You need to consider how you would update fields in the union. Obviously, you can't let users do that without validation. They could do inconsistent updates. Consider doing something like this:
package mifare
const (
MDFTStandarDataFile = 0x00
MDFTBackupDataFile = 0x01
MDFTValueFileWithBackup = 0x02
MDFTLinearRecordFileWithBackup = 0x03
MDFTCyclicRecordFileWithBackup = 0x04
)
type StandardFile struct {
FileSize uint32
}
type ValueFile struct {
LowerLimit int32
UpperLimit int32
LimitedCreditValue int32
LimitedCreditEnabled uint8
}
type LinearRecordFile struct {
Record_size uint32
MaxNumberOfRecords uint32
CurrentNumberOfRecords uint32
}
type DESFireFileSettings struct {
FileType uint8
CommunicationSettings uint8
AccessRights uint16
settings struct {
StandardFile
ValueFile
LinearRecordFile
}
}
func (fs *DESFireFileSettings) StandardFile() (StandardFile, error) {
// if not valid for FileType, return error
return fs.settings.StandardFile, nil
}
func (fs *DESFireFileSettings) SetStandardFile(standardFile StandardFile) error {
// if not valid for FileType, return error
fs.settings.StandardFile = standardFile
return nil
}
func (fs *DESFireFileSettings) ValueFile() (ValueFile, error) {
// if not valid for FileType, return error
return fs.settings.ValueFile, nil
}
func (fs *DESFireFileSettings) SetValueFile(valueFile ValueFile) error {
// if not valid for FileType, return error
fs.settings.ValueFile = valueFile
return nil
}
func (fs *DESFireFileSettings) LinearRecordFile() (LinearRecordFile, error) {
// if not valid for FileType, return error
return fs.settings.LinearRecordFile, nil
}
func (fs *DESFireFileSettings) SetLinearRecordFile(linearRecordFile LinearRecordFile) error {
// if not valid for FileType, return error
fs.settings.LinearRecordFile = linearRecordFile
return nil
}