simplify code (#234)

* simplify code, format makefile

* simplify code

core/collection/rollingwindow.go

@@ -74,7 +74,10 @@ func (rw *RollingWindow) span() int {
 
 func (rw *RollingWindow) updateOffset() {
 	span := rw.span()
-	if span > 0 {
+	if span <= 0 {
+		return
+	}
+
 	offset := rw.offset
 	start := offset + 1
 	steps := start + span
@@ -95,7 +98,6 @@ func (rw *RollingWindow) updateOffset() {
 	rw.offset = (offset + span) % rw.size
 	rw.lastTime = timex.Now()
 }
-}
 
 type Bucket struct {
 	Sum   float64