Downloads chart: Add helper function for computing closest point on line

pkg/web_app/lib/src/widget/downloads_chart/computations.dart

@@ -41,3 +41,40 @@ Iterable<String> prepareRanges(List<VersionRangeCount> rangeDownloads) {
 
   return (ranges: ranges, weekLists: result.reversed.toList());
 }
+
+/// Calculates closest point from [point] to line defined by [startPoint] and
+/// [endPoint].
+///
+/// This assumes that the closest point is on the line, that is within the
+/// two endpoints. Returns `(double.maxFinite, double.maxFinite)` if this is
+/// not the case.
+(num, num) closestPointOnLine(
+    (num, num) startPoint, (num, num) endPoint, (num, num) point) {
+  final directionVector =
+      (endPoint.$1 - startPoint.$1, endPoint.$2 - startPoint.$2);
+
+  if (directionVector.$1 == 0 && directionVector.$2 == 0) {
+    return startPoint;
+  }
+
+  final vector = (point.$1 - startPoint.$1, point.$2 - startPoint.$2);
+
+  // The dot product ((v · d) / (d · d)) where v = vector and d = directionVector
+  final t = ((vector.$1 * directionVector.$1 + vector.$2 * directionVector.$2) /
+      (directionVector.$1 * directionVector.$1 +
+          directionVector.$2 * directionVector.$2));
+
+  if (t < 0 || t > 1) {
+    // Closest point is before or after the line. This should not happen in
+    // our use case.
+    return (double.maxFinite, double.maxFinite);
+  }
+
+  // t * d
+  final projectionVOntoD = (t * directionVector.$1, t * directionVector.$2);
+  final closestPoint = (
+    startPoint.$1 + projectionVOntoD.$1,
+    startPoint.$2 + projectionVOntoD.$2
+  );
+  return (closestPoint.$1, closestPoint.$2);
+}

pkg/web_app/test/widget/downloads_chart/downloads_chart_test.dart

@@ -78,4 +78,78 @@ void main() {
       expect(w3[i], [0, 0, 0, 0, 0, 0]);
     }
   });
+
+  group('closestPointOnLine tests', () {
+    test('point on the line', () {
+      final lineStart = (0, 0);
+      final lineEnd = (10, 10);
+      final point = (5, 5);
+      final closest = closestPointOnLine(lineStart, lineEnd, point);
+      expect(closest, (5.0, 5.0));
+    });
+
+    test('point before the line', () {
+      final lineStart = (0, 0);
+      final lineEnd = (10, 10);
+      final point = (-2, -5);
+      final closest = closestPointOnLine(lineStart, lineEnd, point);
+      expect(closest, (double.maxFinite, double.maxFinite));
+    });
+
+    test('point after the line', () {
+      final lineStart = (0, 0);
+      final lineEnd = (10, 10);
+      final point = (15, 15);
+      final closest = closestPointOnLine(lineStart, lineEnd, point);
+      expect(closest, (double.maxFinite, double.maxFinite));
+    });
+
+    test('point off the line', () {
+      final lineStart = (0, 0);
+      final lineEnd = (10, 10);
+      final point = (5, 3);
+      final closest = closestPointOnLine(lineStart, lineEnd, point);
+      expect(closest, (4.0, 4.0));
+    });
+
+    test('vertical line', () {
+      final lineStart = (1, 2);
+      final lineEnd = (1, 10);
+      final point = (5, 5);
+      final closest = closestPointOnLine(lineStart, lineEnd, point);
+      expect(closest, (1.0, 5.0));
+    });
+
+    test('horizontal line', () {
+      final lineStart = (2, 1);
+      final lineEnd = (10, 1);
+      final point = (5, 5);
+      final closest = closestPointOnLine(lineStart, lineEnd, point);
+      expect(closest, (5.0, 1.0));
+    });
+
+    test('same start and end points', () {
+      final lineStart = (5, 5);
+      final lineEnd = (5, 5);
+      final point = (10, 10);
+      final closest = closestPointOnLine(lineStart, lineEnd, point);
+      expect(closest, (5.0, 5.0));
+    });
+
+    test('line with negative coordinates', () {
+      final lineStart = (-5, -5);
+      final lineEnd = (5, 5);
+      final point = (0, 10);
+      final closest = closestPointOnLine(lineStart, lineEnd, point);
+      expect(closest, (5.0, 5.0));
+    });
+
+    test('line with negative and positive coordinates', () {
+      final lineStart = (-5, 5);
+      final lineEnd = (5, -5);
+      final point = (0, 0);
+      final closest = closestPointOnLine(lineStart, lineEnd, point);
+      expect(closest, (0.0, 0.0));
+    });
+  });
 }