const (
    letters = 26
    numbers = 900 // 100–999
    block   = letters * letters * letters * numbers // 26^3 * 900
)

func Encode(code string) (uint32, error) {
    // Verwacht formaat: L1L2-NNN-L3-C  => lengte 10, posities: 0,1,3,4,5,7,9
    if len(code) != 10 || code[2] != '-' || code[6] != '-' || code[8] != '-' {
        return 0, errors.New("invalid format, expected LL-NNN-L-C")
    }

    l1, l2, l3 := code[0], code[1], code[7]

    // letters checken
    // forbidden alleen op l1,l2
    // nummer parsen uit code[3:6]

    L1 := int(l1 - 'A')
    L2 := int(l2 - 'A')
    L3 := int(l3 - 'A')
    N  := num - 100

    idx := uint32((((L1*letters + L2)*letters + L3) * numbers) + N)

    if code[9] != checksumLetter(idx) {
        return 0, errors.New("invalid checksum")
    }

    return idx, nil
}

func Decode(idx uint32) (string, error) {
    if idx >= block {
        return "", errors.New("index out of range")
    }

    x := int(idx)

    L1 := x / (letters * letters * numbers)
    r1 := x % (letters * letters * numbers)

    L2 := r1 / (letters * numbers)
    r2 := r1 % (letters * numbers)

    L3 := r2 / numbers
    N  := r2 % numbers

    l1 := byte('A' + L1)
    l2 := byte('A' + L2)
    l3 := byte('A' + L3)

    if isForbiddenPair(l1, l2) {
        return "", errors.New("forbidden letter combination")
    }

    c := checksumLetter(idx)

    return fmt.Sprintf("%c%c-%03d-%c-%c", l1, l2, N+100, l3, c), nil
}