app/vmselect/promql: adjust integrate() calculations to be more sim…

…ilar to calculations from InfluxDB: attempt #2

Updates #701

app/vmselect/promql/rollup.go

@@ -1683,28 +1683,21 @@ func rollupDistinct(rfa *rollupFuncArg) float64 {
 }
 
 func rollupIntegrate(rfa *rollupFuncArg) float64 {
-	prevTimestamp := rfa.prevTimestamp
-
 	// There is no need in handling NaNs here, since they must be cleaned up
 	// before calling rollup funcs.
 	values := rfa.values
 	timestamps := rfa.timestamps
-	if len(values) == 0 {
-		if math.IsNaN(rfa.prevValue) {
-			return nan
-		}
-		return 0
-	}
 	prevValue := rfa.prevValue
+	prevTimestamp := rfa.currTimestamp - rfa.window
 	if math.IsNaN(prevValue) {
+		if len(values) == 0 {
+			return nan
+		}
 		prevValue = values[0]
 		prevTimestamp = timestamps[0]
 		values = values[1:]
 		timestamps = timestamps[1:]
 	}
-	if len(values) == 0 {
-		return 0
-	}
 
 	var sum float64
 	for i, v := range values {
@@ -1714,6 +1707,8 @@ func rollupIntegrate(rfa *rollupFuncArg) float64 {
 		prevTimestamp = timestamp
 		prevValue = v
 	}
+	dt := float64(rfa.currTimestamp - prevTimestamp) / 1e3
+	sum += prevValue * dt
 	return sum
 }
 

app/vmselect/promql/rollup_test.go

@@ -431,7 +431,7 @@ func TestRollupNewRollupFuncSuccess(t *testing.T) {
 	f("stdvar_over_time", 945.7430555555555)
 	f("first_over_time", 123)
 	f("last_over_time", 34)
-	f("integrate", 5.237)
+	f("integrate", 0.817)
 	f("distinct_over_time", 8)
 	f("ideriv", 0)
 	f("decreases_over_time", 5)
@@ -970,7 +970,7 @@ func TestRollupFuncsNoWindow(t *testing.T) {
 		}
 		rc.Timestamps = getTimestamps(rc.Start, rc.End, rc.Step)
 		values := rc.Do(nil, testValues, testTimestamps)
-		valuesExpected := []float64{nan, 2.064, 1.677, 1.156, 0.34}
+		valuesExpected := []float64{nan, 2.148, 1.593, 1.156, 1.36}
 		timestampsExpected := []int64{0, 40, 80, 120, 160}
 		testRowsEqual(t, values, rc.Timestamps, valuesExpected, timestampsExpected)
 	})