StringAddition_Exercise.playground

import Cocoa

struct MyNumber {
    private var string:String!
    
    var lenght:Int {
        get {
            return string.characters.count
        }
    }
    init(string: String) {
        self.string = string
    }
    init(length: Int) {
        self.string = String(count: length, repeatedValue: Character(" "))
    }
}
extension MyNumber {
    func add(number: MyNumber) -> MyNumber {
        
        let maxLength = max(self.lenght, number.lenght)
        let lengthDiff = abs(self.lenght - number.lenght)
        let zeros = String(count: lengthDiff, repeatedValue: Character(" "))
        var s1 = " "
        var s2 = " "
        
        if self.lenght > number.lenght {
            s1 = self.string
            s2 = zeros + number.string
        } else {
            s1 = number.string
            s2 = zeros + self.string
        }
        
        var result = String(count: maxLength, repeatedValue: Character(" "))
        var rest = 0

        s1
        s2
        
        for i in (maxLength - 1).stride(through: 0, by: -1) {
            let i1 = s1.startIndex.advancedBy(i)
            let sum = s1[i1].integerValue + s2[i1].integerValue + rest
            rest = sum/10
            result.removeAtIndex(i1)
            result.insert(String(sum).characters.last!, atIndex: i1)
        }
        
        if rest > 0 {
            rest
            result = String(rest) + result.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceCharacterSet())
        }
        return MyNumber(string: result)
    }
}
extension Character {
    var integerValue:Int {
        return Int(String(self)) ?? 0
    }
}

let n1 = MyNumber(string: "9999")
let n2 = MyNumber(string: "999")

let r = n1.add(n2)
r.string