syncthing/luhn/luhn.go
2014-07-04 16:16:50 +02:00

69 lines
1.5 KiB
Go

// Package luhn generates and validates Luhn mod N check digits.
package luhn
import (
"fmt"
"strings"
)
// An alphabet is a string of N characters, representing the digits of a given
// base N.
type Alphabet string
var (
Base32 Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
)
// 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, 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
} else {
factor = 2
}
addend = (addend / n) + (addend % n)
sum += addend
}
remainder := sum % n
checkCodepoint := (n - remainder) % n
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, 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
}