Fix stddev function (#224)

* Correct stddev function usage

* Simplify code

* Fix CI error

src/modules/judge/judge/func.go

@@ -129,9 +129,8 @@ func (f AvgFunction) Compute(vs []*dataobj.HistoryData) (leftValue dataobj.JsonF
 
 type StddevFunction struct {
 	Function
-	Limit      int
-	Operator   string
-	RightValue float64
+	Num   int
+	Limit int
 }
 
 func (f StddevFunction) Compute(vs []*dataobj.HistoryData) (leftValue dataobj.JsonFloat, isTriggered bool) {
@@ -150,8 +149,12 @@ func (f StddevFunction) Compute(vs []*dataobj.HistoryData) (leftValue dataobj.Js
 		num += math.Pow(float64(vs[i].Value)-mean, 2)
 	}
 
-	leftValue = dataobj.JsonFloat(math.Sqrt(num / float64(f.Limit)))
-	isTriggered = checkIsTriggered(leftValue, f.Operator, f.RightValue)
+	std := math.Sqrt(num / float64(f.Limit))
+	upperBound := mean + std*float64(f.Num)
+	lowerBound := mean - std*float64(f.Num)
+
+	leftValue = vs[0].Value
+	isTriggered = checkIsTriggered(leftValue, "<", lowerBound) || checkIsTriggered(leftValue, ">", upperBound)
 	return
 }
 
@@ -365,7 +368,7 @@ func ParseFuncFromString(str string, span []interface{}, operator string, rightV
 	case "avg":
 		fn = &AvgFunction{Limit: limit, Operator: operator, RightValue: rightValue}
 	case "stddev":
-		fn = &StddevFunction{Limit: limit, Operator: operator, RightValue: rightValue}
+		fn = &StddevFunction{Limit: limit, Num: span[1].(int)}
 	case "diff":
 		fn = &DiffFunction{Limit: limit, Operator: operator, RightValue: rightValue}
 	case "pdiff":

src/modules/judge/judge/judge.go

@@ -182,13 +182,13 @@ func Judge(stra *model.Stra, exps []model.Exp, historyData []*dataobj.HistoryDat
 func judgeItemWithStrategy(stra *model.Stra, historyData []*dataobj.HistoryData, exp model.Exp, firstItem *dataobj.JudgeItem, now int64) (leftValue dataobj.JsonFloat, isTriggered bool) {
 	straFunc := exp.Func
 
-	straParam := []interface{}{}
+	var straParam []interface{}
 	if firstItem.Step == 0 {
 		logger.Errorf("wrong step:%+v", firstItem)
 		return
 	}
 
-	limit := stra.AlertDur / int(firstItem.Step)
+	limit := stra.AlertDur / firstItem.Step
 	if limit <= 0 {
 		limit = 1
 	}
@@ -196,7 +196,7 @@ func judgeItemWithStrategy(stra *model.Stra, historyData []*dataobj.HistoryData,
 	straParam = append(straParam, limit)
 
 	switch straFunc {
-	case "happen":
+	case "happen", "stddev":
 		if len(exp.Params) < 1 {
 			logger.Errorf("stra:%d exp:%+v stra param is null", stra.Id, exp)
 			return