Adv2016/D15.1/Program.cs

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

namespace D15._1
{
    class Program
    {
        public class R : StreamReader, IEnumerable
        {
            public R(Stream stream) : base(stream) { }
            public IEnumerator GetEnumerator()
            {
                while (true)
                {
                    var line = ReadLine();
                    if (line == null) break;
                    yield return line;
                }
            }
        }

        static void Main(string[] args)
        {
            List<(int positions, int position)> discs = new List<(int, int)>();

            var regex = new Regex(@"Disc #\d has (?<n>\d+) positions; at time=0, it is at position (?<t>\d+).");
            using (var file = new R(File.OpenText(args[0]).BaseStream))
            {
                foreach (string line in file)
                {
                    var result = regex.Match(line);
                    discs.Add((int.Parse(result.Groups["n"].Value), int.Parse(result.Groups["t"].Value)));
                }
            }

            Console.WriteLine($"Part 1 answer is : {RunMachine(discs)}");
            discs.Add((11, 0));
            Console.WriteLine($"Part 2 answer is : {RunMachine(discs)}");
        }

        private static int RunMachine(List<(int positions, int position)> discs)
        {
            for (int startTime = 0; ; ++startTime)
            {
                int ball = 0;
                foreach (var (positions, position) in discs)
                {
                    ball++;
                    var t = startTime + ball;
                    if ((position + t) % positions != 0)
                    {
                        ball = 0;
                        break;
                    }
                }

                if (ball == discs.Count)
                    return startTime;
            }
        }
    }
}