container/ring: Replace Iter() with Do().

Faster in most cases, and not prone to memory leaks. Named "Do" to match with similarly named method on Vector.

R=gri
CC=golang-dev
https://golang.org/cl/4134046

src/pkg/container/ring/ring.go

@@ -138,16 +138,13 @@ func (r *Ring) Len() int {
 }
 
 
-func (r *Ring) Iter() <-chan interface{} {
-	c := make(chan interface{})
-	go func() {
+// Do calls function f on each element of the ring, in forward order.
+// The behavior of Do is undefined if f changes *r.
+func (r *Ring) Do(f func(interface{})) {
 	if r != nil {
-			c <- r.Value
+		f(r.Value)
 		for p := r.Next(); p != r; p = p.next {
-				c <- p.Value
+			f(p.Value)
 		}
 	}
-		close(c)
-	}()
-	return c
 }

src/pkg/container/ring/ring_test.go

@@ -35,12 +35,12 @@ func verify(t *testing.T, r *Ring, N int, sum int) {
 	// iteration
 	n = 0
 	s := 0
-	for p := range r.Iter() {
+	r.Do(func(p interface{}) {
 		n++
 		if p != nil {
 			s += p.(int)
 		}
-	}
+	})
 	if n != N {
 		t.Errorf("number of forward iterations == %d; expected %d", n, N)
 	}
@@ -128,16 +128,6 @@ func makeN(n int) *Ring {
 	return r
 }
 
-
-func sum(r *Ring) int {
-	s := 0
-	for p := range r.Iter() {
-		s += p.(int)
-	}
-	return s
-}
-
-
 func sumN(n int) int { return (n*n + n) / 2 }