container/list: Add InsertBefore and InsertAfter methods.

R=rsc
APPROVED=rsc
DELTA=84  (68 added, 4 deleted, 12 changed)
OCL=33493
CL=33499

src/pkg/container/list/list.go

@@ -70,31 +70,55 @@ func (l *List) Remove(e *Element) {
 	l.len--;
 }
 
-func (l *List) insertFront(e *Element) {
-	e.prev = nil;
-	e.next = l.front;
-	l.front = e;
-	if e.next != nil {
-		e.next.prev = e;
+func (l *List) insertBefore(e *Element, mark *Element) {
+	if mark.prev == nil {
+		// new front of the list
+		l.front = e;
 	} else {
-		l.back = e;
+		mark.prev.next = e;
 	}
+	e.prev = mark.prev;
+	mark.prev = e;
+	e.next = mark;
 	l.len++;
 }
 
-func (l *List) insertBack(e *Element) {
-	e.next = nil;
-	e.prev = l.back;
-	l.back = e;
-	if e.prev != nil {
-		e.prev.next = e;
+func (l *List) insertAfter(e *Element, mark *Element) {
+	if mark.next == nil {
+		// new back of the list
+		l.back = e;
 	} else {
-		l.front = e;
+		mark.next.prev = e;
 	}
+	e.next = mark.next;
+	mark.next = e;
+	e.prev = mark;
 	l.len++;
 }
 
-// PushFront inserts the value at the front of the list, and returns a new Element containing it.
+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 {
 	if l.id == nil {
 		l.Init();
@@ -104,7 +128,7 @@ func (l *List) PushFront(value interface {}) *Element {
 	return e
 }
 
-// PushBack inserts the value at the back of the list, and returns a new Element containing it.
+// 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 {
 	if l.id == nil {
 		l.Init();
@@ -114,6 +138,26 @@ func (l *List) PushBack(value interface {}) *Element {
 	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 {
+	if mark.id != l.id {
+		return nil
+	}
+	e := &Element{ nil, nil, l.id, value };
+	l.insertBefore(e, mark);
+	return e
+}
+
+// InsertAfter inserts the value immediately after mark and returns a new Element containing the value.
+func (l *List) InsertAfter(value interface {}, mark *Element) *Element {
+	if mark.id != l.id {
+		return nil
+	}
+	e := &Element{ nil, nil, l.id, value };
+	l.insertAfter(e, mark);
+	return e
+}
+
 // MoveToFront moves the element to the front of the list.
 func (l *List) MoveToFront(e *Element) {
 	if e.id != l.id || l.front == e {

src/pkg/container/list/list_test.go

@@ -94,6 +94,26 @@ func TestList(t *testing.T) {
 	l.MoveToBack(e3);  // should be no-op
 	checkListPointers(t, l, []*Element{ e1, e4, e3 });
 
+	e2 = l.InsertBefore(2, e1);  // insert before front
+	checkListPointers(t, l, []*Element{ e2, e1, e4, e3 });
+	l.Remove(e2);
+	e2 = l.InsertBefore(2, e4);  // insert before middle
+	checkListPointers(t, l, []*Element{ e1, e2, e4, e3 });
+	l.Remove(e2);
+	e2 = l.InsertBefore(2, e3);  // insert before back
+	checkListPointers(t, l, []*Element{ e1, e4, e2, e3 });
+	l.Remove(e2);
+
+	e2 = l.InsertAfter(2, e1);  // insert after front
+	checkListPointers(t, l, []*Element{ e1, e2, e4, e3 });
+	l.Remove(e2);
+	e2 = l.InsertAfter(2, e4);  // insert after middle
+	checkListPointers(t, l, []*Element{ e1, e4, e2, e3 });
+	l.Remove(e2);
+	e2 = l.InsertAfter(2, e3);  // insert after back
+	checkListPointers(t, l, []*Element{ e1, e4, e3, e2 });
+	l.Remove(e2);
+
 	// Clear all elements by iterating
 	for e := range l.Iter() {
 		l.Remove(e);