Add Point Comparator Function and Tests

point.go

@@ -81,6 +81,18 @@ func (p *Point) Subtract(other Point) {
 	p.Y -= other.Y
 }
 
+// Compare the point to another, returning a delta coordinate.
+//
+// If the two points are equal the result has X=0 Y=0. Otherwise the X and Y
+// return values will be positive or negative numbers of how you could modify
+// the current Point to be equal to the other.
+func (p *Point) Compare(other Point) Point {
+	return Point{
+		X: other.X - p.X,
+		Y: other.Y - p.Y,
+	}
+}
+
 // MarshalText to convert the point into text so that a render.Point may be used
 // as a map key and serialized to JSON.
 func (p *Point) MarshalText() ([]byte, error) {

point_test.go

@@ -58,3 +58,42 @@ func TestPointInside(t *testing.T) {
 		}
 	}
 }
+
+// Test the Compare function of Point.
+func TestPointDelta(t *testing.T) {
+	var tests = []struct {
+		A render.Point // source
+		B render.Point // comparator
+		D render.Point // expected delta value
+	}{
+		{
+			A: render.NewPoint(0, 0),
+			B: render.NewPoint(10, 10),
+			D: render.NewPoint(10, 10),
+		},
+		{
+			A: render.NewPoint(128, 128),
+			B: render.NewPoint(128, 128),
+			D: render.NewPoint(0, 0),
+		},
+		{
+			A: render.NewPoint(128, 128),
+			B: render.NewPoint(127, 129),
+			D: render.NewPoint(-1, 1),
+		},
+		{
+			A: render.NewPoint(200, 500),
+			B: render.NewPoint(180, 528),
+			D: render.NewPoint(-20, 28),
+		},
+	}
+
+	for _, test := range tests {
+		actual := test.A.Compare(test.B)
+		if actual != test.D {
+			t.Errorf("Failed: (%s).Compare(%s) expected %s but got %s",
+				test.A, test.B, test.D, actual,
+			)
+		}
+	}
+}