fixes influxdata#378 divide by zero in derivative code

CHANGELOG.md

@@ -39,6 +39,8 @@ format a TICKscript file according to a common standard.
 
 ### Bugfixes
 
+- [#378](https://github.com/influxdata/kapacitor/issues/378): Fix issue where derivative would divide by zero.
+
 
 ## v0.11.0 [2016-03-22]
 

derivative.go

@@ -101,6 +101,10 @@ func (d *DerivativeNode) derivative(prev, curr models.Fields, prevTime, currTime
 	}
 
 	elapsed := float64(currTime.Sub(prevTime))
+	if elapsed == 0 {
+		d.logger.Printf("E! cannot perform derivative elapsed time was 0")
+		return 0, false
+	}
 	diff := f1 - f0
 	// Drop negative values for non-negative derivatives
 	if d.d.NonNegativeFlag && diff < 0 {

integrations/data/TestStream_DerivativeZeroElapsed.srpl

@@ -0,0 +1,36 @@
+dbname
+rpname
+packets value=1000 0000000001
+dbname
+rpname
+packets value=1001 0000000002
+dbname
+rpname
+packets value=1002 0000000002
+dbname
+rpname
+packets value=1003 0000000004
+dbname
+rpname
+packets value=1004 0000000005
+dbname
+rpname
+packets value=1006 0000000006
+dbname
+rpname
+packets value=1007 0000000007
+dbname
+rpname
+packets value=1007 0000000008
+dbname
+rpname
+packets value=1008 0000000009
+dbname
+rpname
+packets value=1009 0000000010
+dbname
+rpname
+packets value=1010 0000000011
+dbname
+rpname
+packets value=1011 0000000012

integrations/streamer_test.go

@@ -87,6 +87,35 @@ stream
 	testStreamerWithOutput(t, "TestStream_Derivative", script, 15*time.Second, er, nil, false)
 }
 
+func TestStream_DerivativeZeroElapsed(t *testing.T) {
+
+	var script = `
+stream
+	|from().measurement('packets')
+	|derivative('value')
+	|window()
+		.period(10s)
+		.every(10s)
+	|count('value')
+	|httpOut('TestStream_DerivativeZeroElapsed')
+`
+	er := kapacitor.Result{
+		Series: imodels.Rows{
+			{
+				Name:    "packets",
+				Tags:    nil,
+				Columns: []string{"time", "count"},
+				Values: [][]interface{}{[]interface{}{
+					time.Date(1971, 1, 1, 0, 0, 10, 0, time.UTC),
+					9.0,
+				}},
+			},
+		},
+	}
+
+	testStreamerWithOutput(t, "TestStream_DerivativeZeroElapsed", script, 15*time.Second, er, nil, false)
+}
+
 func TestStream_DerivativeUnit(t *testing.T) {
 
 	var script = `