lib/protocol, vendor: Import luhn code directly

I've changed it incompatibly to fix a correctness bug. Nonetheless, we
should remain incorrect indefinitely.

lib/protocol/luhn.go

@@ -0,0 +1,53 @@
+// Copyright (C) 2014 The Protocol Authors.
+
+package protocol
+
+import (
+	"fmt"
+	"strings"
+)
+
+// An alphabet is a string of N characters, representing the digits of a given
+// base N.
+type luhnAlphabet string
+
+var (
+	luhnBase32 luhnAlphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
+)
+
+// generate returns a check digit for the string s, which should be composed
+// of characters from the Alphabet a.
+func (a luhnAlphabet) generate(s string) (rune, error) {
+	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
+}
+
+// luhnValidate 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 luhnAlphabet) luhnValidate(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
+}