add godoc for RollingWindow (zeromicro#351)

core/collection/rollingwindow.go

@@ -8,8 +8,10 @@ import (
 )
 
 type (
+	// RollingWindowOption let callers customize the RollingWindow.
 	RollingWindowOption func(rollingWindow *RollingWindow)
 
+	// RollingWindow defines a rolling window to calculate the events in buckets with time interval.
 	RollingWindow struct {
 		lock          sync.RWMutex
 		size          int
@@ -21,6 +23,8 @@ type (
 	}
 )
 
+// NewRollingWindow returns a RollingWindow that with size buckets and time interval,
+// use opts to customize the RollingWindow.
 func NewRollingWindow(size int, interval time.Duration, opts ...RollingWindowOption) *RollingWindow {
 	if size < 1 {
 		panic("size must be greater than 0")
@@ -38,13 +42,15 @@ func NewRollingWindow(size int, interval time.Duration, opts ...RollingWindowOpt
 	return w
 }
 
+// Add adds value to current bucket.
 func (rw *RollingWindow) Add(v float64) {
 	rw.lock.Lock()
 	defer rw.lock.Unlock()
 	rw.updateOffset()
 	rw.win.add(rw.offset, v)
 }
 
+// Reduce runs fn on all buckets, ignore current bucket if ignoreCurrent was set.
 func (rw *RollingWindow) Reduce(fn func(b *Bucket)) {
 	rw.lock.RLock()
 	defer rw.lock.RUnlock()
@@ -81,7 +87,7 @@ func (rw *RollingWindow) updateOffset() {
 	offset := rw.offset
 	// reset expired buckets
 	for i := 0; i < span; i++ {
-		rw.win.resetBucket(offset + 1 + i)
+		rw.win.resetBucket(offset + i + 1)
 	}
 
 	rw.offset = (offset + span) % rw.size
@@ -90,6 +96,7 @@ func (rw *RollingWindow) updateOffset() {
 	rw.lastTime = now - (now-rw.lastTime)%rw.interval
 }
 
+// Bucket defines the bucket that holds sum and num of additions.
 type Bucket struct {
 	Sum   float64
 	Count int64
@@ -135,6 +142,7 @@ func (w *window) resetBucket(offset int) {
 	w.buckets[offset%w.size].reset()
 }
 
+// IgnoreCurrentBucket lets the Reduce call ignore current bucket.
 func IgnoreCurrentBucket() RollingWindowOption {
 	return func(w *RollingWindow) {
 		w.ignoreCurrent = true