if debugging then print lag to stderr

src/device/input/button/v01.go

@@ -5,6 +5,7 @@ import (
 	"log"
 	"mayhem-party/src/device/input/raw"
 	"os"
+	"time"
 )
 
 var debugging = os.Getenv("DEBUG") == "true"
@@ -37,9 +38,16 @@ func (v01 V01) Read() []Button {
 	if err := json.Unmarshal(line, &msg); err != nil {
 		log.Printf("%v: %s", err, line)
 	}
+	v01.telemetry(msg)
 	return msg.buttons()
 }
 
+func (v01 V01) telemetry(msg v01Msg) {
+	if debugging {
+		log.Printf("%s|%dms", msg.U, time.Now().UnixNano()/int64(time.Millisecond)-msg.T)
+	}
+}
+
 func (msg v01Msg) buttons() []Button {
 	buttons := make([]Button, len(msg.Y)+len(msg.N))
 	for i := range msg.Y {

src/device/input/button/v01_test.go

@@ -1,12 +1,14 @@
 package button_test
 
 import (
+	"fmt"
 	"mayhem-party/src/device/input/button"
 	"testing"
+	"time"
 )
 
 func TestV01(t *testing.T) {
-	src := constSrc(`{"T":1,"U":"bel","Y":"abc","N":"cde"}`)
+	src := constSrc(fmt.Sprintf(`{"T":%v,"U":"bel","Y":"abc","N":"cde"}`, time.Now().UnixNano()/int64(time.Millisecond)-50))
 	t.Logf("(%v) %s", len(src), src.Read())
 	v01 := button.NewV01(src)
 	got := v01.Read()