Create FILE_ID_DESCRIPTOR in Golang

Question

I'm trying to create this structure in Golang -

typedef struct FILE_ID_DESCRIPTOR {
  DWORD        dwSize;
  FILE_ID_TYPE Type;
  union {
    LARGE_INTEGER FileId;
    GUID          ObjectId;
    FILE_ID_128   ExtendedFileId;
  } DUMMYUNIONNAME;
} FILE_ID_DESCRIPTOR, *LPFILE_ID_DESCRIPTOR;

As you can see, this depends on union of three in which unknown is FILE_ID_128 and another structure called FILE_ID_TYPE so I stumbled upon this, which is FILE_ID_128s and the _FILE_ID_TYPE structure is here and here

typedef enum _FILE_ID_TYPE {
  FileIdType,
  ObjectIdType,
  ExtendedFileIdType,
  MaximumFileIdType
} FILE_ID_TYPE, *PFILE_ID_TYPE;

And doing after doing some research, I found out that enum is const in Go but in Go, we can not have a named const so maybe a struct(?) and union are byte slices. But I could not find what MaximumFileIdType meant and how to implement that in Go. Thanks in advance.

Also, reference that I am following.

Answer 1

Here is ready to use code (taken from here, with fix applyed: in repo the worng size of struct is set, which results in invalid prameter (87) error):

/*Must put file id into ID as big endian; 
little endian form will be placed in descriptor 
automatically when calling id.Descriptor()*/
type ID [16]byte 

func (id ID) Descriptor() Descriptor {
    var structType uint32
    if id.IsInt64() {
        structType = FileType
    } else {
        structType = ExtendedFileIDType
    }

    return Descriptor{
        Size: 24, // (1)
        Type: structType,
        Data: id.LittleEndian(),
    }
}

func (id ID) IsInt64() bool {
    upper := int64(id[0])<<56 | int64(id[1])<<48 | int64(id[2])<<40 | int64(id[3])<<32 | int64(id[4])<<24 | int64(id[5])<<16 | int64(id[6])<<8 | int64(id[7])
    return upper == 0
}

func (id ID) LittleEndian() (value [16]byte) {
    for i := 0; i < 16; i++ {
        value[i] = id[15-i]
    }
    return
}

type Descriptor struct {
    Size uint32
    Type uint32
    Data [16]byte
}

const (
    FileType = iota
    ObjectIDType
    ExtendedFileIDType
)

Some explanation:

(1) size always must be 24 since it must be equal to sizeof(FILE_ID_DESCRIPTOR), which is calculated as follows:

sizeof(DWORD) + sizeof(enum) + sizeof(union)

since DWORD is uint32 the sizeof(DWORD) is 4 bytes; in C++ enums are represented as uint32 too; and sizeof(union) equals the size of largest member of union, which is FILE_ID_128 in our case. size of 128 bit value is 16 bytes. Therefore the above expression can be substituted with:

4 + 4 + 16 = 24