1 files changed, 112 insertions, 126 deletions

diff --git a/src/pkg/container/list/list.go b/src/pkg/container/list/list.go
index a3fd4b39f..562a5badb 100644
--- a/src/pkg/container/list/list.go
+++ b/src/pkg/container/list/list.go
@@ -11,201 +11,187 @@
 //
 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) }
-
-// Front returns the first element in the list.
-func (l *List) Front() *Element { return l.front }
+func New() *List { return new(List).Init() }
 
-// 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.Init()
 	}
-	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)
+// 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)
 }
 
-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 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)
 	}
-	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.insertValue(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.insertValue(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.insertValue(v, mark.prev)
 }
 
-// InsertAfter inserts the value immediately after mark and returns a new Element containing the value.
-func (l *List) InsertAfter(value interface{}, mark *Element) *Element {
+// 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
 	}
-	e := &Element{nil, nil, l, value}
-	l.insertAfter(e, mark)
-	return e
+	// see comment in List.Remove about initialization of l
+	return l.insertValue(v, 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
-		}
+// 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 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.insertValue(e.Value, &l.root)
 	}
 }