84 lines
2.2 KiB
Go
84 lines
2.2 KiB
Go
// Copyright (C) 2014 The Syncthing Authors.
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//
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// This program is free software: you can redistribute it and/or modify it
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// under the terms of the GNU General Public License as published by the Free
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// Software Foundation, either version 3 of the License, or (at your option)
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// any later version.
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//
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// This program is distributed in the hope that it will be useful, but WITHOUT
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// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
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// more details.
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//
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// You should have received a copy of the GNU General Public License along
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// with this program. If not, see <http://www.gnu.org/licenses/>.
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// Package luhn generates and validates Luhn mod N check digits.
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package luhn
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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 Alphabet string
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var (
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Base32 Alphabet = "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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func (a Alphabet) Generate(s string) (rune, error) {
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if err := a.check(); err != nil {
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return 0, err
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}
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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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// Validate 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 Alphabet) Validate(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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// check returns an error if the given alphabet does not consist of unique characters
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func (a Alphabet) check() error {
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cm := make(map[byte]bool, len(a))
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for i := range a {
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if cm[a[i]] {
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return fmt.Errorf("Digit %q non-unique in alphabet %q", a[i], a)
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}
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cm[a[i]] = true
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}
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return nil
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}
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