01. Registration

namespace _01.Registration;

internal class Program
{
    static void Main(string[] args)
    {
        string username = Console.ReadLine();

        string input;

        while ( (input = Console.ReadLine()) != "Registration" )
        {
            string[] command = input.Split();

            if (command[0] == "Letters")
            {
                if (command[1] == "Lower")
                {
                    username = username.ToLower();
                }

                else if (command[1] == "Upper")
                {
                    username = username.ToUpper();
                }

                Console.WriteLine(username);
            }

            else if (command[0] == "Reverse")
            {
                int startIndex = int.Parse(command[1]);

                int endIndex = int.Parse(command[2]);

                if ( (IsValidIndex(startIndex, username.Length)) && (IsValidIndex(endIndex, username.Length)) )
                {
                    string reversedSubstring = ReverseSubstring(username, startIndex, endIndex);
                    Console.WriteLine(reversedSubstring);
                }
            }

            else if (command[0] == "Substring")
            {
                string substring = command[1];

                if (username.Contains(substring))
                {
                    username = username.Replace(substring, "");
                    Console.WriteLine(username);
                }

                else
                {
                    Console.WriteLine($"The username {username} doesn't contain {substring}.");
                }
            }

            else if (command[0] == "Replace")
            {
                char charToReplace = char.Parse(command[1]);

                username = username.Replace(charToReplace, '-');

                Console.WriteLine(username);
            }

            else if (command[0] == "IsValid")
            {
                char charToCheck = char.Parse(command[1]);

                if (username.Contains(charToCheck))
                {
                    Console.WriteLine("Valid username.");
                }

                else
                {
                    Console.WriteLine($"{charToCheck} must be contained in your username.");
                }
            }
        }
    }

    static string ReverseSubstring(string str, int startIndex, int endIndex)
    {
        char[] charArray = new char[endIndex - startIndex + 1];

        for (int i = 0; i <= endIndex - startIndex; i++)
        {
            charArray[i] = str[startIndex + i];
        }

        Array.Reverse(charArray);

        return new string(charArray);
    }

    static bool IsValidIndex(int index, int length)
    {
        return (index >= 0) && (index < length);
    }
}