Step by Step solution about How to resolve the algorithm Validate International Securities Identification Number step by step in the C# programming language

The provided C# code defines two classes: IsinValidator and Program. It is designed to validate International Securities Identification Numbers (ISINs). Let's break down the code in detail:

IsinValidator Class:

• The IsinValidator class contains static methods for validating ISINs.
• IsValidIsin(string isin): This is the main method that returns a boolean indicating whether the input string is a valid ISIN. It combines two tests:
  • IsinRegex.IsMatch(isin): This checks if the input string matches the regular expression pattern defined by IsinRegex.
  • LuhnTest(Digitize(isin)): This checks if the digits extracted from the ISIN pass the Luhn algorithm test, which is a checksum used to validate financial numbers.
• IsinRegex: A static Regex object defining the regular expression pattern for valid ISINs. It matches strings that start with two uppercase letters, followed by nine uppercase letters or digits, and ending with a single digit.
• Digitize(string isin): This method converts the input ISIN string into a string of digits by mapping each character to its numeric value. Characters 'A' through 'Z' are mapped to 10-35. Digits '0' through '9' are mapped to their corresponding decimal values.
• LuhnTest(string number): This method performs the Luhn algorithm on the input string of digits. It reverses the string, converts each character to an integer, calculates a summand for each digit, and sums the results. If the result is divisible by 10, the test passes.
• Summand(int digit, int i): This method calculates a summand for a given digit based on its position. The formula is digit + (position modulo 2) * (digit - digit divided by 5 multiplied by 9).
• DigitValue(char c): This method maps a character to its numeric value. Characters 'A' through 'Z' are mapped to 10-35. Digits '0' through '9' are mapped to their corresponding decimal values.

Program Class:

• The Program class contains a Main method that serves as the entry point of the program.
• In the Main method:
  • An array of ISINs (isins) is defined.
  • For each ISIN in the array, it calls IsinValidator.IsValidIsin to determine if the ISIN is valid.
  • It prints whether each ISIN is valid or not valid to the console.

Sample Input and Output:

The example provided in the code includes an array of ISINs. The output for this input should look something like this:

US0378331005 is valid
US0373831005 is not valid
U50378331005 is not valid
US03378331005 is not valid
AU0000XVGZA3 is valid
AU0000VXGZA3 is not valid
FR0000988040 is valid

using System;
using System.Linq;
using System.Text.RegularExpressions;

namespace ValidateIsin
{
    public static class IsinValidator
    {
        public static bool IsValidIsin(string isin) => 
            IsinRegex.IsMatch(isin) && LuhnTest(Digitize(isin));

        private static readonly Regex IsinRegex = 
            new Regex("^[A-Z]{2}[A-Z0-9]{9}\\d$", RegexOptions.Compiled);

        private static string Digitize(string isin) =>
            string.Join("", isin.Select(c => $"{DigitValue(c)}"));

        private static bool LuhnTest(string number) => 
            number.Reverse().Select(DigitValue).Select(Summand).Sum() % 10 == 0;

        private static int Summand(int digit, int i) =>
            digit + (i % 2) * (digit - digit / 5 * 9);

        private static int DigitValue(char c) =>
            c >= '0' && c <= '9' 
                ? c - '0' 
                : c - 'A' + 10;
   }
	
   public class Program
   {
        public static void Main() 
        {
            string[] isins = 
            {
                "US0378331005",
                "US0373831005",
                "U50378331005",
                "US03378331005",
                "AU0000XVGZA3",
                "AU0000VXGZA3",
                "FR0000988040"
            };

            foreach (string isin in isins) {
                string validOrNot = IsinValidator.IsValidIsin(isin) ? "valid" : "not valid";
                Console.WriteLine($"{isin} is {validOrNot}");
            }
        }
    }
}