String of decimal digits used to represent an arbitrarily large counter

DecimalString

struct DecimalString {
    var value: String
    
    private init(_ value: String) {
        self.value = value
    }
    
    init(digits string: String) {
        let digits = CharacterSet.decimalDigits
        self.value = string.unicodeScalars.filter { digits.contains($0) } .map { String($0) } .joined()
    }
    
    func next() -> DecimalString {
        var carry  = UInt32(1)
        var digits = value.unicodeScalars.map { $0.value - UnicodeScalar("0").value }
        
        for idx in digits.indices.reversed() {
            let nextDigit = digits[idx] + carry
            digits[idx] = nextDigit % 10
            carry = nextDigit / 10
            
            if carry == 0 { break }
        }
        
        if carry != 0 {
            digits.insert(1, at: 0)
        }
        
        return DecimalString(digits.map { String($0) }.joined())
    }
}

let d = DecimalString(digits: "ABC99")
print(d.next())


print(d)