How to create big number

LargeNumber

struct BigInt {
    var value:String

    func multiply(right:BigInt) ->BigInt {
        var a1 = value.characters.reverse().map {Int(String($0))!}
        var a2 = right.value.characters.reverse().map {Int(String($0))!}
        var product = [Int](count:a1.count + a2.count,repeatedValue:0)
        for iterNumber1 in 0..<a1.count {
            for iterNumber2 in 0..<a2.count{
                let idxIter = iterNumber1 + iterNumber2
                product[idxIter] = a1[iterNumber1] * a2[iterNumber2] + (idxIter >= product.count ? 0 :product[idxIter])
                if product[idxIter] > 9 {
                    product[idxIter + 1] = (product[idxIter / 10] + (idxIter + 1 >= product.count ? 0 :product[idxIter + 1]))
                    product[idxIter] -= (product[idxIter] / 10) * 10
                    
                }
            }
        }
        product = Array(product.reverse())
        print(product.count)
        return BigInt(value:product.map {String($0)}.joinWithSeparator("") )
}
    
}
func * (left:BigInt,right:BigInt) ->BigInt {return left.multiply(right)}
let int1 = BigInt(value: "32123439888883883838")
let int2 = BigInt(value: "439938892929929299299292")
let product = int1 * int2