lib/protocol: faster Luhn algorithm and better testing (#6475)

The previous implementation was very generic; its tests didn't cover the
actual alphabet for device IDs.

Benchmark results on amd64:

name         old time/op    new time/op     delta
Luhnify-8      1.00µs ± 1%     0.28µs ± 4%   -72.38%  (p=0.000 n=9+10)
Unluhnify-8     992ns ± 2%      274ns ± 1%   -72.39%  (p=0.000 n=10+9)

lib/protocol/luhn_test.go

@@ -3,46 +3,24 @@
 package protocol
 
 import (
+	"strings"
 	"testing"
 )
 
-func TestGenerate(t *testing.T) {
-	// Base 6 Luhn
-	a := luhnAlphabet("abcdef")
-	c, err := a.generate("abcdef")
+func TestLuhn32(t *testing.T) {
+	c, err := luhn32("AB725E4GHIQPL3ZFGT")
 	if err != nil {
 		t.Fatal(err)
 	}
-	if c != 'e' {
-		t.Errorf("Incorrect check digit %c != e", c)
+	if c != 'G' {
+		t.Errorf("Incorrect check digit %c != G", 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)
+	_, err = luhn32("3734EJEKMRHWPZQTWYQ1")
 	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")
+	if !strings.Contains(err.Error(), "'1'") {
+		t.Errorf("luhn32 should have errored on digit '1', got %v", err)
 	}
 }