Playing with Date, DateInterval, DateComponents, Calendar

Date_Extensions

extension Date {
    init?(iso:String) {
        let d = ISO8601DateFormatter()
        guard let date = d.date(from: iso) else {
            return nil
        }
        self = date
    }
    
    func year() -> String {
        let formatter = DateFormatter()
        formatter.dateFormat = "yyyy"
        return formatter.string(from: self)
    }
}

extension DateInterval {
    init?(from startDate:String, to endDate:String) {
        guard let firstDate = Date(iso: startDate),
            let secondDate = Date(iso: endDate) else {
                return nil
        }
        self = DateInterval(start: firstDate, end: secondDate)
    }
    
    func number(of:Calendar.Component) -> Int? {
        let calendar = Calendar.current
        
        // Replace the hour (time) of both dates with 00:00
        let date1 = calendar.startOfDay(for: self.start)
        let date2 = calendar.startOfDay(for: self.end)
        
        let components = calendar.dateComponents([of], from: date1, to: date2)
        switch of {
        case .year:
            return components.year
        case .day:
            return components.day
        case .month:
            return components.month
        case .era:
            return components.era
        case .hour:
            return components.hour
        case .minute:
            return components.minute
        case .second:
            return components.second
        case .weekday:
            return components.weekday
        case .weekdayOrdinal:
            return components.weekdayOrdinal
        case .quarter:
            return components.quarter
        case .weekOfMonth:
            return components.weekOfMonth
        case .weekOfYear:
            return components.weekOfYear
        case .yearForWeekOfYear:
            return components.yearForWeekOfYear
        case .nanosecond:
            return components.nanosecond
        default:
            return nil
        }
        
    }
    
    func dateArray(intervalType:Calendar.Component, interval:Int) -> [Date]? {
        let calendar = Calendar.current
        let date1 = calendar.startOfDay(for: self.start)
        var dates = [date1]
        var dateComponent = DateComponents()
        switch intervalType {
        case .year:
            dateComponent.year = interval
        case .day:
            dateComponent.day = interval
        case .month:
            dateComponent.month = interval
        case .era:
            dateComponent.era = interval
        case .hour:
            dateComponent.hour = interval
        case .minute:
            dateComponent.minute = interval
        case .second:
            dateComponent.second = interval
        case .weekday:
            dateComponent.weekday = interval
        case .weekdayOrdinal:
            dateComponent.weekdayOrdinal = interval
        case .quarter:
            dateComponent.quarter = interval
        case .weekOfMonth:
            dateComponent.weekOfMonth = interval
        case .weekOfYear:
            dateComponent.weekOfYear = interval
        case .yearForWeekOfYear:
            dateComponent.yearForWeekOfYear = interval
        case .nanosecond:
            dateComponent.nanosecond = interval
        default:
            return nil
        }
         // replace with
        repeat {
            dates.append(calendar.date(byAdding: dateComponent, to: dates.last!)!)
        }
        while self.contains(dates.last!)
        if !self.contains(dates.last!) {
            dates.removeLast()
        }
        return dates
    }
    
    func years() -> [String]? {
        guard let dates = self.dateArray(intervalType: .year, interval: 10) else { return nil}
        let d = dates.map({$0.year()})
        return d

    }
    
    
    
    
}

let int = DateInterval(from: "1870-01-01T12:00:00Z", to: "2020-01-01T12:00:00Z")
int?.number(of: .year)
int?.number(of: .day)
int?.number(of: .nanosecond)
int?.dateArray(intervalType: .year, interval: 10)
int?.dateArray(intervalType: .month, interval: 120)
int?.years() // returns [String]?