import UIKit

private let reuseIdentifier = "DateCell"

class CalendarViewController: UICollectionViewController {
    let calendar = Calendar.current
    var date: Date = Date().startOfMonth()

    var currentMonth: Int {
        get { return calendar.component(.month, from: date) }
    }

    var currentMonthSymbol: String {
        get { return calendar.monthSymbols[currentMonth - 1] }
    }

    var currentYear: Int {
        get { return calendar.component(.year, from: date) }
    }

    var offset: Int {
        get {
            let weekday = calendar.component(.weekday, from: date)
            if weekday == 1 { return 6 }
            return weekday - 2
        }
    }

    @objc func didTapPrev(_ sender: Any) {
        date = Calendar.current.date(byAdding: .month, value: -1, to: date)!
        smoothReload()
        setTitle()
    }

    @objc func didTapNext(_ sender: Any) {
        date = Calendar.current.date(byAdding: .month, value: 1, to: date)!
        smoothReload()
        setTitle()
    }

    @objc func didTapSearch(_ sender: Any) {
        performSegue(withIdentifier: "ShowSearch", sender: sender)
    }

    override func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
        if kind == UICollectionView.elementKindSectionHeader {
            return collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "CalendarHeader", for: indexPath)
        }
        fatalError("Unexpected kind")
    }

    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return calendar.range(of: .day, in: .month, for: date)!.count + offset
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! DateCell
        cell.backgroundColor = UIColor(white: 1, alpha: 0)

        if indexPath.row < offset {
            cell.dateLabel.text = ""
            cell.taskCountLabel.text = ""
            cell.layer.borderWidth = 0
        } else {
            cell.setDate(calendar.date(byAdding: .day, value: indexPath.row - offset, to: date)!)
            cell.layer.borderWidth = 0.5
            cell.layer.borderColor = UIColor.lightGray.cgColor

            if cell.date!.isSameDateAs(Date()) {
                cell.backgroundColor = UIColor(red: 1, green: 0, blue: 0, alpha: 0.5)
            } else if calendar.isDateInWeekend(cell.date!) {
                cell.backgroundColor = UIColor(red: 1, green: 0, blue: 0, alpha: 0.2)
            }
        }

        return cell
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if let dateVC = segue.destination as? DateViewController {
            let cell = sender as! DateCell
            dateVC.date = cell.date!
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        setTitle()

        let prevButton = UIBarButtonItem(title: "Prev", style: .plain, target: self, action: #selector(didTapPrev(_:)))
        self.navigationItem.setLeftBarButton(prevButton, animated: true)

        let nextButton = UIBarButtonItem(title: "Next", style: .plain, target: self, action: #selector(didTapNext(_:)))
        let searchButton = UIBarButtonItem(barButtonSystemItem: .search, target: self, action: #selector(didTapSearch(_:)))
        self.navigationItem.setRightBarButtonItems([nextButton, searchButton], animated: true)

        let swipeLeft = UISwipeGestureRecognizer(target: self, action: #selector(didTapNext))
        swipeLeft.direction = .left
        self.view.addGestureRecognizer(swipeLeft)

        let swipeRight = UISwipeGestureRecognizer(target: self, action: #selector(didTapPrev))
        swipeRight.direction = .right
        self.view.addGestureRecognizer(swipeRight)
    }

    func setTitle() {
        self.title = "\(currentMonthSymbol) \(currentYear)"
    }
}

extension CalendarViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let width = collectionView.bounds.width / 7.0
        let height = width

        return CGSize(width: width, height: height)
    }
}

extension Date {
    func startOfMonth() -> Date {
        let components = Calendar.current.dateComponents([.year, .month], from: Calendar.current.startOfDay(for: self))
        return Calendar.current.date(from: components)!
    }

    func isSameDateAs(_ other: Date) -> Bool {
        return Calendar.current.startOfDay(for: self) == Calendar.current.startOfDay(for: other)
    }
}

extension UICollectionViewController {
    func smoothReload() {
        self.collectionView.performBatchUpdates({
            self.collectionView.reloadSections(NSIndexSet(index: 0) as IndexSet)
        }, completion: { (finished: Bool) -> Void in })
    }
}