@ -131,19 +139,18 @@ func TestList(t *testing.T) {
|
||||
l.Remove(e)
|
||||
}
|
||||
checkListPointers(t, l, []*Element{})
|
||||
checkListLen(t, l, 0)
|
||||
}
|
||||
|
||||
func checkList(t *testing.T, l *List, es []interface{}) {
|
||||
if l.Len() != len(es) {
|
||||
t.Errorf("list has len=%v, want %v", l.Len(), len(es))
|
||||
if !checkListLen(t, l, len(es)) {
|
||||
return
|
||||
}
|
||||
|
||||
i := 0
|
||||
for e := l.Front(); e != nil; e = e.Next() {
|
||||
le := e.Value.(int)
|
||||
if le != es[i] {
|
||||
t.Errorf("elt #%d has value=%v, want %v", i, le, es[i])
|
||||
t.Errorf("elt[%d].Value = %v, want %v", i, le, es[i])
|
||||
}
|
||||
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