FieldReducers: Fix median (#98184)

Co-authored-by: Kristina Durivage <kristina.durivage@grafana.com>
Co-authored-by: Leon Sorokin <leeoniya@gmail.com>

packages/grafana-data/src/transformations/fieldReducer.test.ts

@@ -254,6 +254,29 @@ describe('Stats Calculators', () => {
     expect(reduce(someNulls, ReducerID.count)).toEqual(4);
   });
 
+  it('median should ignoreNulls by default', () => {
+    const someNulls = createField('y', [3, null, 2, 1, 4]);
+    expect(reduce(someNulls, ReducerID.median)).toEqual(2.5);
+  });
+
+  it('median should use fieldConfig nullValueMode.Ignore and not count nulls', () => {
+    const someNulls = createField('y', [3, null, 2, 1, 4]);
+    someNulls.config.nullValueMode = NullValueMode.Ignore;
+    expect(reduce(someNulls, ReducerID.median)).toEqual(2.5);
+  });
+
+  it('median should use fieldConfig nullValueMode.Null and count nulls', () => {
+    const someNulls = createField('y', [3, null, 2, 1, 4]);
+    someNulls.config.nullValueMode = NullValueMode.Null;
+    expect(reduce(someNulls, ReducerID.median)).toEqual(2);
+  });
+
+  it('median should use fieldConfig nullValueMode.AsZero and count nulls as zero', () => {
+    const someNulls = createField('y', [3, null, 2, 1, 4]);
+    someNulls.config.nullValueMode = NullValueMode.AsZero;
+    expect(reduce(someNulls, ReducerID.median)).toEqual(2);
+  });
+
   it('can reduce to percentiles', () => {
     // This `Array.from` will build an array of elements from 1 to 99
     const percentiles = [...Array.from({ length: 99 }, (_, i) => i + 1)];

packages/grafana-data/src/transformations/fieldReducer.ts

@@ -283,7 +283,8 @@ export const fieldReducers = new Registry<FieldReducerInfo>(() => [
     id: ReducerID.median,
     name: 'Median',
     description: 'Median Value',
-    standard: true,
+    standard: false,
+    reduce: calculateMedian,
     aliasIds: ['median'],
     preservesUnits: true,
   },
@@ -584,6 +585,7 @@ export function doStandardCalcs(field: Field, ignoreNulls: boolean, nullAsZero:
   if (isNumber(calcs.firstNotNull) && isNumber(calcs.diff)) {
     calcs.diffperc = (calcs.diff / calcs.firstNotNull) * 100;
   }
+
   return calcs;
 }
 
@@ -703,3 +705,32 @@ function calculatePercentile(field: Field, percentile: number, ignoreNulls: bool
   const index = Math.round((sorted.length - 1) * percentile);
   return sorted[index];
 }
+
+function calculateMedian(field: Field<number>, ignoreNulls: boolean, nullAsZero: boolean): FieldCalcs {
+  const numbers: number[] = [];
+
+  for (let i = 0; i < field.values.length; i++) {
+    let currentValue = field.values[i];
+
+    if (currentValue == null) {
+      if (ignoreNulls) {
+        continue;
+      }
+      if (nullAsZero) {
+        currentValue = 0;
+      }
+    }
+
+    numbers.push(currentValue);
+  }
+
+  numbers.sort((a, b) => a - b);
+
+  const mid = Math.floor(numbers.length / 2);
+
+  if (numbers.length % 2 === 0) {
+    return { median: (numbers[mid - 1] + numbers[mid]) / 2 };
+  } else {
+    return { median: numbers[mid] };
+  }
+}