Adv2016/D14.1/Program.cs

using System;
using System.Collections.Generic;
using System.Security.Cryptography;
using System.Text;

namespace D14._1
{
    class Program
    {
        static byte[] ToB(string str) => Encoding.ASCII.GetBytes(str);
        static string ToS(byte[] b) => BitConverter.ToString(b).Replace("-", "").ToLower();

        static bool HasQuintuplet(string str, char c)
        {
            for (int i = 4; i < str.Length; ++i)
                if (str[i - 4] == str[i - 3] && str[i - 3] == str[i - 2] && str[i - 2] == str[i - 1] && str[i - 1] == str[i] && str[i] == c)
                    return true;
            return false;
        }

        static char? HasTriplet(string str)
        {
            for (int i = 2; i < str.Length; ++i)
                if (str[i - 2] == str[i - 1] && str[i - 1] == str[i])
                    return str[i];
            return null;
        }

        static void Main(string[] args)
        {
            if (args.Length < 1) throw new ArgumentException();
            string input = args[0];

            int index1 = GetHashes(input, 0);
            Console.WriteLine($"Index for Part 1 is : {index1}");

            int index2 = GetHashes(input, 2016);
            Console.WriteLine($"Index for Part 2 is : {index2}");
        }

        static Dictionary<int, string> cache = new Dictionary<int, string>();
        private static int GetHashes(string input, int stretching)
        {
            cache.Clear();
            int index = -1;

            using (var md5 = MD5.Create())
            {
                var found = 0;

                while (found < 64)
                {
                    index++;

                    string hash = TryGetHash(input, index, md5, stretching);

                    var triplet = HasTriplet(hash);
                    if (triplet.HasValue == false) continue;

                    for (var i = index + 1; i < index + 1000; ++i)
                    {
                        string h = TryGetHash(input, i, md5, stretching);
                        if (HasQuintuplet(h, triplet.Value))
                        {
                            found++;
                            break;
                        }
                    }
                }
            }

            return index;
        }

        private static string TryGetHash(string input, int index, MD5 md5, int stretching)
        {
            if (cache.ContainsKey(index)) return cache[index];
            var hash = GetHash(input, index, md5, stretching);
            cache.Add(index, hash);
            return hash;
        }

        private static string GetHash(string input, int index, MD5 md5, int stretching)
        {
            var hash = ToS(md5.ComputeHash(ToB($"{input}{index}")));
            for (var i = 0; i < stretching; ++i) hash = ToS(md5.ComputeHash(ToB(hash)));
            return hash;
        }
    }
}