syncthing/lib/protocol/luhn.go

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// Copyright (C) 2014 The Protocol Authors.
package protocol
import "fmt"
var luhnBase32 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
func codepoint32(b byte) int {
switch {
case 'A' <= b && b <= 'Z':
return int(b - 'A')
case '2' <= b && b <= '7':
return int(b + 26 - '2')
default:
return -1
}
}
// luhn32 returns a check digit for the string s, which should be composed
// of characters from the alphabet luhnBase32.
// Doesn't follow the actual Luhn algorithm
// see https://forum.syncthing.net/t/v0-9-0-new-node-id-format/478/6 for more.
func luhn32(s string) (rune, error) {
factor := 1
sum := 0
const n = 32
for i := range s {
codepoint := codepoint32(s[i])
if codepoint == -1 {
return 0, fmt.Errorf("digit %q not valid in alphabet %q", s[i], luhnBase32)
}
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(luhnBase32[checkCodepoint]), nil
}