Downloads chart: Add helper function for determining point in polygon

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

@@ -125,3 +125,51 @@ bool isPointOnPathWithTolerance(
   }
   return false;
 }
+
+/// Determines if a point is inside a polygon.
+///
+/// Uses the ray casting algorithm to determine if a given point lies inside
+/// a polygon defined by a list of vertices. The polygon is assumed to be
+/// closed and non-self-intersecting.
+///
+/// Returns `true` if the point is inside the polygon or exactly on a vertex or
+/// an edge, and `false` otherwise.
+bool isPointInPolygon(List<(double, double)> polygon, (double, double) point) {
+  if (polygon.length < 3) {
+    return false;
+  }
+
+  int intersections = 0;
+  final (px, py) = point;
+
+  // Check if the point is on an edge
+  if (isPointOnPathWithTolerance(polygon, point, 0.001)) {
+    return true;
+  }
+
+  for (int i = 0; i < polygon.length; i++) {
+    final (x1, y1) = polygon[i];
+    final (x2, y2) = polygon[(i + 1) % polygon.length];
+
+    // Check if the point is on a vertex
+    if ((px == x1 && py == y1) || (px == x2 && py == y2)) {
+      return true;
+    }
+
+    if (py > min(y1, y2) && py <= max(y1, y2)) {
+      double intersectX;
+      if (y1 == y2) {
+        // horizontal edge
+        continue;
+      } else {
+        intersectX = x1 + (py - y1) * (x2 - x1) / (y2 - y1);
+      }
+
+      if (px < intersectX) {
+        intersections++;
+      }
+    }
+  }
+
+  return intersections % 2 == 1;
+}

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

@@ -182,4 +182,46 @@ void main() {
       expect(isPointOnPathWithTolerance(chart, point, 0.001), isTrue);
     });
   });
+
+  group('isPointInPolygon', () {
+    test('Basic inside/outside, vertex, edge, and invalid input', () {
+      final square = [(0.0, 0.0), (0.0, 2.0), (2.0, 2.0), (2.0, 0.0)];
+      final invalidPolygon = [(0.0, 0.0)];
+
+      final insidePoint = (1.0, 1.0);
+      final outsidePoint = (3.0, 1.0);
+      final vertexPoint = (0.0, 0.0);
+      final edgePoint = (1.0, 0.0);
+
+      expect(isPointInPolygon(square, insidePoint), isTrue);
+      expect(isPointInPolygon(square, outsidePoint), isFalse);
+      expect(isPointInPolygon(square, vertexPoint), isTrue);
+      expect(isPointInPolygon(square, edgePoint), isFalse);
+      expect(isPointInPolygon(invalidPolygon, insidePoint), isFalse);
+    });
+
+    test('Complex polygon', () {
+      final complexPolygon = [
+        (0.0, 0.0),
+        (0.0, 4.0),
+        (2.0, 2.0),
+        (4.0, 4.0),
+        (4.0, 0.0)
+      ];
+      final insidePoint = (1.0, 1.0);
+      final outsidePoint = (2.0, 3.0);
+      final vertexPoint = (1.0, 1.0);
+      final edgePoint = (1.0, 3.0);
+      final edgePoint2 = (3.0, 3.0);
+      final outsidePointOnEdgeExtension = (3.0, 4.0);
+
+      expect(isPointInPolygon(complexPolygon, insidePoint), isTrue);
+      expect(isPointInPolygon(complexPolygon, outsidePoint), isFalse);
+      expect(isPointInPolygon(complexPolygon, vertexPoint), isTrue);
+      expect(isPointInPolygon(complexPolygon, edgePoint), isTrue);
+      expect(isPointInPolygon(complexPolygon, edgePoint2), isTrue);
+      expect(isPointInPolygon(complexPolygon, outsidePointOnEdgeExtension),
+          isFalse);
+    });
+  });
 }