Getting first week of the Year NSCalendar

Question

I am trying to put the dates of the beginning of the week in a app. However in 2017, this results in un-expected behavior, as this year has the week '1' 2 times in it,

starting Sunday Jan 1st, and Sun Dec 31. My program finds the latter. Giving me the result of '01-01-2018' where is should be, '02-01-2017'

How can i make sure my program finds the first date available?

this is my code:

getFirstDay(Int(weekNummer)!, year: yearWrap)

func getFirstDay(weekNumber:Int, year:Int)->String?{
    let Calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
    let dayComponent = NSDateComponents()
    dayComponent.weekOfYear = weekNumber
    dayComponent.weekday = 2
    dayComponent.year = year
    var date = Calendar.dateFromComponents(dayComponent)
    if ((weekNumber == 1 || weekNumber == 52 || weekNumber == 53) && Calendar.components(.Month, fromDate: date!).month != 1 ){
        dayComponent.year = year - 1
        date = Calendar.dateFromComponents(dayComponent)
    }
    let dateFormatter = NSDateFormatter()
    dateFormatter.dateFormat = "dd-MM-yyyy"
    return String(dateFormatter.stringFromDate(date!))
}

Answer 1

Let do NSCalendar the complete date math

func firstDayOfWeek(week: Int, inYear year : Int) -> NSString {
  let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
  let components = NSDateComponents()
  components.weekOfYear = week
  components.yearForWeekOfYear = year
  components.weekday = 2 // First weekday of the week (Sunday = 1)
  let dateFormatter = NSDateFormatter()
  dateFormatter.dateFormat = "dd-MM-yyyy"
  let date = calendar.dateFromComponents(components)!
  return dateFormatter.stringFromDate(date)
}

firstDayOfWeek(1, inYear : 2017) // 02-01-2017
firstDayOfWeek(1, inYear : 2018) // 01-01.2018