//: Playground - noun: a place where people can play

import UIKit
import PlaygroundSupport

class ViewController: UIViewController {
    
    let W: CGFloat = 400
    let H: CGFloat = 400
    let E: CGFloat = 20
    let dx: Float = 0.25
    let graph = CAShapeLayer()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        drawAxis()
        
        let slider = UISlider(frame: CGRect(x: 0, y: H, width: W, height: 30))
        slider.addTarget(self, action: #selector(self.sliderChanged(sender:)), for: UIControl.Event.valueChanged)
        slider.minimumValue = -Float(W) / Float(2 * E)
        slider.maximumValue =  Float(W) / Float(2 * E)
        self.view.addSubview(slider)
    }
    
    func drawAxis() {
        let gline = UIBezierPath()
        gline.move(to:    CGPoint(x: 0, y: H / 2))
        gline.addLine(to: CGPoint(x: W, y: H / 2))
        gline.move(to:    CGPoint(x: W / 2, y: H))
        gline.addLine(to: CGPoint(x: W / 2, y: 0))
        
        let axis = CAShapeLayer()
        axis.strokeColor = UIColor.black.cgColor
        axis.path = gline.cgPath
        view.layer.addSublayer(axis)
        
        let OLabel = UILabel(frame: CGRect(x: W / 2, y: H / 2, width: 12, height: 12))
        OLabel.text = "O"
        self.view.addSubview(OLabel)
    }
    
    
    @objc func sliderChanged(sender : UISlider) {
        graph.removeFromSuperlayer()
        
        let line = UIBezierPath()
        var x: Float = -Float(W) / Float(2 * E)
        while x < Float(W) / Float(2 * E) {
            let P  = defFunction(with: sender.value, and: x)
            let P_ = defFunction(with: sender.value, and: x + dx)
            
            line.move(to: P); line.addLine(to: P_)
            x += dx
        }
        graph.strokeColor = UIColor.blue.cgColor
        graph.lineWidth = 1.0
        graph.path = line.cgPath
        view.layer.addSublayer(graph)
    }
    
    func defFunction(with k: Float, and x: Float) -> CGPoint {
        // def of function //
        let y = k * x * x
        
        return CGPoint(x: W / 2 + CGFloat(x) * E, y: H / 2 - CGFloat(y) * E)
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
    
}

let viewController = ViewController()
viewController.view.backgroundColor = UIColor.white

PlaygroundPage.current.liveView = viewController
PlaygroundPage.current.needsIndefiniteExecution = true