Transformations / Reducers: Remove setting delta at value (#103742)

* Remove setting delta at value

* Add more tests for delta calculation

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

@@ -55,7 +55,7 @@ describe('Stats Calculators', () => {
   it('should calculate basic stats', () => {
     const stats = reduceField({
       field: basicTable.fields[0],
-      reducers: [ReducerID.first, ReducerID.last, ReducerID.mean, ReducerID.count, ReducerID.diffperc],
+      reducers: [ReducerID.first, ReducerID.last, ReducerID.mean, ReducerID.count, ReducerID.diffperc, ReducerID.delta],
     });
 
     expect(stats.first).toEqual(10);
@@ -63,6 +63,7 @@ describe('Stats Calculators', () => {
     expect(stats.mean).toEqual(15);
     expect(stats.count).toEqual(2);
     expect(stats.diffperc).toEqual(100);
+    expect(stats.delta).toEqual(10);
   });
 
   it('should handle undefined field data without crashing', () => {
@@ -201,7 +202,14 @@ describe('Stats Calculators', () => {
 
     const stats = reduceField({
       field: createField('x', info[0].data),
-      reducers: [ReducerID.first, ReducerID.last, ReducerID.firstNotNull, ReducerID.lastNotNull, ReducerID.diffperc],
+      reducers: [
+        ReducerID.first,
+        ReducerID.last,
+        ReducerID.firstNotNull,
+        ReducerID.lastNotNull,
+        ReducerID.diffperc,
+        ReducerID.delta,
+      ],
     });
 
     expect(stats[ReducerID.first]).toEqual(NaN);
@@ -209,6 +217,7 @@ describe('Stats Calculators', () => {
     expect(stats[ReducerID.firstNotNull]).toEqual(200);
     expect(stats[ReducerID.lastNotNull]).toEqual(200);
     expect(stats[ReducerID.diffperc]).toEqual(0);
+    expect(stats[ReducerID.delta]).toEqual(0);
 
     const reducers = [ReducerID.lastNotNull, ReducerID.firstNotNull];
     for (const input of info) {
@@ -277,6 +286,12 @@ describe('Stats Calculators', () => {
     expect(reduce(someNulls, ReducerID.median)).toEqual(2);
   });
 
+  it('delta should not baseline at initial value', () => {
+    const someNulls = createField('y', [67, 71, 66]);
+    someNulls.config.nullValueMode = NullValueMode.AsZero;
+    expect(reduce(someNulls, ReducerID.delta)).toEqual(4);
+  });
+
   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

@@ -527,15 +527,9 @@ export function doStandardCalcs(field: Field, ignoreNulls: boolean, nullAsZero:
           if (calcs.lastNotNull! > currentValue) {
             // counter reset
             calcs.previousDeltaUp = false;
-            if (i === data.length - 1) {
-              // reset on last
-              calcs.delta += currentValue;
-            }
           } else {
             if (calcs.previousDeltaUp) {
               calcs.delta += step; // normal increment
-            } else {
-              calcs.delta += currentValue; // account for counter reset
             }
             calcs.previousDeltaUp = true;
           }