Angle calculations between two vectors

govector_test.go

@@ -119,16 +119,39 @@ func TestIntersection(t *testing.T) {
 		t.Errorf("Intersection Error")
 	}
 
-    p1 = Point2d{20, 5}
-    p2 = Point2d{5, 0}
+	p1 = Point2d{20, 5}
+	p2 = Point2d{5, 0}
 
-    v1 = Vector2d{10, 0}
-    v2 = Vector2d{0, 10}
+	v1 = Vector2d{10, 0}
+	v2 = Vector2d{0, 10}
 
-    i, p = Intersection(p1, p2, v1, v2)
-    fmt.Printf("%v %v\n", i, p)
-    if i {
-        t.Errorf("Intersection Error")
-    }
+	i, p = Intersection(p1, p2, v1, v2)
+	fmt.Printf("%v %v\n", i, p)
+	if i {
+		t.Errorf("Intersection Error")
+	}
+}
+
+func TestAngle(t *testing.T) {
+	v1 := Vector2d{10, 0}
+	v2 := Vector2d{0, 10}
+
+	a := Angle(v2, v1)
+
+	fmt.Printf("%v %v %f\n", v1, v2, a*rad2deg)
+	if math.Abs(a-math.Pi/2) > epsilon {
+		t.Errorf("Angle Error")
+	}
+
+	v1 = Vector2d{5, 0}
+	v2 = Vector2d{10, 10}
+
+	a = Angle(v1, v2)
+
+	fmt.Printf("%v %v %f\n", v1, v2, a*rad2deg)
+
+	if math.Abs(a-math.Pi/4) > epsilon {
+		t.Errorf("Angle Error")
+	}
 
 }

vectorpoint.go

@@ -22,6 +22,7 @@ type Point2d struct {
 }
 
 const epsilon = 1e-7
+const rad2deg = 180 / math.Pi
 
 func (p1 Point2d) Sub(p2 Point2d) Vector2d {
 	return Vector2d{p1.X - p2.X, p1.Y - p2.Y}
@@ -80,3 +81,8 @@ func Move(d1, d2 Point2d, velocity float64, timeDelta float64) Point2d {
 func Distance(p1, p2 Point2d) float64 {
 	return p1.Sub(p2).Mag()
 }
+
+func Angle(v1, v2 Vector2d) float64 {
+	x := (v1.Dot(v2) / (v1.Mag() * v2.Mag()))
+	return math.Acos(x)
+}