public void PolygonArea()
{
PointD p0 = new PointD(0, 0), p1 = new PointD(1, 1), p2 = new PointD(2, 0);
PointD p3 = new PointD(0, 2), p4 = new PointD(2, 2);
// triangles in both orientations
Assert.AreEqual(-1, GeoAlgorithms.PolygonArea(p0, p1, p2));
Assert.AreEqual(+1, GeoAlgorithms.PolygonArea(p2, p1, p0));
// squares in both orientations
Assert.AreEqual(-4, GeoAlgorithms.PolygonArea(p0, p3, p4, p2));
Assert.AreEqual(+4, GeoAlgorithms.PolygonArea(p2, p4, p3, p0));
// collinear points and star shape
Assert.AreEqual(0, GeoAlgorithms.PolygonArea(p0, p1, p4));
Assert.AreEqual(0, GeoAlgorithms.PolygonArea(p0, p1, p3, p1, p2, p1, p4, p1));
}