Account for more edge cases

Also adjust code so we're handling these edge cases
the right way

ee/app/assets/javascripts/burndown_chart/burndown_chart_data.js

@@ -53,13 +53,13 @@ export default class BurndownChartData {
       // weight count on an given date. To mitigate this, we reset the current
       // date's counters to 0 and carry forward the negative count to a future
       // date until the total is positive again.
-      if (openIssuesCount <= 0 || openIssuesWeight <= 0) {
-        carriedIssuesCount = openIssuesCount;
-        carriedIssuesWeight = openIssuesWeight;
+      if (openIssuesCount + carriedIssuesCount < 0 || openIssuesWeight + carriedIssuesWeight < 0) {
+        carriedIssuesCount += openIssuesCount;
+        carriedIssuesWeight += openIssuesWeight;
 
         openIssuesCount = 0;
         openIssuesWeight = 0;
-      } else if (carriedIssuesCount < 0 || carriedIssuesWeight < 0) {
+      } else {
         openIssuesCount += carriedIssuesCount;
         openIssuesWeight += carriedIssuesWeight;
 

ee/spec/frontend/burndown_chart/burndown_chart_data_spec.js

@@ -87,23 +87,77 @@ describe('BurndownChartData', () => {
       });
     });
 
-    describe('when first two days of milestone have negative issue count', () => {
+    describe('when days in milestone have negative counts', () => {
+      describe('and the first two days have a negative count', () => {
         beforeAll(() => {
+          milestoneEvents.length = 0;
           milestoneEvents.push(
             { created_at: '2017-03-01T00:00:00.000Z', weight: 2, action: 'closed' },
             { created_at: '2017-03-01T00:00:00.000Z', weight: 2, action: 'closed' },
+            { created_at: '2017-03-02T00:00:00.000Z', weight: 2, action: 'created' },
+            { created_at: '2017-03-03T00:00:00.000Z', weight: 2, action: 'created' },
+            { created_at: '2017-03-03T00:00:00.000Z', weight: 2, action: 'created' },
+          );
+        });
+
+        it('generates an array where the first two days counts are zero', () => {
+          expect(burndownChartData.generate()).toEqual([
+            ['2017-03-01', 0, 0],
+            ['2017-03-02', 0, 0],
+            ['2017-03-03', 1, 2],
+          ]);
+        });
+      });
+
+      describe('and the middle day has a negative count', () => {
+        // This scenario is unlikely to occur as this implies there are more
+        // closed issues than total issues, but we account for it anyway as a
+        // potential edge case.
+
+        beforeAll(() => {
+          milestoneEvents.length = 0;
+          milestoneEvents.push(
+            { created_at: '2017-03-01T00:00:00.000Z', weight: 2, action: 'created' },
+            { created_at: '2017-03-02T00:00:00.000Z', weight: 2, action: 'closed' },
+            { created_at: '2017-03-02T00:00:00.000Z', weight: 2, action: 'closed' },
+            { created_at: '2017-03-03T00:00:00.000Z', weight: 2, action: 'created' },
+            { created_at: '2017-03-03T00:00:00.000Z', weight: 2, action: 'created' },
+          );
+        });
+
+        it('generates an array where the middle day count is zero', () => {
+          expect(burndownChartData.generate()).toEqual([
+            ['2017-03-01', 1, 2],
+            ['2017-03-02', 0, 0],
+            ['2017-03-03', 1, 2],
+          ]);
+        });
+      });
+
+      describe('and the last day has a negative count', () => {
+        // This scenario is unlikely to occur as this implies there are more
+        // closed issues than total issues, but we account for it anyway as a
+        // potential edge case.
+
+        beforeAll(() => {
+          milestoneEvents.length = 0;
+          milestoneEvents.push(
             { created_at: '2017-03-01T00:00:00.000Z', weight: 2, action: 'closed' },
             { created_at: '2017-03-02T00:00:00.000Z', weight: 2, action: 'created' },
+            { created_at: '2017-03-02T00:00:00.000Z', weight: 2, action: 'closed' },
+            { created_at: '2017-03-03T00:00:00.000Z', weight: 2, action: 'created' },
+            { created_at: '2017-03-03T00:00:00.000Z', weight: 2, action: 'closed' },
           );
         });
 
-      it('sets first two dates data to 0 and carries forward negative total to the third day', () => {
+        it('generates an array where all counts are zero', () => {
           expect(burndownChartData.generate()).toEqual([
             ['2017-03-01', 0, 0],
             ['2017-03-02', 0, 0],
-          ['2017-03-03', 2, 4],
+            ['2017-03-03', 0, 0],
           ]);
         });
       });
     });
+  });
 });