How to calculate points on a line which is perpendicular to another line in Swift?

I'm writing code in Swift and I want to draw CAShapeLayers with BezierPaths as it's presented in the image below.

How can I calculate C & D points if points A & B are known, distance between A & B is known, and length between points C & D is also known? Also, B is exactly mid-way between C & D.

After C and D points are calculated, a line between them would be drawn using UIBezierPath like this:

let path: UIBezierPath = UIBezierPath()
path.move(to: CGPoint(x: Cx, y: Cy))
path.addLine(to: CGPoint(x: Dx, y: Dy))
layer.path = path.cgPath

Thanks for your help and time.

Here's the math that I would use :)

First you can calculate the angle in the Cartesian plane from A to B:

theta_AB = atan2(By-Ay, Bx-Ax)

The angle of the perpendicular line CD is just theta + pi/2

theta_CD = theta_AB + pi/2

Then assume that d is the known distance between C and D.

Then you have:

Cx = Bx + cos(theta_CD) * d/2
Cy = By + sin(theta_CD) * d/2
Dx = Bx - cos(theta_CD) * d/2
Dy = By - sin(theta_CD) * d/2

So putting it all together with (hopefully) correct Swift syntax, you'd have:

let theta_AB = atan2(By-Ay, Bx-Ax);
let theta_CD = theta_AB + Double.pi/2.0;
let Cx = Bx + cos(theta_CD) * d/2;
let Cy = By + sin(theta_CD) * d/2;
let Dx = Bx - cos(theta_CD) * d/2;
let Dy = By - sin(theta_CD) * d/2;
let C : CGPoint = CGPoint(x: Cx, y: Cy);
let D : CGPoint = CGPoint(x: Dx, y: Dy);

Or something like that