adding AngleTo method to Vec

geometry.go

@@ -169,6 +169,12 @@ func (u Vec) Normal() Vec {
 	return Vec{-u.Y, u.X}
 }
 
+// Returns angle between two vectors
+func (u Vec) AngleTo(v Vec) {
+	u, v = u.Unit(), v.Unit()
+	return math.Acos(u.Dot(v))
+}
+
 // Dot returns the dot product of vectors u and v.
 func (u Vec) Dot(v Vec) float64 {
 	return u.X*v.X + u.Y*v.Y