[#79] subnet/id: Add function which resets ID to zero subnet

Add `MakeZero` function which makes `ID` instance to refer to zero
subnet.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

subnet/id/id.go

@@ -87,3 +87,10 @@ func (x *ID) SetNumber(num uint32) {
 func IsZero(id ID) bool {
 	return id.Equals(nil)
 }
+
+// MakeZero makes ID to refer to zero subnet. Arg must not be nil (it is already zero).
+//
+// Makes no sense to call on zero value (e.g. declared using var).
+func MakeZero(id *ID) {
+	id.SetNumber(0)
+}

subnet/id/id_test.go

@@ -92,3 +92,15 @@ func TestSubnetIDEncoding(t *testing.T) {
 		require.True(t, id2.Equals(id))
 	})
 }
+
+func TestMakeZero(t *testing.T) {
+	var id subnetid.ID
+
+	id.SetNumber(13)
+
+	require.False(t, subnetid.IsZero(id))
+
+	subnetid.MakeZero(&id)
+
+	require.True(t, subnetid.IsZero(id))
+}