namespace Messages
{
    public class InvalidPasswordException : System.Exception
    {
        public string EncryptedText { get; }
        public string Password { get; }

        public InvalidPasswordException() { }
        public InvalidPasswordException(string message) : base(message) { }
        public InvalidPasswordException(string message, string encryptedText, string password) : base(message)
        {
            EncryptedText = encryptedText;
            Password = password;
        }
    }

    public class Rot13Encryption : IEncryption
    {
        public string Encrypt(string text)
        {
            if (text == null) return null;
            var chars = new char[text.Length];
            for (int i = 0; i < text.Length; i++) chars[i] = Rot13(text[i]);
            return new string(chars);
        }

        public string Decrypt(string text)
        {
            if (text == null) return null;
            var chars = new char[text.Length];
            for (int i = 0; i < text.Length; i++) chars[i] = Rot13(text[i]);
            return new string(chars);
        }

        public char GetMark() => 'R';

        private char Rot13(char letter)
        {
            if (letter >= 'A' && letter <= 'Z')
                letter = (char)('A' + (letter - 'A' + 13) % 26);
            else if (letter >= 'a' && letter <= 'z')
                letter = (char)('a' + (letter - 'a' + 13) % 26);
            return letter;
        }
    }

    public class PasswordBasedEncryption : IEncryption
    {
        private readonly string _password;

        public PasswordBasedEncryption(string password)
        {
            if (string.IsNullOrEmpty(password))
                throw new InvalidPasswordException();
            _password = password;
        }

        public string Encrypt(string text)
        {
            if (text == null) return null;
            var chars = new char[text.Length];
            for (int i = 0; i < text.Length; i++)
            {
                int a = text[i];
                int b = _password[i % _password.Length];
                chars[i] = (char)(a + b);
            }
            return new string(chars);
        }

        public string Decrypt(string text)
        {
            if (text == null) return null;
            var chars = new char[text.Length];
            for (int i = 0; i < text.Length; i++)
            {
                int a = text[i];
                int b = _password[i % _password.Length];
                int c = a - b;
                if (c < 0)
                    throw new InvalidPasswordException("Invalid password for decryption.", text, _password);
                chars[i] = (char)c;
            }
            return new string(chars);
        }

        public char GetMark() => 'P';
    }
}