mpt: export func for decoding node with type

`NodeObject` can contain auxilliary fields and shouldn't be used from outside.

pkg/core/mpt/base.go

@@ -1,6 +1,8 @@
 package mpt
 
 import (
+	"fmt"
+
 	"github.com/nspcc-dev/neo-go/pkg/crypto/hash"
 	"github.com/nspcc-dev/neo-go/pkg/io"
 	"github.com/nspcc-dev/neo-go/pkg/util"
@@ -82,3 +84,23 @@ func encodeNodeWithType(n Node, w *io.BinWriter) {
 	w.WriteB(byte(n.Type()))
 	n.EncodeBinary(w)
 }
+
+// DecodeNodeWithType decodes node together with it's type.
+func DecodeNodeWithType(r *io.BinReader) Node {
+	var n Node
+	switch typ := NodeType(r.ReadB()); typ {
+	case BranchT:
+		n = new(BranchNode)
+	case ExtensionT:
+		n = new(ExtensionNode)
+	case HashT:
+		n = new(HashNode)
+	case LeafT:
+		n = new(LeafNode)
+	default:
+		r.Err = fmt.Errorf("invalid node type: %x", typ)
+		return nil
+	}
+	n.DecodeBinary(r)
+	return n
+}

pkg/core/mpt/node.go

@@ -4,7 +4,6 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"errors"
-	"fmt"
 
 	"github.com/nspcc-dev/neo-go/pkg/io"
 	"github.com/nspcc-dev/neo-go/pkg/util"
@@ -43,21 +42,7 @@ func (n NodeObject) EncodeBinary(w *io.BinWriter) {
 
 // DecodeBinary implements io.Serializable.
 func (n *NodeObject) DecodeBinary(r *io.BinReader) {
-	typ := NodeType(r.ReadB())
-	switch typ {
-	case BranchT:
-		n.Node = new(BranchNode)
-	case ExtensionT:
-		n.Node = new(ExtensionNode)
-	case HashT:
-		n.Node = new(HashNode)
-	case LeafT:
-		n.Node = new(LeafNode)
-	default:
-		r.Err = fmt.Errorf("invalid node type: %x", typ)
-		return
-	}
-	n.Node.DecodeBinary(r)
+	n.Node = DecodeNodeWithType(r)
 }
 
 // UnmarshalJSON implements json.Unmarshaler.