lib/protocol: Optimize luhn and chunk functions

These functions were very naive and slow. We haven't done much about
them because they pretty much don't matter at all for Syncthing
performance. They are however called very often in the discovery server
and these optimizations have a huge effect on the CPU load on the
public discovery servers.

The code isn't exactly obvious, but we have good test coverage on all
these functions.

benchmark                 old ns/op     new ns/op     delta
BenchmarkLuhnify-8        12458         1045          -91.61%
BenchmarkUnluhnify-8      12598         1074          -91.47%
BenchmarkChunkify-8       10792         104           -99.04%

benchmark                 old allocs     new allocs     delta
BenchmarkLuhnify-8        18             1              -94.44%
BenchmarkUnluhnify-8      18             1              -94.44%
BenchmarkChunkify-8       44             2              -95.45%

benchmark                 old bytes     new bytes     delta
BenchmarkLuhnify-8        1278          64            -94.99%
BenchmarkUnluhnify-8      1278          64            -94.99%
BenchmarkChunkify-8       42552         128           -99.70%

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4346

lib/protocol/deviceid_test.go

@@ -150,3 +150,41 @@ func TestNewDeviceIDMarshalling(t *testing.T) {
 		t.Error("Mismatch in old -> new direction")
 	}
 }
+
+var resStr string
+
+func BenchmarkLuhnify(b *testing.B) {
+	str := "ABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJAB"
+	var err error
+	for i := 0; i < b.N; i++ {
+		resStr, err = luhnify(str)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkUnluhnify(b *testing.B) {
+	str, _ := luhnify("ABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJAB")
+	var err error
+	for i := 0; i < b.N; i++ {
+		resStr, err = unluhnify(str)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkChunkify(b *testing.B) {
+	str := "ABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJAB"
+	for i := 0; i < b.N; i++ {
+		resStr = chunkify(str)
+	}
+}
+
+func BenchmarkUnchunkify(b *testing.B) {
+	str := chunkify("ABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJAB")
+	for i := 0; i < b.N; i++ {
+		resStr = unchunkify(str)
+	}
+}