add scrobbling of played tracks

backend/util/stopwatch.go

@@ -0,0 +1,38 @@
+package util
+
+import "time"
+
+type Stopwatch struct {
+	running bool
+	started time.Time
+	elapsed time.Duration
+}
+
+func (s *Stopwatch) Start() {
+	if s.running {
+		return
+	}
+	s.started = time.Now()
+	s.running = true
+}
+
+func (s *Stopwatch) Stop() {
+	if !s.running {
+		return
+	}
+	s.elapsed += time.Since(s.started)
+	s.running = false
+}
+
+func (s *Stopwatch) Elapsed() time.Duration {
+	e := s.elapsed
+	if s.running {
+		e += time.Since(s.started)
+	}
+	return e
+}
+
+func (s *Stopwatch) Reset() {
+	s.running = false
+	s.elapsed = time.Duration(0)
+}