Polygon Area Tests

polygon-area-tests.cs

// Testing code via Stack Overflow answer at:
// https://stackoverflow.com/a/16281192/581414

void Main()
{
	var square = new List<Point>{
		new Point(1, 1),
		new Point(1, 10),
		new Point(10,10),
		new Point(10,1),
	};
	
	square.Area().Dump("Square 9x9 => area should be 81");
	
	square.Add(new Point(1, 1)); // close polygon
	square.Area().Dump("Square 9x9 => area should be 81");
	
    var triangle = new List<Point>{
		new Point(1, 1),
		new Point(1, 10),
		new Point(10,10),
	};
	
	triangle.Area().Dump("Right-angled triangle => area should be 81/2 = 40.5");
	
	triangle.Add(new Point(1, 1)); // close polygon
	triangle.Area().Dump("Right-angled triangle => area should be 81/2 = 40.5");
}

public static class ExtensionMethods {
	public static double Area(this List<Point> points)
	{
		return Math.Abs(points.Take(points.Count - 1)
	   .Select((p, i) => (points[i + 1].X - p.X) * (points[i + 1].Y + p.Y))
	   .Sum() / 2);
	}
}

public class Point 
{
	public List<double> Coordinates {get; set;} = new();
	
	public Point (double x, double y)
	{
		Coordinates.Add(x);
		Coordinates.Add(y);
	}
	
	public double X {get { return Coordinates[0]; }}
	public double Y {get { return Coordinates[1]; }}
}