import UIKit

class ViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource {


    @IBOutlet weak var collectionView: UICollectionView!
    @IBOutlet weak var label: UILabel!

    let now = Date()
    var cal = Calendar.current
    let dateFormatter = DateFormatter()
    var components = DateComponents()

    override func viewDidLoad() {
        super.viewDidLoad()

        cal.locale = Locale(identifier: "ja")
        dateFormatter.locale = Locale(identifier: "ja_JP")
        dateFormatter.dateFormat = "yyyy年M月"
        components.year = cal.component(.year, from: now)
        components.month = cal.component(.month, from: now)
        components.day = 1
        calculation()
    }

    func calculation(){
        let firstDayOfMonth = cal.date(from: components)
        label.text = dateFormatter.string(from: firstDayOfMonth!)
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 37
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
         let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath)
        let firstDayOfMonth = cal.date(from: components)
        let firstWeekday = cal.component(.weekday, from: firstDayOfMonth!)

        let weekdayAdding = 2 - firstWeekday
        for subview in cell.contentView.subviews {
            subview.removeFromSuperview()
        }

        let daysCountInMonth = cal.range(of: .day, in: .month, for: firstDayOfMonth!)?.count
        if (indexPath.row + weekdayAdding) >= 1 && (indexPath.row + weekdayAdding) <= daysCountInMonth! {
            cell.backgroundColor = #colorLiteral(red: 0.937254902, green: 0.937254902, blue: 0.9568627451, alpha: 1)
            let label = UILabel()
            label.font = UIFont(name: "Arial", size: 17)
            label.text = "\(indexPath.row + weekdayAdding)"
            label.sizeToFit()
            label.center = cell.contentView.center
            cell.contentView.addSubview(label)
        }
        else{
            cell.backgroundColor = #colorLiteral(red: 1, green: 1, blue: 1, alpha: 1)
        }
        return cell
    }

}