Handle 0 downloads in growth function (#8741)

Author: szakarias

app/lib/service/download_counts/package_trends.dart

@@ -35,6 +35,10 @@ double computeRelativeGrowthRate(List<int> totalDownloads) {
       recentDownloads.reduce((prev, element) => prev + element) /
           recentDownloads.length;
 
+  if (averageRecentDownloads == 0) {
+    return 0;
+  }
+
   // We reverse the recentDownloads list for regression, since the first entry
   // is the newest point in time. By reversing, we pass the data in
   // chronological order.

app/test/service/download_counts/package_trends_test.dart

@@ -11,6 +11,7 @@ void main() {
       expect(calculateLinearRegressionSlope([10.0, 20.0, 30.0]), 10.0);
       expect(calculateLinearRegressionSlope([30.0, 20.0, 10.0]), -10.0);
       expect(calculateLinearRegressionSlope([10.0, 10.0, 10.0]), 0);
+      expect(calculateLinearRegressionSlope([0.0, 0.0, 0.0]), 0);
     });
 
     test('return 0.0 if denominator is very small', () {
@@ -20,11 +21,16 @@ void main() {
   });
 
   group('computeRelativeGrowthRate', () {
-    test('returns 0.0 for stable downloads meeting threshold', () {
+    test('returns 0.0 for stable downloads', () {
       final downloads = List<int>.generate(analysisWindowDays, (i) => 2000);
       expect(computeRelativeGrowthRate(downloads), 0.0);
     });
 
+    test('returns 0.0 for 0 downloads', () {
+      final downloads = List<int>.generate(analysisWindowDays, (i) => 0);
+      expect(computeRelativeGrowthRate(downloads), 0.0);
+    });
+
     test('calculates positive relative growth rate for positive trend', () {
       // Input list (newest first):  [1645, 1635, ..., 1355] (30 values)
       // Average = 1500 for the first 30 values. Slope: 10.