extension UIColor {
    class var lightBlue: UIColor {
        return UIColor(red: 92.0 / 255, green: 192.0 / 255, blue: 210.0 / 255, alpha: 1.0)
    }
    
    class var lightRed: UIColor {
        return UIColor(red: 195.0 / 255, green: 123.0 / 255, blue: 175.0 / 255, alpha: 1.0)
    }
}


extension Date {
    func string(format: String) -> String {
        let formatter = DateFormatter()
        formatter.dateFormat = format
        return formatter.string(from: self)
    }
}

import Foundation

final class MonthDateManager {
    
    private let calendar = Calendar.current
    private (set) var days: [Date] = []
    private var firstDate: Date! {
        didSet {
           days = createDaysForMonth()
        }
    }
    
    var monthString: String {
        return firstDate.string(format: "YYYY/MM")
    }
    
    init() {
        var component = calendar.dateComponents([.year, .month], from: Date())
        component.day = 1
        firstDate = calendar.date(from: component)
        days = createDaysForMonth()
    }
    
    func createDaysForMonth() -> [Date] {
        // 月の初日の曜日
        let dayOfTheWeek = calendar.component(.weekday, from: firstDate) - 1
        // 月の日数
        let numberOfWeeks = calendar.range(of: .weekOfMonth, in: .month, for: firstDate)!.count
        // その月に表示する日数
        let numberOfItems = numberOfWeeks * 7 

        return (0..<numberOfItems).map { i in
            var dateComponents = DateComponents()
            dateComponents.day = i - dayOfTheWeek
            return calendar.date(byAdding: dateComponents, to: firstDate)!
        }
    }
    
    func nextMonth() {
        firstDate = calendar.date(byAdding: .month, value: 1, to: firstDate)
    }
    
    func prevMonth() {
        firstDate = calendar.date(byAdding: .month, value: -1, to: firstDate)
    }
}

import UIKit

final class CalendarCell: UICollectionViewCell {
    
    private var label: UILabel = {
        let it = UILabel()
        it.textAlignment = .center
        it.translatesAutoresizingMaskIntoConstraints = false
        return it
    }()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        contentView.addSubview(label)
        
        NSLayoutConstraint.activate([
            label.topAnchor.constraint(equalTo: contentView.topAnchor),
            label.bottomAnchor.constraint(equalTo: contentView.bottomAnchor),
            label.leftAnchor.constraint(equalTo: contentView.leftAnchor),
            label.rightAnchor.constraint(equalTo: contentView.rightAnchor),
            ])
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    func configure(model: Model) {
         label.text = model.text
         label.textColor = model.textColor
    }
}


extension CalendarCell {

    struct Model {
        var text: String = ""
        var textColor: UIColor = .black
    }
}

extension CalendarCell.Model {
    
    init(date: Date) {
        let weekday = Calendar.current.component(.weekday, from: date)
        if weekday == 1 {
            textColor = .lightRed
        } else if weekday == 7 {
            textColor = .lightBlue
        } else {
            textColor = .gray
        }
        text = date.string(format: "d")
    }
}

final class ViewController: UIViewController {
    
    private let dateManager = MonthDateManager()
    private let weeks = ["日","月", "火", "水", "木", "金", "土"]
    private let itemSize: CGFloat = (UIScreen.main.bounds.width - 60) / 7
    
    private lazy var calenderCollectionView: UICollectionView = {
        let layout = UICollectionViewFlowLayout()
        layout.scrollDirection = .vertical
        layout.minimumLineSpacing = 10
        layout.minimumInteritemSpacing = 10
        layout.itemSize = CGSize(width: itemSize, height: 50)
        let collectionView = UICollectionView(frame: self.view.frame, collectionViewLayout: layout)
        collectionView.backgroundColor = .white
        collectionView.register(CalendarCell.self, forCellWithReuseIdentifier: "cell")
        collectionView.delegate = self
        collectionView.dataSource = self
        return collectionView
    }()
    
    private func setUpNavigationBar() {
        navigationController?.navigationBar.barTintColor = UIColor(red: 255/255, green: 132/255, blue: 214/255, alpha: 1)
        navigationController?.navigationBar.tintColor = .white
        navigationController?.navigationBar.titleTextAttributes = [.foregroundColor : UIColor.white]
        
        navigationItem.rightBarButtonItem = UIBarButtonItem(
            title: "next",
            style: .plain,
            target: self,
            action: #selector(actionNext)
        )
        
        navigationItem.leftBarButtonItem = UIBarButtonItem(
            title: "back",
            style: .plain,
            target: self,
            action: #selector(actionBack)
        )
        title = dateManager.monthString
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .white
        calenderCollectionView.frame.size.width = view.bounds.width
        calenderCollectionView.frame.size.height = 500
        view.addSubview(calenderCollectionView)
        setUpNavigationBar()
    }
    
    @objc private func actionNext() {
        dateManager.nextMonth()
        calenderCollectionView.reloadData()
        title = dateManager.monthString
    }
    
    @objc private func actionBack() {
        dateManager.prevMonth()
        calenderCollectionView.reloadData()
        title = dateManager.monthString
    }
}

extension ViewController: UICollectionViewDataSource {
    
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 2
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return section == 0 ? weeks.count : dateManager.days.count
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as! CalendarCell
        if indexPath.section == 0 {
            let day = weeks[indexPath.row]
            let model = CalendarCell.Model(text: day, textColor: .black)
            cell.configure(model: model)
        } else {
            let date = dateManager.days[indexPath.row]
            cell.configure(model: CalendarCell.Model(date: date))
        }
        return cell
    }
}

extension ViewController: UICollectionViewDelegate {
    
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        if indexPath.section == 0 {
            return
        }
        title = dateManager.days[indexPath.row].string(format: "YYYY/MM/dd")
    }
}