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/deviceid.go

@@ -10,7 +10,6 @@ import (
 	"fmt"
 	"strings"
 
-	"github.com/calmh/luhn"
 	"github.com/syncthing/syncthing/lib/sha256"
 )
 
@@ -158,7 +157,7 @@ func luhnify(s string) (string, error) {
 	for i := 0; i < 4; i++ {
 		p := s[i*13 : (i+1)*13]
 		copy(res[i*(13+1):], p)
-		l, err := luhn.Base32.Generate(p)
+		l, err := luhnBase32.generate(p)
 		if err != nil {
 			return "", err
 		}
@@ -176,7 +175,7 @@ func unluhnify(s string) (string, error) {
 	for i := 0; i < 4; i++ {
 		p := s[i*(13+1) : (i+1)*(13+1)-1]
 		copy(res[i*13:], p)
-		l, err := luhn.Base32.Generate(p)
+		l, err := luhnBase32.generate(p)
 		if err != nil {
 			return "", err
 		}

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
+}

lib/protocol/luhn_test.go

@@ -0,0 +1,48 @@
+// Copyright (C) 2014 The Protocol Authors.
+
+package protocol
+
+import (
+	"testing"
+)
+
+func TestGenerate(t *testing.T) {
+	// Base 6 Luhn
+	a := luhnAlphabet("abcdef")
+	c, err := a.generate("abcdef")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c != 'e' {
+		t.Errorf("Incorrect check digit %c != e", c)
+	}
+
+	// Base 10 Luhn
+	a = luhnAlphabet("0123456789")
+	c, err = a.generate("7992739871")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c != '3' {
+		t.Errorf("Incorrect check digit %c != 3", c)
+	}
+}
+
+func TestInvalidString(t *testing.T) {
+	a := luhnAlphabet("ABC")
+	_, err := a.generate("7992739871")
+	t.Log(err)
+	if err == nil {
+		t.Error("Unexpected nil error")
+	}
+}
+
+func TestValidate(t *testing.T) {
+	a := luhnAlphabet("abcdef")
+	if !a.luhnValidate("abcdefe") {
+		t.Errorf("Incorrect validation response for abcdefe")
+	}
+	if a.luhnValidate("abcdefd") {
+		t.Errorf("Incorrect validation response for abcdefd")
+	}
+}