Consistency changes to container/* packages for iteration.

container/list:
  - change Iter to go over the list values

container/ring:
  - add Iter, drop Forward/Backward

container/vector:
  - add channel direction constraints

R=rsc,gri
APPROVED=rsc
DELTA=86  (23 added, 40 deleted, 23 changed)
OCL=33935
CL=34132

7 files changed, 46 insertions, 63 deletions

diff --git a/src/pkg/container/list/list.go b/src/pkg/container/list/list.go
index 3b77ced59..64000e463 100755
--- a/src/pkg/container/list/list.go
+++ b/src/pkg/container/list/list.go
@@ -18,6 +18,16 @@ type Element struct {
 	Value interface {};
 }
 
+// Next returns the next list element or nil.
+func (e *Element) Next() *Element {
+	return e.next
+}
+
+// Prev returns the previous list element or nil.
+func (e *Element) Prev() *Element {
+	return e.prev
+}
+
 // List represents a doubly linked list.
 type List struct {
 	front, back *Element;
@@ -181,18 +191,15 @@ func (l *List) Len() int {
 	return l.len
 }
 
-func (l *List) iterate(c chan <- *Element) {
-	var next *Element;
-	for e := l.front; e != nil; e = next {
-		// Save next in case reader of c changes e.
-		next = e.next;
-		c <- e;
+func (l *List) iterate(c chan<- interface {}) {
+	for e := l.front; e != nil; e = e.next {
+		c <- e.Value;
 	}
 	close(c);
 }
 
-func (l *List) Iter() <-chan *Element {
-	c := make(chan *Element);
+func (l *List) Iter() <-chan interface {} {
+	c := make(chan interface {});
 	go l.iterate(c);
 	return c
 }
diff --git a/src/pkg/container/list/list_test.go b/src/pkg/container/list/list_test.go
index 4a291e918..741aa5516 100755
--- a/src/pkg/container/list/list_test.go
+++ b/src/pkg/container/list/list_test.go
@@ -114,8 +114,21 @@ func TestList(t *testing.T) {
 	checkListPointers(t, l, []*Element{ e1, e4, e3, e2 });
 	l.Remove(e2);
 
-	// Clear all elements by iterating
+	// Check standard iteration.
+	sum := 0;
 	for e := range l.Iter() {
+		if i, ok := e.(int); ok {
+			sum += i;
+		}
+	}
+	if sum != 4 {
+		t.Errorf("sum over l.Iter() = %d, want 4", sum);
+	}
+
+	// Clear all elements by iterating
+	var next *Element;
+	for e := l.Front(); e != nil; e = next {
+		next = e.Next();
 		l.Remove(e);
 	}
 	checkListPointers(t, l, []*Element{});
diff --git a/src/pkg/container/ring/ring.go b/src/pkg/container/ring/ring.go
index 0cd41cb41..fc1aef96d 100644
--- a/src/pkg/container/ring/ring.go
+++ b/src/pkg/container/ring/ring.go
@@ -138,37 +138,16 @@ func (r *Ring) Len() int {
 }
 
 
-// Forward returns a channel for forward iteration through a ring.
-// Iteration is undefined if the ring is changed during iteration.
-//
-func (r *Ring) Forward() <-chan *Ring {
-	c := make(chan *Ring);
+func (r *Ring) Iter() <-chan interface {} {
+	c := make(chan interface {});
 	go func() {
 		if r != nil {
-			c <- r;
+			c <- r.Value;
 			for p := r.Next(); p != r; p = p.next {
-				c <- p;
-			}
-		}
-		close(c);
-	}();
-	return c;
-}
-
-
-// Backward returns a channel for backward iteration through a ring.
-// Iteration is undefined if the ring is changed during iteration.
-//
-func (r *Ring) Backward() <-chan *Ring {
-	c := make(chan *Ring);
-	go func() {
-		if r != nil {
-			c <- r;
-			for p := r.Prev(); p != r; p = p.prev {
-				c <- p;
+				c <- p.Value;
 			}
 		}
 		close(c);
 	}();
-	return c;
+	return c
 }
diff --git a/src/pkg/container/ring/ring_test.go b/src/pkg/container/ring/ring_test.go
index 8ecbacd14..4f81d55aa 100644
--- a/src/pkg/container/ring/ring_test.go
+++ b/src/pkg/container/ring/ring_test.go
@@ -32,13 +32,13 @@ func verify(t *testing.T, r *Ring, N int, sum int) {
 		t.Errorf("r.Len() == %d; expected %d", n, N);
 	}
 
-	// forward iteration
+	// iteration
 	n = 0;
 	s := 0;
-	for p := range r.Forward() {
+	for p := range r.Iter() {
 		n++;
-		if p.Value != nil {
-			s += p.Value.(int);
+		if p != nil {
+			s += p.(int);
 		}
 	}
 	if n != N {
@@ -48,22 +48,6 @@ func verify(t *testing.T, r *Ring, N int, sum int) {
 		t.Errorf("forward ring sum = %d; expected %d", s, sum);
 	}
 
-	// backward iteration
-	n = 0;
-	s = 0;
-	for p := range r.Backward() {
-		n++;
-		if p.Value != nil {
-			s += p.Value.(int);
-		}
-	}
-	if n != N {
-		t.Errorf("number of backward iterations == %d; expected %d", n, N);
-	}
-	if sum >= 0 && s != sum {
-		t.Errorf("backward ring sum = %d; expected %d", s, sum);
-	}
-
 	if r == nil {
 		return;
 	}
@@ -147,8 +131,8 @@ func makeN(n int) *Ring {
 
 func sum(r *Ring) int {
 	s := 0;
-	for p := range r.Forward() {
-		s += p.Value.(int);
+	for p := range r.Iter() {
+		s += p.(int);
 	}
 	return s;
 }
diff --git a/src/pkg/container/vector/intvector.go b/src/pkg/container/vector/intvector.go
index f599ce185..076f9982d 100644
--- a/src/pkg/container/vector/intvector.go
+++ b/src/pkg/container/vector/intvector.go
@@ -101,7 +101,7 @@ func (p *IntVector) Less(i, j int) bool {
 
 
 // Iterate over all elements; driver for range
-func (p *IntVector) iterate(c chan int) {
+func (p *IntVector) iterate(c chan<- int) {
 	for i, v := range p.a {
 		c <- v.(int)
 	}
@@ -110,7 +110,7 @@ func (p *IntVector) iterate(c chan int) {
 
 
 // Channel iterator for range.
-func (p *IntVector) Iter() chan int {
+func (p *IntVector) Iter() <-chan int {
 	c := make(chan int);
 	go p.iterate(c);
 	return c;
diff --git a/src/pkg/container/vector/stringvector.go b/src/pkg/container/vector/stringvector.go
index 4f6d74e29..2ead95c70 100644
--- a/src/pkg/container/vector/stringvector.go
+++ b/src/pkg/container/vector/stringvector.go
@@ -100,7 +100,7 @@ func (p *StringVector) Less(i, j int) bool {
 
 
 // Iterate over all elements; driver for range
-func (p *StringVector) iterate(c chan string) {
+func (p *StringVector) iterate(c chan<- string) {
 	for i, v := range p.a {
 		c <- v.(string)
 	}
@@ -109,7 +109,7 @@ func (p *StringVector) iterate(c chan string) {
 
 
 // Channel iterator for range.
-func (p *StringVector) Iter() chan string {
+func (p *StringVector) Iter() <-chan string {
 	c := make(chan string);
 	go p.iterate(c);
 	return c;
diff --git a/src/pkg/container/vector/vector.go b/src/pkg/container/vector/vector.go
index 5b5cad21c..ba5e881d1 100644
--- a/src/pkg/container/vector/vector.go
+++ b/src/pkg/container/vector/vector.go
@@ -228,7 +228,7 @@ func (p *Vector) Swap(i, j int) {
 
 
 // Iterate over all elements; driver for range
-func (p *Vector) iterate(c chan Element) {
+func (p *Vector) iterate(c chan<- Element) {
 	for i, v := range p.a {
 		c <- v
 	}
@@ -237,7 +237,7 @@ func (p *Vector) iterate(c chan Element) {
 
 
 // Channel iterator for range.
-func (p *Vector) Iter() chan Element {
+func (p *Vector) Iter() <-chan Element {
 	c := make(chan Element);
 	go p.iterate(c);
 	return c;