Small patch to allow resetting the btree.

btree.go

@@ -298,6 +298,20 @@ func (n *node) maybeSplitChild(i, maxItems int) bool {
 	return true
 }
 
+// reset will free all nodes within the given cow ctx.
+func (n *node) reset(c *copyOnWriteContext) {
+	for i := len(n.items) - 1; i >= 0; i-- {
+		if len(n.children) > 0 {
+			n.children[i+1].reset(c)
+		}
+	}
+	if len(n.children) > 0 {
+		n.children[0].reset(c)
+	} else {
+		c.freeNode(n)
+	}
+}
+
 // insert inserts an item into the subtree rooted at this node, making sure
 // no nodes in the subtree exceed maxItems items.  Should an equivalent item be
 // be found/replaced by insert, it will be returned.
@@ -618,6 +632,20 @@ func (t *BTree) Clone() (t2 *BTree) {
 	return &out
 }
 
+// Reset removes all items from the btree.
+func (t *BTree) Reset() {
+	if t.root == nil || len(t.root.items) == 0 {
+		return
+	}
+	t.root = t.root.mutableFor(t.cow)
+	t.root.reset(t.cow)
+	for _, n := range t.root.children {
+		t.cow.freeNode(n)
+	}
+	t.cow.freeNode(t.root)
+	t.root, t.length = nil, 0
+}
+
 // maxItems returns the max number of items to allow per node.
 func (t *BTree) maxItems() int {
 	return t.degree*2 - 1

btree_test.go

@@ -431,6 +431,48 @@ func BenchmarkDelete(b *testing.B) {
 	}
 }
 
+func BenchmarkDeleteAll(b *testing.B) {
+	b.StopTimer()
+	insertP := perm(benchmarkTreeSize)
+	b.StartTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		tr := New(*btreeDegree)
+		for _, v := range insertP {
+			tr.ReplaceOrInsert(v)
+		}
+
+		dels := make([]Item, 0, tr.Len())
+		tr.Ascend(ItemIterator(func(b Item) bool {
+			dels = append(dels, b)
+			return true
+		}))
+		for _, del := range dels {
+			tr.Delete(del)
+		}
+		if tr.Len() > 0 {
+			b.Fatal(`len must be zero`)
+		}
+	}
+}
+
+func BenchmarkReset(b *testing.B) {
+	b.StopTimer()
+	insertP := perm(benchmarkTreeSize)
+	b.StartTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		tr := New(*btreeDegree)
+		for _, v := range insertP {
+			tr.ReplaceOrInsert(v)
+		}
+		tr.Reset()
+		if tr.Len() > 0 {
+			b.Fatal(`len must be zero`)
+		}
+	}
+}
+
 func BenchmarkGet(b *testing.B) {
 	b.StopTimer()
 	insertP := perm(benchmarkTreeSize)