Added CompleteCode function.

serial/serial.go

@@ -19,15 +19,16 @@ const (
 )
 
 var (
-	ErrInvalidFormat          = errors.New("invalid format, expected LLL-NNN-LC")
-	ErrInvalidLetters         = errors.New("invalid letters")
-	ErrForbiddenLetterTriplet = errors.New("forbidden letter combination")
-	ErrInvalidNumber          = errors.New("invalid number")
-	ErrNumberOutOfRange       = errors.New("number out of range")
-	ErrInvalidChecksum        = errors.New("invalid checksum")
-	ErrIndexOutOfRange        = errors.New("index out of range")
-	ErrRandomSourceFailed     = errors.New("random source failed")
-	ErrNoAvailableIndex       = errors.New("no available index found")
+	ErrInvalidFormat           = errors.New("invalid format, expected LLL-NNN-LC")
+	ErrInvalidFormatNoChecksum = errors.New("invalid format, expected LLL-NNN-L")
+	ErrInvalidLetters          = errors.New("invalid letters")
+	ErrForbiddenLetterTriplet  = errors.New("forbidden letter combination")
+	ErrInvalidNumber           = errors.New("invalid number")
+	ErrNumberOutOfRange        = errors.New("number out of range")
+	ErrInvalidChecksum         = errors.New("invalid checksum")
+	ErrIndexOutOfRange         = errors.New("index out of range")
+	ErrRandomSourceFailed      = errors.New("random source failed")
+	ErrNoAvailableIndex        = errors.New("no available index found")
 )
 
 type RandomCodeOptions struct {
@@ -112,6 +113,40 @@ func Decode(idx uint32) (string, error) {
 	return code, nil
 }
 
+func CompleteCode(codeWithoutChecksum string) (string, uint32, error) {
+	if len(codeWithoutChecksum) != 9 || codeWithoutChecksum[3] != '-' || codeWithoutChecksum[7] != '-' {
+		return "", 0, ErrInvalidFormatNoChecksum
+	}
+
+	l1, l2, l3, l4 := codeWithoutChecksum[0], codeWithoutChecksum[1], codeWithoutChecksum[2], codeWithoutChecksum[8]
+	n1, n2, n3 := codeWithoutChecksum[4], codeWithoutChecksum[5], codeWithoutChecksum[6]
+
+	if !isUpperAlphaASCII(l1) || !isUpperAlphaASCII(l2) || !isUpperAlphaASCII(l3) || !isUpperAlphaASCII(l4) {
+		return "", 0, ErrInvalidLetters
+	}
+	if IsForbiddenTriplet(l1, l2, l3) {
+		return "", 0, ErrForbiddenLetterTriplet
+	}
+	if !isNumber(n1) || !isNumber(n2) || !isNumber(n3) {
+		return "", 0, ErrInvalidNumber
+	}
+
+	L1 := uint32(l1 - 'A')
+	L2 := uint32(l2 - 'A')
+	L3 := uint32(l3 - 'A')
+	L4 := uint32(l4 - 'A')
+	num := uint32(n1-'0')*100 + uint32(n2-'0')*10 + uint32(n3-'0')
+	if num < 100 || num > 999 {
+		return "", 0, ErrNumberOutOfRange
+	}
+	N := num - 100
+
+	idx := uint32((((((L1*letters+L2)*letters+L3)*letters + L4) * numbers) + N))
+	checksum := checksumLetter(idx)
+
+	return codeWithoutChecksum + string(checksum), idx, nil
+}
+
 func RandomCode(isInUse ...func(uint32) bool) (string, uint32, error) {
 	options := RandomCodeOptions{}
 	if len(isInUse) > 0 {

serial/serial_test.go

@@ -192,6 +192,38 @@ func TestEncodeRejectsNonDigitInNumber(t *testing.T) {
 	}
 }
 
+func TestCompleteCodeRoundtrip(t *testing.T) {
+	original, err := serial.Decode(123456)
+	if err != nil {
+		t.Fatalf("Decode failed: %v", err)
+	}
+
+	withoutChecksum := original[:len(original)-1]
+	full, idx, err := serial.CompleteCode(withoutChecksum)
+	if err != nil {
+		t.Fatalf("CompleteCode failed: %v", err)
+	}
+
+	if full != original {
+		t.Fatalf("completed code mismatch: got %q, want %q", full, original)
+	}
+
+	idx2, err := serial.Encode(full)
+	if err != nil {
+		t.Fatalf("Encode failed for completed code %q: %v", full, err)
+	}
+	if idx2 != idx {
+		t.Fatalf("idx mismatch: got %d, want %d", idx, idx2)
+	}
+}
+
+func TestCompleteCodeRejectsInvalidFormat(t *testing.T) {
+	_, _, err := serial.CompleteCode("AAA-10-A")
+	if !errors.Is(err, serial.ErrInvalidFormatNoChecksum) {
+		t.Fatalf("expected invalid no-checksum format error, got: %v", err)
+	}
+}
+
 func TestRandomCodeRoundtrip(t *testing.T) {
 	for i := 0; i < 100; i++ {
 		code, idx, err := serial.RandomCode()