[#356] refs: Add functions to work with zero subnet

Add helper functions which provide ease of use with subnet zero IDs.

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

refs/types.go

@@ -183,6 +183,16 @@ func (s *SubnetID) GetValue() uint32 {
 	return 0
 }
 
+// IsZeroSubnet returns true iff the SubnetID refers to zero subnet.
+func IsZeroSubnet(id *SubnetID) bool {
+	return id.GetValue() == 0
+}
+
+// MakeZeroSubnet makes the SubnetID to refer to zero subnet.
+func MakeZeroSubnet(id *SubnetID) {
+	id.SetValue(0)
+}
+
 func (v *Version) GetMajor() uint32 {
 	if v != nil {
 		return v.major

refs/types_test.go

@@ -0,0 +1,20 @@
+package refs_test
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/refs"
+	"github.com/stretchr/testify/require"
+)
+
+func TestZeroSubnet(t *testing.T) {
+	id := new(refs.SubnetID)
+
+	require.True(t, refs.IsZeroSubnet(id))
+
+	id.SetValue(1)
+	require.False(t, refs.IsZeroSubnet(id))
+
+	refs.MakeZeroSubnet(id)
+	require.True(t, refs.IsZeroSubnet(id))
+}