implement Threebonacci

ThreeFibonacciViewController.swift

//
//  ThreeFibonacciViewController.swift
//


// For the purposes of this problem, a Threebonacci sequence is a twist on the 
// Fibonacci sequence where you would sum the previous three elements to get the 
// next element. That is, if the Threebonacci function is defined as t, where n is the
// index of the element involved, t(n) = t(n-1) + t(n-2) + t(n-3). 
// You can assume t(0) = 0, t(1) = 1, and t(2) = 1. With this information, please 
// complete the tableview in the following UIViewController so that each 
// UITableViewCell displays its Threebonacci value corresponding with its row index. 
// An example is as follows: 0 1 1 2 4 and so on so forth.

import UIKit

class ThreeFibonacciViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {
    @IBOutlet weak var tableView: UITableView!
    
    var threebonacciArray = [0,1,1]
    let totalNumberOfThreebonacci = 1000000
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        tableView.delegate = self
        tableView.dataSource = self
        
        calculateThreebonacci()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        
    }

    func numberOfSections(in tableView: UITableView) -> Int {
        return 1
    }
    
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return threebonacciArray.count
    }
    
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

        let cell = tableView.dequeueReusableCell(withIdentifier: "Default", for: indexPath)
    
        // if we need a row that's greater than the number of results in the array,
        // we need to calculate 
        if indexPath.row < threebonacciArray.count 
        {
            cell.textLabel.text = String(threebonacciArray[indexPath.row])
        } else {
            cell.textLabel.text = "great place to put spinner"
        }
        
        return cell
    }
    
    func calculateThreebonacci() {
        
        DispatchQueue.global(qos: .background).async { [weak self] in 
            
            for index in 3...totalNumberOfThreebonacci { 
                
                let tNminusThree = self?.threeebonacciArray[index-3] ?? 0
                let tNminusTwo = self?.threebonacciArray[index-2]
                let tNminusOne = self?.threebonacciArray[index-1]
            
                let newResult = tNminusThree + tNminusTwo + tNminusOne
                self?.threebonacciArray.append(newResult)
                
                // insert row if the bottom of the table view is visible

            }

            // or get to end of calculating and reloadData on the table
            DispatchQueue.main.async {
                self?.tableView.reloadData()   
            }
        }
    }
}