Alerting: Support median in reduce expressions (#91119)

* Alerting: support median in reduce expressions

pkg/expr/mathexp/reduce.go

@@ -3,6 +3,7 @@ package mathexp
 import (
 	"fmt"
 	"math"
+	"sort"
 
 	"github.com/grafana/grafana-plugin-sdk-go/data"
 )
@@ -14,17 +15,18 @@ type ReducerFunc = func(fv *Float64Field) *float64
 type ReducerID string
 
 const (
-	ReducerSum   ReducerID = "sum"
-	ReducerMean  ReducerID = "mean"
-	ReducerMin   ReducerID = "min"
-	ReducerMax   ReducerID = "max"
-	ReducerCount ReducerID = "count"
-	ReducerLast  ReducerID = "last"
+	ReducerSum    ReducerID = "sum"
+	ReducerMean   ReducerID = "mean"
+	ReducerMin    ReducerID = "min"
+	ReducerMax    ReducerID = "max"
+	ReducerCount  ReducerID = "count"
+	ReducerLast   ReducerID = "last"
+	ReducerMedian ReducerID = "median"
 )
 
 // GetSupportedReduceFuncs returns collection of supported function names
 func GetSupportedReduceFuncs() []ReducerID {
-	return []ReducerID{ReducerSum, ReducerMean, ReducerMin, ReducerMax, ReducerCount, ReducerLast}
+	return []ReducerID{ReducerSum, ReducerMean, ReducerMin, ReducerMax, ReducerCount, ReducerLast, ReducerMedian}
 }
 
 func Sum(fv *Float64Field) *float64 {
@@ -98,6 +100,32 @@ func Last(fv *Float64Field) *float64 {
 	return fv.GetValue(fv.Len() - 1)
 }
 
+func Median(fv *Float64Field) *float64 {
+	values := make([]float64, 0, fv.Len())
+	for i := 0; i < fv.Len(); i++ {
+		v := fv.GetValue(i)
+		if v == nil || math.IsNaN(*v) {
+			nan := math.NaN()
+			return &nan
+		}
+		values = append(values, *v)
+	}
+
+	if len(values) == 0 {
+		nan := math.NaN()
+		return &nan
+	}
+
+	sort.Float64s(values)
+	mid := len(values) / 2
+	if len(values)%2 == 0 {
+		v := (values[mid-1] + values[mid]) / 2
+		return &v
+	} else {
+		return &values[mid]
+	}
+}
+
 func GetReduceFunc(rFunc ReducerID) (ReducerFunc, error) {
 	switch rFunc {
 	case ReducerSum:
@@ -112,6 +140,8 @@ func GetReduceFunc(rFunc ReducerID) (ReducerFunc, error) {
 		return Count, nil
 	case ReducerLast:
 		return Last, nil
+	case ReducerMedian:
+		return Median, nil
 	default:
 		return nil, fmt.Errorf("reduction %v not implemented", rFunc)
 	}