[#58] object: Allow to set marshal data

Now it is possible set marshaled data to reduce memory
allocations.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

util/proto/marshal.go

@@ -19,6 +19,11 @@ type (
 		StableMarshal([]byte) []byte
 		StableSize() int
 	}
+
+	setMarshalData interface {
+		SetMarshalData([]byte)
+		StableSize() int
+	}
 )
 
 func BytesMarshal(field int, buf, v []byte) int {
@@ -263,6 +268,26 @@ func NestedStructureMarshal[T stableMarshaller](field int64, buf []byte, v T) in
 	return offset + n
 }
 
+// NestedStructureSetMarshalData calculates offset for field in parentData
+// and calls SetMarshalData for nested structure.
+//
+// Returns marshalled data length of nested structure.
+func NestedStructureSetMarshalData(field int64, parentData []byte, v setMarshalData) int {
+	n := v.StableSize()
+	if n == 0 {
+		return 0
+	}
+
+	buf := make([]byte, binary.MaxVarintLen64)
+	prefix := protowire.EncodeTag(protowire.Number(field), protowire.BytesType)
+	offset := binary.PutUvarint(buf, prefix)
+	offset += binary.PutUvarint(buf, uint64(n))
+
+	v.SetMarshalData(parentData[offset : offset+n])
+
+	return offset + n
+}
+
 func NestedStructureSize[T stableMarshaller](field int64, v T) (size int) {
 	n := v.StableSize()
 	if n == 0 {