Add Two double string

//split string to two part
func splitNumber(_ str: String) -> (String, String){
    let arr = Array(str)
    if let indexDot = arr.firstIndex(of: "."){
        return (String(arr[0 ..< indexDot]), String(arr[indexDot + 1 ..< str.count]))
    }
    return (str, "")
}

//add two valid double string
func addDoubleString(_ str1: String, _ str2: String) -> String{
    let pair1 = splitNumber(str1)
    let pair2 = splitNumber(str2)
    let decimalResult = addDecimal(pair1.1, pair2.1)
    return addInteger(pair1.0, pair2.0, decimalResult.1) + decimalResult.0
}

//this function add two decimal string and return the int carry over part
func addDecimal(_ str1 : String, _ str2: String) -> (String , Int){
    var st1 = str1
    var st2 = str2
    while st1.count != st2.count{
        if str1.count > str2.count {st2.append("0")}
        if str2.count > str1.count {st1.append("0")}
    }

    let decimalRes = addString(st1, st2, 0)
    return (decimalRes.0.count > 0 ? "." + decimalRes.0 : "", decimalRes.1)
}

//this function add two integer with carry
func addInteger(_ str1 : String, _ str2: String, _ inputCarry :Int) -> String{
    let res = addString(str1, str2, inputCarry)
    return (res.1 > 0 ? String(res.1) : "") + res.0
}


//this function add two integer number and return carry
func addString(_ str1 : String, _ str2: String, _ inputCarry :Int) -> (String , Int){ // res and carry
    let arr1 = Array(str1)
    let arr2 = Array(str2)
    var p1 = arr1.count - 1
    var p2 = arr2.count - 1
    var res = ""
    var carry = inputCarry
    while p1 >= 0 || p2 >= 0 {
        let int1 = (p1 >= 0 ? Int(String(arr1[p1]))! : 0 )
        let int2 = (p2 >= 0 ? Int(String(arr2[p2]))! : 0)
        let sum = int1 + int2 + carry
        res = String(sum % 10) + res
        carry = sum / 10
        p1 -= 1
        p2 -= 1
    }
    return (res , carry)
}



print(addDoubleString("1.5", "2.50"))