treapstore: Add Collection.VisitItemsAscend.

common/data/treapstore/store.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 // Package treapstore is a lightweight append-only in-memory key-value store
 // built on top a treap (tree + heap) implementation.
 //
 // treapstore is specifically focused on supporting the in-memory datastore
 // implementation at "github.com/luci/gae/impl/memory".
 package treapstore
@@ skipping 219 matching lines @@
 }
 
 func (c *Collection) setRoot(root *gtreap.Treap) {
         if c.rootLock != nil {
                 c.rootLock.Lock()
                 defer c.rootLock.Unlock()
         }
         c.root = root
 }
 
-// Iterator returns an iterator over the Store, starting at the supplied pivot
-// item.
+// VisitAscend traverses the Collection ascendingly, invoking visitor for each
+// visited item.
+//
+// If visitor returns false, iteration will stop prematurely.
+//
+// VisitAscend is a more efficient traversal than using an Iterator, and is
+// useful in times when entry-by-entry iteration is not required.
+func (c *Collection) VisitAscend(pivot gtreap.Item, visitor gtreap.ItemVisitor) {
+        c.currentRoot().VisitAscend(pivot, visitor)
+}
+
+// Iterator returns an iterator over the Collection, starting at the supplied
+// pivot item.
 func (c *Collection) Iterator(pivot gtreap.Item) *gtreap.Iterator {
         root := c.currentRoot()
         if root == nil {
                 root = emptyTreap
         }
         return root.Iterator(pivot)
 }