Adv2016/D04.1/Program.cs

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

namespace D04._1
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 1) throw new ArgumentException();
            if (File.Exists(args[0]) == false) throw new FileNotFoundException();

            int answer = 0;

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

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

                    int[] frequencies = new int['z' - 'a' + 1];
                    foreach (char c in name) frequencies[c - 'a']++;
                    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) answer += id;
                }
            }

            Console.WriteLine($"The answer is : {answer}");
        }
    }
}