feat(alerting): add backend support for median reducer

pkg/services/alerting/conditions/reducer.go

@@ -3,6 +3,8 @@ package conditions
 import (
 	"math"
 
+	"sort"
+
 	"github.com/grafana/grafana/pkg/tsdb"
 	"gopkg.in/guregu/null.v3"
 )
@@ -71,6 +73,24 @@ func (s *SimpleReducer) Reduce(series *tsdb.TimeSeries) null.Float {
 				break
 			}
 		}
+	case "median":
+		var values []float64
+		for _, v := range series.Points {
+			if v[0].Valid {
+				allNull = false
+				values = append(values, v[0].Float64)
+			}
+		}
+		if len(values) >= 1 {
+			sort.Float64s(values)
+			length := len(values)
+			if length%2 == 1 {
+				value = values[(length-1)/2]
+			} else {
+				value = (values[(length/2)-1] + values[length/2]) / 2
+			}
+		}
+
 	}
 
 	if allNull {

pkg/services/alerting/conditions/reducer_test.go

@@ -41,6 +41,20 @@ func TestSimpleReducer(t *testing.T) {
 			So(result, ShouldEqual, float64(3000))
 		})
 
+		Convey("median odd amount of numbers", func() {
+			result := testReducer("median", 1, 2, 3000)
+			So(result, ShouldEqual, float64(2))
+		})
+
+		Convey("median even amount of numbers", func() {
+			result := testReducer("median", 1, 2, 4, 3000)
+			So(result, ShouldEqual, float64(3))
+		})
+
+		Convey("median with one values", func() {
+			result := testReducer("median", 1)
+			So(result, ShouldEqual, float64(1))
+		})
 	})
 }