56 lines
1.3 KiB
Go
56 lines
1.3 KiB
Go
// Copyright (C) 2014 The Protocol Authors.
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package protocol
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import (
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"fmt"
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"strings"
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)
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// An alphabet is a string of N characters, representing the digits of a given
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// base N.
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type luhnAlphabet string
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var (
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luhnBase32 luhnAlphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
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)
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// generate returns a check digit for the string s, which should be composed
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// of characters from the Alphabet a.
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// Doesn't follow the actual Luhn algorithm
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// see https://forum.syncthing.net/t/v0-9-0-new-node-id-format/478/6 for more.
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func (a luhnAlphabet) generate(s string) (rune, error) {
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factor := 1
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sum := 0
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n := len(a)
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for i := range s {
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codepoint := strings.IndexByte(string(a), s[i])
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if codepoint == -1 {
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return 0, fmt.Errorf("Digit %q not valid in alphabet %q", s[i], a)
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}
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addend := factor * codepoint
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if factor == 2 {
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factor = 1
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} else {
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factor = 2
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}
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addend = (addend / n) + (addend % n)
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sum += addend
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}
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remainder := sum % n
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checkCodepoint := (n - remainder) % n
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return rune(a[checkCodepoint]), nil
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}
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// luhnValidate returns true if the last character of the string s is correct, for
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// a string s composed of characters in the alphabet a.
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func (a luhnAlphabet) luhnValidate(s string) bool {
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t := s[:len(s)-1]
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c, err := a.generate(t)
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if err != nil {
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return false
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}
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return rune(s[len(s)-1]) == c
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}
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