Luhn error checking

luhn/luhn.go

@@ -1,7 +1,10 @@
 // Package luhn generates and validates Luhn mod N check digits.
 package luhn
 
-import "strings"
+import (
+	"fmt"
+	"strings"
+)
 
 // An alphabet is a string of N characters, representing the digits of a given
 // base N.
@@ -13,13 +16,20 @@ var (
 
 // Generate returns a check digit for the string s, which should be composed
 // of characters from the Alphabet a.
-func (a Alphabet) Generate(s string) rune {
+func (a Alphabet) Generate(s string) (rune, error) {
+	if err:=a.check();err!=nil{
+		return 0,err
+	}
+
 	factor := 1
 	sum := 0
 	n := len(a)
 
 	for i := range s {
 		codepoint := strings.IndexByte(string(a), s[i])
+		if codepoint == -1 {
+			return 0, fmt.Errorf("Digit %q not valid in alphabet %q", s[i], a)
+		}
 		addend := factor * codepoint
 		if factor == 2 {
 			factor = 1
@@ -31,13 +41,28 @@ func (a Alphabet) Generate(s string) rune {
 	}
 	remainder := sum % n
 	checkCodepoint := (n - remainder) % n
-	return rune(a[checkCodepoint])
+	return rune(a[checkCodepoint]), nil
 }
 
 // Validate returns true if the last character of the string s is correct, for
 // a string s composed of characters in the alphabet a.
 func (a Alphabet) Validate(s string) bool {
 	t := s[:len(s)-1]
-	c := a.Generate(t)
+	c, err := a.Generate(t)
+	if err != nil {
+		return false
+	}
 	return rune(s[len(s)-1]) == c
 }
+
+// check returns an error if the given alphabet does not consist of unique characters
+func (a Alphabet) check() error {
+	cm := make(map[byte]bool, len(a))
+	for i := range a {
+		if cm[a[i]] {
+			return fmt.Errorf("Digit %q non-unique in alphabet %q", a[i], a)
+		}
+		cm[a[i]] = true
+	}
+	return nil
+}