//
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 }
-
-// Back returns the last element in the list.
-func (l *List) Back() *Element { return l.back }
+// Len returns the number of elements of list l.
+func (l *List) Len() int { return l.len }
-// Remove removes the element from the list
-// and returns its Value.
-func (l *List) Remove(e *Element) interface{} {
- l.remove(e)
- e.list = nil // do what remove does not
- return e.Value
+// 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 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
+// Back returns the last element of list l or nil.
+func (l *List) Back() *Element {
+ if l.len == 0 {
+ return nil
}
-
- e.prev = nil
- e.next = nil
- l.len--
+ return l.root.prev
}
-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
+// 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
}
- e.prev = mark.prev
- mark.prev = e
- e.next = mark
- l.len++
}
-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
+// 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
}
-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)
+// 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
}
-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
+// 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)
}
- l.insertAfter(e, l.back)
+ return e.Value
}
-// 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
+// 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)
}
-// 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
+// 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)
}
-// 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)
}
}
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 p := e.prev; p != prev {
+ t.Errorf("elt[%d](%p).prev = %p, want %p", i, e, p, prev)
+ }
+ if p := e.Prev(); p != Prev {
+ t.Errorf("elt[%d](%p).Prev() = %p, want %p", i, e, p, Prev)
}
+
+ next := root
+ Next := (*Element)(nil)
if i < len(es)-1 {
- e_next = es[i+1]
+ next = es[i+1]
+ Next = next
}
- if e.prev != e_prev {
- t.Errorf("elt #%d (%v) has prev=%v, want %v", i, e, e.prev, e_prev)
+ if n := e.next; n != next {
+ t.Errorf("elt[%d](%p).next = %p, want %p", i, e, n, next)
}
- if e.next != e_next {
- t.Errorf("elt #%d (%v) has next=%v, want %v", i, e, e.next, e_next)
+ if n := e.Next(); n != Next {
+ t.Errorf("elt[%d](%p).Next() = %p, want %p", i, e, n, 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)
- }
-}
-
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)
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})
}
}
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
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++
}
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)
+ }
}