Normalize trend by overall average (dart-lang#8767)

Author: szakarias

app/lib/service/download_counts/package_trends.dart

@@ -5,6 +5,7 @@
 import 'dart:math';
 
 const analysisWindowDays = 30;
+const totalTrendWindowDays = 330;
 const minThirtyDaysDownloadThreshold = 30000;
 
 /// Calculates the relative daily growth rate of a package's downloads.
@@ -22,6 +23,9 @@ const minThirtyDaysDownloadThreshold = 30000;
 /// downloads (10% relative growth) than for a package with 10000 average daily
 /// downloads (0.1% relative growth).
 double computeRelativeGrowthRate(List<int> totalDownloads) {
+  if (totalDownloads.isEmpty) {
+    return 0;
+  }
   final List<int> data;
   if (totalDownloads.length < analysisWindowDays) {
     data = [
@@ -38,7 +42,12 @@ double computeRelativeGrowthRate(List<int> totalDownloads) {
       recentDownloads.reduce((prev, element) => prev + element) /
           recentDownloads.length;
 
-  if (averageRecentDownloads == 0) {
+  final m = min(totalDownloads.length, totalTrendWindowDays);
+  final averageTotalDownloads =
+      totalDownloads.sublist(0, m).reduce((prev, element) => prev + element) /
+          m;
+
+  if (averageRecentDownloads == 0 || averageTotalDownloads == 0) {
     return 0;
   }
 
@@ -51,7 +60,7 @@ double computeRelativeGrowthRate(List<int> totalDownloads) {
   // Normalize slope by average downloads to represent relative growth.
   // This measures how much the download count is growing relative to its
   // current volume.
-  return growthRate / averageRecentDownloads;
+  return growthRate / averageTotalDownloads;
 }
 
 /// Computes the slope of the best-fit line for a given list of data points

app/test/service/download_counts/computations_test.dart

@@ -314,10 +314,16 @@ void main() {
                 'fake_download_counts_data_for_trend2.jsonl'));
         await processDownloadCounts(d);
       }
-      final neonTrend = computeTrendScore(
-          [...List.filled(15, 2000), ...List.filled(15, 1000)]);
-      final oxygenTrend = computeTrendScore(
-          [...List.filled(15, 5000), ...List.filled(15, 3000)]);
+      final neonTrend = computeTrendScore([
+        ...List.filled(15, 2000),
+        ...List.filled(15, 1000),
+        ...List.filled(701, -1)
+      ]);
+      final oxygenTrend = computeTrendScore([
+        ...List.filled(15, 5000),
+        ...List.filled(15, 3000),
+        ...List.filled(701, -1)
+      ]);
 
       expect(await computeTrend(),
           {'flutter_titanium': 0.0, 'neon': neonTrend, 'oxygen': oxygenTrend});

app/test/service/download_counts/package_trends_test.dart

@@ -36,9 +36,12 @@ void main() {
     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.
-      final downloads =
-          List<int>.generate(analysisWindowDays * 2, (i) => 1645 - (i * 10));
-      final expectedRate = 10.0 / 1500.0;
+      final downloads = <int>[
+        ...List<int>.generate(analysisWindowDays * 2, (i) => 1645 - (i * 10)),
+        ...List.filled(300, 0)
+      ];
+      final avg = downloads.reduce((prev, element) => prev + element) / 330;
+      final expectedRate = 10.0 / avg;
       expect(computeRelativeGrowthRate(downloads), expectedRate);
     });
 
@@ -107,10 +110,10 @@ void main() {
       final downloads = [100, 50];
       // For relativeGrowth:
       //   Padded data: [100, 50, 0...0] (28 zeros)
-      //   avg = 150/30 = 5
+      //   avg = (100 + 50) / 2 = 75.
       //   growthRate = 63750 / 67425
       final expectedDampening = min(1.0, 150 / 30000);
-      final expectedRelativeGrowth = 63750 / 67425 / 5;
+      final expectedRelativeGrowth = (63750 / 67425) / 75;
       final expectedScore =
           expectedRelativeGrowth * expectedDampening * expectedDampening;
       expect(computeTrendScore(downloads), expectedScore);
@@ -178,7 +181,7 @@ void main() {
   test('Short history, high sum meets threshold -> no dampening', () {
     final downloads = List<int>.filled(15, 2000);
     final expectedDampening = min(1.0, 30000 / 30000);
-    final expectedRelativeGrowth = 6750000 / 67425 / 1000;
+    final expectedRelativeGrowth = (6750000 / 67425) / 2000;
     final expectedScore =
         expectedRelativeGrowth * expectedDampening * expectedDampening;