collatz.swift

func chain(num:Int) -> [Int]  {

    guard num > 0 else {
        fatalError("can not calculate with 0")
    }

    switch num {
    case 1:
        return [1]
        
    case let num where num  % 2 == 0:
        return [num] + chain(num/2)
        
    default:
        return [num] + chain(num*3+1)
    }
}

(1...100).map { (num:Int) -> [Int] in
    return chain(num)
    }.filter { (nums:[Int]) -> Bool in
        nums.count > 15
}.count