Adv2016/D04.2/Program.cs

using System;
using System.IO;
using System.Linq;
using System.Text;

namespace D04._2
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 1) throw new ArgumentException();
            if (File.Exists(args[0]) == false) throw new FileNotFoundException();

            using (var file = File.OpenText(args[0]))
            {
                var sb = new StringBuilder();
                while (true)
                {
                    var line = file.ReadLine();
                    if (line == null) break;

                    int lio = line.LastIndexOf('-'), io = line.IndexOf('[');
                    var name = line.Substring(0, lio);
                    var hash = line.Substring(io + 1).TrimEnd(']');
                    var id = int.Parse(line.Substring(lio + 1, io - lio - 1));

                    var ALPHA = 'z' - 'a' + 1;

                    sb.Clear();
                    int[] frequencies = new int[ALPHA];
                    foreach (char c in name)
                    {
                        if (c == '-')
                        {
                            sb.Append(' ');
                            continue;
                        }
                        int lid = c - 'a';
                        frequencies[lid]++;
                        sb.Append((char)('a' + (lid + id) % ALPHA));
                    }

                    var letters = string.Join("", frequencies.Select((f, index) => (((char)(index + 'a'), f) as (char letter, int freq)?).Value)
                        .OrderByDescending(kvp => kvp.freq)
                        .ThenBy(kvp => kvp.letter)
                        .Take(5)
                        .Select(kvp => kvp.letter));

                    if (letters == hash && sb.ToString().Contains("north"))
                        Console.WriteLine($"The answer is : {id}");
                }
            }
        }
    }
}