utils/iterator.go

@@ -14,13 +14,15 @@ type symmDiffIterator struct {
 	eqPathIndex int       // Count index of last pair of equal paths, to detect an updated key
 }
 
-// NewDifferenceIterator constructs a trie.NodeIterator that iterates over the exclusive elements in b that
-// are not in a. Returns the iterator, and a pointer to an integer recording the number
-// of nodes seen.
+// NewSymmetricDifferenceIterator constructs a trie.NodeIterator that iterates over the symmetric difference
+// of elements in a and b, i.e., the elements in a that are not in b, and vice versa.
+// Returns the iterator, and a pointer to an integer recording the number of nodes seen.
 func NewSymmetricDifferenceIterator(a, b trie.NodeIterator) (*symmDiffIterator, *int) {
 	it := &symmDiffIterator{
 		a: iterState{a, true},
 		b: iterState{b, true},
+		// common paths are detected by a distance <=1 from this index, so put it out of reach
+		eqPathIndex: -2,
 	}
 	return it, &it.count
 }
@@ -91,9 +93,15 @@ func (it *symmDiffIterator) AddResolver(resolver trie.NodeResolver) {
 }
 
 func (it *symmDiffIterator) Next(bool) bool {
-	// NodeIterators start in a "pre-valid" state, so you have to call Next before they point to a
-	// valid node. This delays advancing the sub-iterators until they are so initialized by seek.
-	if it.count != 0 {
+	// NodeIterators start in a "pre-valid" state, so the first Next advances to a valid node.
+	if it.count == 0 {
+		if it.a.Next(true) {
+			it.count++
+		}
+		if it.b.Next(true) {
+			it.count++
+		}
+	} else {
 		if it.curr().Next(true) {
 			it.count++
 		}
@@ -120,7 +128,7 @@ func (it *symmDiffIterator) seek() {
 		cmp := bytes.Compare(it.a.Path(), it.b.Path())
 		if cmp == 0 {
 			it.eqPathIndex = it.count
-			cmp = compareValues(&it.a, &it.b)
+			cmp = compareNodes(&it.a, &it.b)
 		}
 		switch cmp {
 		case -1:
@@ -147,8 +155,10 @@ func (it *symmDiffIterator) Error() error {
 	return it.b.Error()
 }
 
-// Compares nodes with equal paths by value
-func compareValues(a, b trie.NodeIterator) int {
+func compareNodes(a, b trie.NodeIterator) int {
+	// if cmp := bytes.Compare(a.Path(), b.Path()); cmp != 0 {
+	// 	return cmp
+	// }
 	if a.Leaf() && !b.Leaf() {
 		return -1
 	} else if b.Leaf() && !a.Leaf() {

utils/iterator_test.go

@@ -14,13 +14,13 @@ type kvs struct{ k, v string }
 
 var (
 	testdata1 = []kvs{
-		{"bar", "b"},
 		{"barb", "ba"},
 		{"bard", "bc"},
 		{"bars", "bb"},
+		{"bar", "b"},
 		{"fab", "z"},
-		{"foo", "a"},
 		{"food", "ab"},
+		{"foo", "a"},
 	}
 
 	testdata2 = []kvs{
@@ -38,22 +38,30 @@ var (
 func TestSymmetricDifferenceIterator(t *testing.T) {
 	t.Run("with no difference", func(t *testing.T) {
 		db := trie.NewDatabase(rawdb.NewMemoryDatabase())
-		tree := trie.NewEmpty(db)
-		di, count := utils.NewSymmetricDifferenceIterator(tree.NodeIterator(nil), tree.NodeIterator(nil))
+		triea := trie.NewEmpty(db)
+		di, count := utils.NewSymmetricDifferenceIterator(triea.NodeIterator(nil), triea.NodeIterator(nil))
 		for di.Next(true) {
 			t.Errorf("iterator should not yield any elements")
 		}
 		assert.Equal(t, 0, *count)
 
-		tree.MustUpdate([]byte("foo"), []byte("bar"))
-		di, count = utils.NewSymmetricDifferenceIterator(tree.NodeIterator(nil), tree.NodeIterator(nil))
+		triea.MustUpdate([]byte("foo"), []byte("bar"))
+		di, count = utils.NewSymmetricDifferenceIterator(triea.NodeIterator(nil), triea.NodeIterator(nil))
 		for di.Next(true) {
 			t.Errorf("iterator should not yield any elements")
 		}
 		assert.Equal(t, 4, *count)
+
+		trieb := trie.NewEmpty(db)
+		di, count = utils.NewSymmetricDifferenceIterator(triea.NodeIterator([]byte("food")), trieb.NodeIterator(nil))
+		for di.Next(true) {
+			t.Errorf("iterator should not yield any elements")
+			t.Logf("%s", di.LeafKey())
+		}
+		assert.Equal(t, 0, *count)
 	})
 
-	t.Run("with one difference", func(t *testing.T) {
+	t.Run("small difference", func(t *testing.T) {
 		dba := trie.NewDatabase(rawdb.NewMemoryDatabase())
 		triea := trie.NewEmpty(dba)
 
@@ -62,14 +70,31 @@ func TestSymmetricDifferenceIterator(t *testing.T) {
 		trieb.MustUpdate([]byte("foo"), []byte("bar"))
 
 		di, count := utils.NewSymmetricDifferenceIterator(triea.NodeIterator(nil), trieb.NodeIterator(nil))
+		leaves := 0
 		for di.Next(true) {
 			if di.Leaf() {
-				assert.Equal(t, []byte("foo"), di.LeafKey())
-				assert.Equal(t, []byte("bar"), di.LeafBlob())
 				assert.False(t, di.CommonPath())
+				assert.Equal(t, "foo", string(di.LeafKey()))
+				assert.Equal(t, "bar", string(di.LeafBlob()))
+				leaves++
 			}
 		}
+		assert.Equal(t, 1, leaves)
 		assert.Equal(t, 2, *count)
+
+		trieb.MustUpdate([]byte("quux"), []byte("bars"))
+		di, count = utils.NewSymmetricDifferenceIterator(triea.NodeIterator(nil), trieb.NodeIterator([]byte("quux")))
+		leaves = 0
+		for di.Next(true) {
+			if di.Leaf() {
+				assert.False(t, di.CommonPath())
+				assert.Equal(t, "quux", string(di.LeafKey()))
+				assert.Equal(t, "bars", string(di.LeafBlob()))
+				leaves++
+			}
+		}
+		assert.Equal(t, 1, leaves)
+		assert.Equal(t, 1, *count)
 	})
 
 	dba := trie.NewDatabase(rawdb.NewMemoryDatabase())