Adv2018/D12.2/Program.cs

using System;
using System.Collections.Generic;
using System.IO;

namespace D12._2
{
    class Rules : Dictionary<string, char> { }
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 1) return;
            if (File.Exists(args[0]) == false) return;
            var file = File.OpenText(args[0]);

            var state = file.ReadLine().Substring(@"initial state: ".Length);
            file.ReadLine();

            var ruleset = new Rules();

            do
            {
                var line = file.ReadLine();
                if (line == null) break;

                var s = line.Split(" => ");
                ruleset.Add(s[0], s[1][0]);

            } while (true);

            int left = 0;
            int last = 0;
            int prev = 0;
            int cycleresult = 0;
            int prevcycleresult = 0;
            int gen = 0;

            while (true)
            {
                string nstate = "";

                for (int j = -2; j < state.Length + 2; ++j)
                    nstate += getState(state, j, ruleset);

                left -= 2;
                state = nstate;

                last = CalcResult(state, left);

                cycleresult = last - prev;

                gen++;
                if (cycleresult == prevcycleresult) break;

                prevcycleresult = cycleresult;
                prev = last;

            }

            var result = (50000000000 - gen) * cycleresult + last;
            Console.WriteLine($"The result is {result}");
        }

        private static int CalcResult(string state, int l)
        {
            int left = l;
            int result = 0;
            foreach (char c in state)
            {
                if (c == '#') result += left;
                left++;
            }

            return result;
        }

        static char getState(string state, int p, Rules rules)
        {
            var sb = "";
            for (var i = -2; i <= 2; ++i)
                sb += p + i < 0 || p + i >= state.Length ? '.' : state[p + i];
            return rules[sb];
        }
    }
}