Add ISBN-10 to ISBN-13 validation and conversion

Enhance NormalizeISBN to validate and convert ISBNs:
- Validate length (10 or 13 digits), return error if invalid
- Convert ISBN-10 to ISBN-13 by prefixing '978' and recalculating checksum
- Add NormalizeISBNSafe for backward compatibility in scanners

This ensures all ISBNs stored in database are valid ISBN-13 format.

internal/utils/isbn.go

@@ -1,12 +1,42 @@
 package utils
 
 import (
+	"errors"
 	"regexp"
 )
 
+var (
+	ErrInvalidISBN = errors.New("ISBN must be 10 or 13 digits")
+)
+
 // NormalizeISBN removes hyphens and spaces from ISBN to standardize format
 // Handles ISBN-10 and ISBN-13 formats
-func NormalizeISBN(isbn string) string {
+// Returns error if ISBN is not 10 or 13 digits after normalization
+// Converts ISBN-10 to ISBN-13 by prefixing with "978" and recalculating checksum
+func NormalizeISBN(isbn string) (string, error) {
+	if isbn == "" {
+		return "", nil
+	}
+
+	// Remove hyphens and spaces, return only digits and X (for ISBN-10)
+	normalized := regexp.MustCompile(`[-\s]`).ReplaceAllString(isbn, "")
+
+	// Check length: must be 10 or 13 digits
+	length := len(normalized)
+	if length == 10 {
+		// Convert ISBN-10 to ISBN-13
+		return convertISBN10To13(normalized)
+	}
+	if length == 13 {
+		return normalized, nil
+	}
+
+	return "", ErrInvalidISBN
+}
+
+// NormalizeISBNSafe removes hyphens and spaces from ISBN without validation
+// Used by scanners where metadata may be incomplete or malformed
+func NormalizeISBNSafe(isbn string) string {
 	if isbn == "" {
 		return ""
 	}
@@ -14,3 +44,24 @@ func NormalizeISBN(isbn string) string {
 	// Remove hyphens and spaces, return only digits and X (for ISBN-10)
 	return regexp.MustCompile(`[-\s]`).ReplaceAllString(isbn, "")
 }
+
+// convertISBN10To13 converts ISBN-10 to ISBN-13 by prefixing "978" and recalculating checksum
+func convertISBN10To13(isbn10 string) (string, error) {
+	// ISBN-10 to ISBN-13: prefix "978" and recalculate checksum
+	// Replace last digit (X becomes 0 for calculation purposes)
+	isbn12 := "978" + isbn10[:9]
+
+	// Calculate ISBN-13 checksum
+	sum := 0
+	for i := 0; i < 12; i++ {
+		digit := int(isbn12[i] - '0')
+		if i%2 == 0 {
+			sum += digit * 1
+		} else {
+			sum += digit * 3
+		}
+	}
+	checksum := (10 - (sum % 10)) % 10
+
+	return isbn12 + string(rune('0'+checksum)), nil
+}