fix Children() and add test (#159)

Author: gballet

stateless.go

@@ -70,11 +70,15 @@ func NewStateless() *StatelessNode {
 }
 
 func (n *StatelessNode) Children() []VerkleNode {
-	var children []VerkleNode
-	for _, child := range n.children {
-		children = append(children, child)
+	var children [256]VerkleNode
+	for i := range children {
+		if n.children[byte(i)] != nil {
+			children[i] = n.children[byte(i)]
+		} else {
+			children[i] = Empty(struct{}{})
+		}
 	}
-	return children
+	return children[:]
 }
 
 func (n *StatelessNode) SetChild(i int, v VerkleNode) error {

stateless_test.go

@@ -33,6 +33,38 @@ import (
 	"testing"
 )
 
+func TestStatelessChildren(t *testing.T) {
+	root := NewStateless()
+	root.Insert(zeroKeyTest, fourtyKeyTest, nil)
+	root.Insert(oneKeyTest, fourtyKeyTest, nil)
+
+	list := root.Children()
+	if len(list) != NodeWidth {
+		t.Fatal("invalid list length")
+	}
+
+	var emptycount = 0
+	for _, v := range list {
+		if _, ok := v.(Empty); ok {
+			emptycount++
+		}
+	}
+	if emptycount != NodeWidth-1 {
+		t.Fatal("invalid number of children")
+	}
+
+	if err := root.SetChild(72, Empty{}); err == nil {
+		t.Fatal("didn't catch a stateful node being inserted in a stateless node")
+	}
+	if err := root.SetChild(512, Empty{}); err == nil {
+		t.Fatal("didn't catch a node being inserted at an invalid index in a stateless node")
+	}
+
+	if err := root.SetChild(3, &StatelessNode{}); err != nil {
+		t.Fatal("error inserting stateless node")
+	}
+}
+
 func TestStatelessDelete(t *testing.T) {
 	root := NewStateless()
 	root.Insert(zeroKeyTest, fourtyKeyTest, nil)