47 lines
1.0 KiB
Go
47 lines
1.0 KiB
Go
// Copyright (C) 2014 The Protocol Authors.
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package protocol
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import "fmt"
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var luhnBase32 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
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func codepoint32(b byte) int {
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switch {
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case 'A' <= b && b <= 'Z':
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return int(b - 'A')
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case '2' <= b && b <= '7':
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return int(b + 26 - '2')
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default:
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return -1
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}
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}
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// luhn32 returns a check digit for the string s, which should be composed
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// of characters from the alphabet luhnBase32.
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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 luhn32(s string) (rune, error) {
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factor := 1
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sum := 0
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const n = 32
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for i := range s {
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codepoint := codepoint32(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], luhnBase32)
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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(luhnBase32[checkCodepoint]), nil
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}
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