using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SpecialOlympics
{
    class Program
    {
        interface IEncoder<T>
        {
            T Encode(T chipher, T key);
        }

        interface IDecoder<T>
        {
            T Decode(T text, T key);
        }

        class OneTimePad : IEncoder<char>, IDecoder<char>, IEncoder<string>, IDecoder<string>
        {
            private static readonly int LENGTH_OF_LATIN = 26;

            private string XorStrings(string text, string key, Func<char,char,char> operation)
            {
                if(text.Count(c => 'A' <= c && c <= 'Z') > key.Length) throw new ArgumentException("Key length should be greater then letters count in text");
                var builder = new StringBuilder(text);
                var numberOfLetters = 0;
                for (var i = 0; i < builder.Length; i++)
                {
                    var c = builder[i];
                    if ('A' <= c && c <= 'Z')
                    {
                        builder[i] = operation(c,key[numberOfLetters++]);
                    }
                }
                return builder.ToString();
            }


            public string Encode(string chipher, string key)
            {
                return XorStrings(chipher, key, Encode);
            }

            public string Decode(string text, string key)
            {
                return XorStrings(text, key, Decode);
            }

            public char Encode(char chipher, char key)
            {
                return (char)('A' + (((chipher - 'A') + (key - 'A')) % LENGTH_OF_LATIN));
            }

            public char Decode(char text, char key)
            {
                var diff = (text - 'A') - (key - 'A');
                return (char)('A' + ((diff < 0 ? LENGTH_OF_LATIN + diff : diff) % LENGTH_OF_LATIN));
            }
        }

        static void Main(string[] args)
        {
            var oneTimePad = new OneTimePad();
            if (args.Length < 3)
            {
                Console.WriteLine("Not enought arguments");
                Console.WriteLine("First argument is action (--encode or --decode), second is key and third is text or chipher text.");
            }
            try
            {
                switch (args[0])
                {
                    case "--encode":
                    case "-e":
                        Console.WriteLine(oneTimePad.Encode(args[2], args[1]));
                        break;
                    case "--decode":
                    case "-d":
                        Console.WriteLine(oneTimePad.Decode(args[2], args[1]));
                        break;
                    default:
                        Console.WriteLine(
                            "First argument is action (--encode or --decode), second is key and third is text or chipher text. Only latin upper case letters are encoded");
                        break;
                }
            }
            catch (Exception e)
            {
                Console.WriteLine("Error occured: {0}",e.Message);
            }
        }
    }
}