test: turn TestKahanSum into scripted test

This saves having a function solely to call kahanSumInc.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

promql/functions.go

@@ -948,15 +948,6 @@ func funcTimestamp(vals []parser.Value, args parser.Expressions, enh *EvalNodeHe
 	return enh.Out, nil
 }
 
-func kahanSum(samples []float64) float64 {
-	var sum, c float64
-
-	for _, v := range samples {
-		sum, c = kahanSumInc(v, sum, c)
-	}
-	return sum + c
-}
-
 func kahanSumInc(inc, sum, c float64) (newSum, newC float64) {
 	t := sum + inc
 	// Using Neumaier improvement, swap if next term larger than sum.

promql/functions_test.go

@@ -15,7 +15,6 @@ package promql
 
 import (
 	"context"
-	"math"
 	"testing"
 	"time"
 
@@ -80,9 +79,3 @@ func TestFunctionList(t *testing.T) {
 		require.True(t, ok, "function %s exists in parser package, but not in promql package", i)
 	}
 }
-
-func TestKahanSum(t *testing.T) {
-	vals := []float64{1.0, math.Pow(10, 100), 1.0, -1 * math.Pow(10, 100)}
-	expected := 2.0
-	require.Equal(t, expected, kahanSum(vals))
-}

promql/testdata/functions.test

@@ -764,6 +764,14 @@ eval instant at 1m avg_over_time(metric10[1m])
 eval instant at 1m sum_over_time(metric10[1m])/count_over_time(metric10[1m])
   {} 0
 
+# Test if very big intermediate values cause loss of detail.
+clear
+load 10s
+  metric 1 1e100 1 -1e100
+
+eval instant at 1m sum_over_time(metric[1m])
+  {} 2
+
 # Tests for stddev_over_time and stdvar_over_time.
 clear
 load 10s