take into account orientation of coordinate system

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 1.1.1
+
+- Change documentation to take into account orientation of coordinate system.
+
 ## 1.1.0
 
 - Use accessor functions to point coordinates instead of specified classes.

lib/src/convex_hull_base.dart

@@ -51,7 +51,10 @@ enum Direction {
 /// [x] and [y] are accessor functions which specify what are the coordinates.
 ///
 /// The returned convex hull starts with the leftmost point and traverses
-/// counter-clockwise.
+/// counter-clockwise for standard orientation (positive x is right, positive
+/// y is up), and clockwise for left-handed orientation (positive x is right,
+/// positive y is *down*). Left-handed orientation is common in many computer
+/// graphics applications.
 ///
 /// Convex hull only works for lists of at least 3 points. If there are less,
 /// the function returns the points without changing anything and logs an info
@@ -68,6 +71,8 @@ Iterable<T> convexHull<T>(Iterable<T> points,
   final sortedPoints = points.map((e) => Point2d(x(e), y(e), e)).toList()
     ..sort();
 
+  // wlog we assume a standard orientation, thus receive the lower half first,
+  // then the upper half. For left-handed orientation, the names would switch.
   final lowerHalf = _halfHull(sortedPoints);
   final upperHalf = _halfHull(sortedPoints.reversed);
 

pubspec.yaml

@@ -1,6 +1,6 @@
 name: convex_hull
 description: Calculates the convex hull of a given set of points. Monotone chain is used as algorithm.
-version: 1.1.0
+version: 1.1.1
 homepage: https://github.com/defo10/convex-hull
 
 environment:

test/convex_hull_test.dart

@@ -49,4 +49,16 @@ void main() {
         x: (e) => e.xCoordinate, y: (e) => e.yCoordinate).map((e) => e.info);
     expect(hull, equals(["1", "2"]));
   });
+
+  test("goes clockwise starting from left", () {
+    final points = <Example>[
+      Example(0, 0, "a"),
+      Example(60, 0, "b"),
+      Example(60, -60, "c"),
+    ];
+
+    final hull = convexHull<Example>(points,
+        x: (e) => e.xCoordinate, y: (e) => e.yCoordinate).map((e) => e.info);
+    expect(hull, equals(["a", "c", "b"]));
+  });
 }