FastCPU-BasedTouchesMoved.swift

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
    guard let newTouchPoint = touches.first?.location(in: self) else { return }

    var lastTouchPoint: CGPoint = .zero

    if let lastIndex = lines.indices.last {
        // get reference to last point
        if let lastPoint = lines[lastIndex].last { lastTouchPoint = lastPoint }

        // add new point
        lines[lastIndex].append(newTouchPoint)
    }

    let rect = calculateRectBetween(lastPoint: lastTouchPoint, newPoint: newTouchPoint)

    setNeedsDisplay(rect)
}