trie: add difference iterator (#3637)

This PR implements a differenceIterator, which allows iterating over trie nodes
that exist in one trie but not in another. This is a prerequisite for most GC
strategies, in order to find obsolete nodes.

trie/iterator_test.go

@@ -99,9 +99,9 @@ func TestNodeIteratorCoverage(t *testing.T) {
 
 	// Gather all the node hashes found by the iterator
 	hashes := make(map[common.Hash]struct{})
-	for it := NewNodeIterator(trie); it.Next(); {
-		if it.Hash != (common.Hash{}) {
-			hashes[it.Hash] = struct{}{}
+	for it := NewNodeIterator(trie); it.Next(true); {
+		if it.Hash() != (common.Hash{}) {
+			hashes[it.Hash()] = struct{}{}
 		}
 	}
 	// Cross check the hashes and the database itself
@@ -116,3 +116,60 @@ func TestNodeIteratorCoverage(t *testing.T) {
 		}
 	}
 }
+
+func TestDifferenceIterator(t *testing.T) {
+	triea := newEmpty()
+	valsa := []struct{ k, v string }{
+		{"bar", "b"},
+		{"barb", "ba"},
+		{"bars", "bb"},
+		{"bard", "bc"},
+		{"fab", "z"},
+		{"foo", "a"},
+		{"food", "ab"},
+		{"foos", "aa"},
+	}
+	for _, val := range valsa {
+		triea.Update([]byte(val.k), []byte(val.v))
+	}
+	triea.Commit()
+
+	trieb := newEmpty()
+	valsb := []struct{ k, v string }{
+		{"aardvark", "c"},
+		{"bar", "b"},
+		{"barb", "bd"},
+		{"bars", "be"},
+		{"fab", "z"},
+		{"foo", "a"},
+		{"foos", "aa"},
+		{"food", "ab"},
+		{"jars", "d"},
+	}
+	for _, val := range valsb {
+		trieb.Update([]byte(val.k), []byte(val.v))
+	}
+	trieb.Commit()
+
+	found := make(map[string]string)
+	di, _ := NewDifferenceIterator(NewNodeIterator(triea), NewNodeIterator(trieb))
+	it := NewIteratorFromNodeIterator(di)
+	for it.Next() {
+		found[string(it.Key)] = string(it.Value)
+	}
+
+	all := []struct{ k, v string }{
+		{"aardvark", "c"},
+		{"barb", "bd"},
+		{"bars", "be"},
+		{"jars", "d"},
+	}
+	for _, item := range all {
+		if found[item.k] != item.v {
+			t.Errorf("iterator value mismatch for %s: got %q want %q", item.k, found[item.k], item.v)
+		}
+	}
+	if len(found) != len(all) {
+		t.Errorf("iterator count mismatch: got %d values, want %d", len(found), len(all))
+	}
+}