Various improvements.

serial/serial.go

@@ -8,7 +8,8 @@ import (
 )
 
 const (
-	MaxIndex = block
+	MaxIndex          = block
+	MaxRandomAttempts = 1024
 
 	letters = 26
 	numbers = 900                                             // 100–999
@@ -23,9 +24,16 @@ var (
 	ErrNumberOutOfRange       = errors.New("number out of range")
 	ErrInvalidChecksum        = errors.New("invalid checksum")
 	ErrIndexOutOfRange        = errors.New("index out of range")
-	ErrRandomGenerationFailed = errors.New("random code generation failed")
+	ErrRandomSourceFailed     = errors.New("random source failed")
+	ErrNoAvailableIndex       = errors.New("no available index found")
 )
 
+type RandomCodeOptions struct {
+	MaxAttempts int
+	IsInUse     func(uint32) bool
+	RandomIndex func(max uint32) (uint32, error)
+}
+
 func Encode(code string) (uint32, error) {
 	// Expected format: L1L2L3-NNN-L4C  => length 10, positions: 0,1,2,4,5,6,8,9
 	if len(code) != 10 || code[3] != '-' || code[7] != '-' {
@@ -103,15 +111,34 @@ func Decode(idx uint32) (string, error) {
 }
 
 func RandomCode(isInUse ...func(uint32) bool) (string, uint32, error) {
-	var inUse func(uint32) bool
+	options := RandomCodeOptions{}
 	if len(isInUse) > 0 {
-		inUse = isInUse[0]
+		options.IsInUse = isInUse[0]
 	}
 
-	for attempts := 0; attempts < 1024; attempts++ {
-		idx, err := randomUint32n(MaxIndex)
+	return RandomCodeWithOptions(options)
+}
+
+func RandomCodeWithOptions(options RandomCodeOptions) (string, uint32, error) {
+	maxAttempts := options.MaxAttempts
+	if maxAttempts <= 0 {
+		maxAttempts = MaxRandomAttempts
+	}
+
+	inUse := options.IsInUse
+	randomIndex := options.RandomIndex
+	if randomIndex == nil {
+		randomIndex = randomUint32n
+	}
+
+	for attempts := 0; attempts < maxAttempts; attempts++ {
+		idx, err := randomIndex(MaxIndex)
 		if err != nil {
-			return "", 0, err
+			return "", 0, fmt.Errorf("%w: %v", ErrRandomSourceFailed, err)
+		}
+
+		if idx >= MaxIndex {
+			return "", 0, fmt.Errorf("%w: index out of range", ErrRandomSourceFailed)
 		}
 
 		if inUse != nil && inUse(idx) {
@@ -129,7 +156,7 @@ func RandomCode(isInUse ...func(uint32) bool) (string, uint32, error) {
 		return "", 0, err
 	}
 
-	return "", 0, ErrRandomGenerationFailed
+	return "", 0, ErrNoAvailableIndex
 }
 
 func checksumLetter(idx uint32) byte {