container/list: added MoveBefore and MoveAfter

Fixes #4940.

R=golang-dev, bradfitz, gri
CC=golang-dev
https://golang.org/cl/12021044

src/pkg/container/list/list.go

@@ -179,6 +179,24 @@ func (l *List) MoveToBack(e *Element) {
 	l.insert(l.remove(e), l.root.prev)
 }
 
+// MoveBefore moves element e to its new position before mark.
+// If e is not an element of l, or e == mark, the list is not modified.
+func (l *List) MoveBefore(e, mark *Element) {
+	if e.list != l || e == mark {
+		return
+	}
+	l.insert(l.remove(e), mark.prev)
+}
+
+// MoveAfter moves element e to its new position after mark.
+// If e is not an element of l, or e == mark, the list is not modified.
+func (l *List) MoveAfter(e, mark *Element) {
+	if e.list != l || e == mark {
+		return
+	}
+	l.insert(l.remove(e), mark)
+}
+
 // 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) {

src/pkg/container/list/list_test.go

@@ -233,3 +233,37 @@ func TestIssue4103(t *testing.T) {
 		t.Errorf("l1.Len() = %d, want 3", n)
 	}
 }
+
+func TestMove(t *testing.T) {
+	l := New()
+	e1 := l.PushBack(1)
+	e2 := l.PushBack(2)
+	e3 := l.PushBack(3)
+	e4 := l.PushBack(4)
+
+	l.MoveAfter(e3, e3)
+	checkListPointers(t, l, []*Element{e1, e2, e3, e4})
+	l.MoveBefore(e2, e2)
+	checkListPointers(t, l, []*Element{e1, e2, e3, e4})
+
+	l.MoveAfter(e3, e2)
+	checkListPointers(t, l, []*Element{e1, e2, e3, e4})
+	l.MoveBefore(e2, e3)
+	checkListPointers(t, l, []*Element{e1, e2, e3, e4})
+
+	l.MoveBefore(e2, e4)
+	checkListPointers(t, l, []*Element{e1, e3, e2, e4})
+	e1, e2, e3, e4 = e1, e3, e2, e4
+
+	l.MoveBefore(e4, e1)
+	checkListPointers(t, l, []*Element{e4, e1, e2, e3})
+	e1, e2, e3, e4 = e4, e1, e2, e3
+
+	l.MoveAfter(e4, e1)
+	checkListPointers(t, l, []*Element{e1, e4, e2, e3})
+	e1, e2, e3, e4 = e1, e4, e2, e3
+
+	l.MoveAfter(e2, e3)
+	checkListPointers(t, l, []*Element{e1, e3, e2, e4})
+	e1, e2, e3, e4 = e1, e3, e2, e4
+}