determine size of rectangle

To determine the size of rectangle, it was required to be able to
determine who would be the neighboring points.  A neighbor is a point
that would make the exterior of the rectangle, not the cross-section.
Finding the neighbors allows for a simple l*w to calculate size.

rectangle_test.go

@@ -27,3 +27,28 @@ func TestIsRect(t *testing.T) {
 
 	}
 }
+
+func TestSize(t *testing.T) {
+	var isRectTests = []struct {
+		r        Rectangle
+		expected float64
+	}{
+		{Rectangle{P1: Point{1, 1}, P2: Point{1, 2}, P3: Point{2, 1}, P4: Point{2, 2}}, 1},
+		{Rectangle{P1: Point{0, 0}, P2: Point{0, 1}, P3: Point{1, 0}, P4: Point{1, 1}}, 1},
+		{Rectangle{P1: Point{0, 0}, P2: Point{0, -1}, P3: Point{-1, 0}, P4: Point{-1, -1}}, 1},
+		{Rectangle{P1: Point{1.5, 1.5}, P2: Point{1.5, 3.5}, P3: Point{3.5, 1.5}, P4: Point{3.5, 3.5}}, 4},
+		{Rectangle{P1: Point{0, 0}, P2: Point{0, 3}, P3: Point{2, 0}, P4: Point{2, 3}}, 6},
+		{Rectangle{P1: Point{0, 0}, P2: Point{0, 100}, P3: Point{100, 0}, P4: Point{100, 100}}, 10000},
+	}
+	for _, rt := range isRectTests {
+		actual := rt.r.Size()
+		if actual != rt.expected {
+			t.Errorf(
+				"failed spiral:\n\texpected: %d\n\t  actual: %d",
+				rt.expected,
+				actual,
+			)
+		}
+
+	}
+}