контрольный выстрел

using System;
using System.Collections.Generic;
using System.Text;

namespace SRP.ControlDigit
{
    public static class ControlDigitAlgo
    {
        public static int Upc(long number)
        {
            return number.GetSum(3, factor => 4 - factor).ControlDigit(10);
        }

        public static int Isbn10(long number)
        {
            var Isbn10ControlDigit = number.GetSum(2, factor => factor + 1).ControlDigit(11);
            if (Isbn10ControlDigit == 10)
            {
                return 'X';
            }
            return Isbn10ControlDigit + '0';
        }

        public static int Luhn(long number)
        {
                return number.GetSumLuhn(2, factor => factor % 2 == 0 ? 1 : 2).ControlDigit(10);
        }
    }

    public static class Extensions
    {
        public static int GetSum(this long number, int factor, Func<int, int> nextPosition)
        {
            var digitsSum = 0;
            do
            {
                digitsSum += factor * (int)(number % 10);
                number /= 10;
                factor = nextPosition(factor);
            } while (number > 0);
            return digitsSum;
		}

        public static int GetSumLuhn(this long number, int factor, Func<int, int> nextPosition)
        {
            var digitsSum = 0;
            while (number > 0)
            {
                if (factor * (int)(number % 10) > 9)
                {
                    digitsSum += factor * (int)(number % 10) - 9;
                }
                else
                {
                    digitsSum += factor * (int)(number % 10);
                }
                number /= 10;
                factor = nextPosition(factor);
            }
            return digitsSum;
        }

        public static int ControlDigit(this int digitsSum, int module)
        {
            if (digitsSum % module == 0) return 0;
            return module - digitsSum % module;
        }
    }
}