Adventofcode Day 4 - Challenge 2

adventofcode-day4-2.cs

using System;
using System.IO;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using System.Linq;

namespace ConsoleApplication
{
    public class Program
    {
        public static void Main(string[] args)
        {
            using(var input = File.OpenText("input.txt")){
                string line;
                while((line = input.ReadLine()) != null)
                {
                    var room = new Room(line);
                    if(room.IsValidRoomName() && room.Name.ToLower().Contains("north")){
                        Console.WriteLine(room.Name + " " + room.SectorId);
                    }
                }
                
            }
        }

    }

    public class Room{
        public Room(string encryptedname){
            this.parseEncryptedName(encryptedname);
            this.Name = DecryptUsingShiftedCypher(this.EncryptedName, this.SectorId);
        }
        public string EncryptedName{
            get;
            set;
        }
        public string Name{
            get;
            set;
        }        
        public int SectorId{
            get;
            set;
        }
        public string CheckSum{
            get;
            set;
        }
        private string DecryptUsingShiftedCypher(string input, int shift){
            return String.Join("", input.Select(i => this.ShiftChar(i, this.SectorId)).ToArray());
        }
        private char ShiftChar(char letter, int shift){
            var fixedshift = shift % 26;
            if(letter==' ') return letter;
            var result = (char)(letter + fixedshift);
            // Subtract 26 on overflow.
            // Add 26 on underflow.
            if (result > 'z')
            {
                result = (char)(result - 26);
            }
            else if (letter < 'a')
            {
                result = (char)(result + 26);
            }            
            return result;
        }
        public bool IsValidRoomName(){
            return this.CheckSum == this.CalculateCheckSum();
        }
        private string CalculateCheckSum(){
            return String.Join("",this.GetGroupsOfLetters(this.EncryptedName.Replace(" ", "")));
        }
        private char[] GetGroupsOfLetters(string input){
            var groups = input.GroupBy(c => c).OrderByDescending(g => g.Count()).ThenBy(g => g.Key).Select(g=>g.Key).Take(5).ToArray();
            return groups;
        }
        private void parseEncryptedName(string encryptedname){
            var pattern = new Regex(@"(.*)\-(\d*)\[(\w*)\]");
            var match = pattern.Match(encryptedname);
            this.EncryptedName = match.Groups[1].Value.Replace("-"," ");
            this.SectorId = Int32.Parse(match.Groups[2].Value);
            this.CheckSum = match.Groups[3].Value;
        }
    }
}