feat: geometric centroid (#38)

* docs: rephrased description

* feat: included geometry method

* feat: simplified centroid

* Update packages/geometry/test/src/geometry_test.dart

Co-authored-by: Allison Ryan <77211884+allisonryan0002@users.noreply.github.com>

* chore: rebase

* refactor: removed throwsAssertionError for throwsA matcher

* docs: rephrased doc comment

Co-authored-by: Allison Ryan <77211884+allisonryan0002@users.noreply.github.com>

packages/geometry/README.md

@@ -3,7 +3,7 @@
 [![style: very good analysis][very_good_analysis_badge]][very_good_analysis_link]
 [![License: MIT][license_badge]][license_link]
 
-Helper package to calculate points of lines, arcs and curves for the pathways of the ball.
+Provides a set of helpers for working with 2D geometry.
 
 [license_badge]: https://img.shields.io/badge/license-MIT-blue.svg
 [license_link]: https://opensource.org/licenses/MIT

packages/geometry/lib/src/geometry.dart

@@ -106,3 +106,12 @@ num factorial(num n) {
     return n * factorial(n - 1);
   }
 }
+
+/// Arithmetic mean position of all the [Vector2]s in a polygon.
+///
+/// For more information read: https://en.wikipedia.org/wiki/Centroid
+Vector2 centroid(List<Vector2> vertices) {
+  assert(vertices.isNotEmpty, 'Vertices must not be empty');
+  final sum = vertices.reduce((a, b) => a + b);
+  return sum / vertices.length.toDouble();
+}

packages/geometry/pubspec.yaml

@@ -1,5 +1,5 @@
 name: geometry
-description: Helper package to calculate points of lines, arcs and curves for the pathways of the ball
+description: Provides a set of helpers for working with 2D geometry.
 version: 1.0.0+1
 publish_to: none
 

packages/geometry/test/src/geometry_test.dart

@@ -166,4 +166,29 @@ void main() {
       });
     });
   });
+
+  group('centroid', () {
+    test('throws AssertionError when vertices are empty', () {
+      expect(() => centroid([]), throwsA(isA<AssertionError>()));
+    });
+
+    test('is correct when one vertex is given', () {
+      expect(centroid([Vector2.zero()]), Vector2.zero());
+    });
+
+    test('is correct when two vertex are given', () {
+      expect(centroid([Vector2.zero(), Vector2(1, 1)]), Vector2(0.5, 0.5));
+    });
+
+    test('is correct when three vertex are given', () {
+      expect(
+        centroid([
+          Vector2.zero(),
+          Vector2(1, 1),
+          Vector2(2, 2),
+        ]),
+        Vector2(1, 1),
+      );
+    });
+  });
 }