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.go

@@ -1,6 +1,9 @@
 package rect
 
-import "math"
+import (
+	"math"
+	"sort"
+)
 
 type Rectangle struct {
 	P1, P2, P3, P4 Point
@@ -22,3 +25,35 @@ func (r Rectangle) IsRect() bool {
 	dd4 := math.Sqrt(math.Abs(cx-r.P4.X)) + math.Sqrt(math.Abs(cy-r.P4.Y))
 	return dd1 == dd2 && dd1 == dd3 && dd1 == dd4
 }
+
+func (r Rectangle) Neighbors(p Point) (Point, Point) {
+	keys := []float64{distance(r.P1, p), distance(r.P2, p), distance(r.P3, p), distance(r.P4, p)}
+	sort.Float64s(keys)
+	n := []Point{}
+	d := distance(r.P1, p)
+	if keys[1] == d || keys[2] == d {
+		n = append(n, r.P1)
+	}
+	d = distance(r.P2, p)
+	if keys[1] == d || keys[2] == d {
+		n = append(n, r.P2)
+	}
+	d = distance(r.P3, p)
+	if keys[1] == d || keys[2] == d {
+		n = append(n, r.P3)
+	}
+	d = distance(r.P4, p)
+	if keys[1] == d || keys[2] == d {
+		n = append(n, r.P4)
+	}
+	return n[0], n[1]
+}
+
+func (r Rectangle) Size() float64 {
+	n1, n2 := r.Neighbors(r.P1)
+	return distance(r.P1, n1) * distance(r.P1, n2)
+}
+
+//
+//func Containment(r1, r2, Rectangle) bool {
+//}

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,
+			)
+		}
+
+	}
+}