Adv2015/D19.1/Program.cs

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

namespace D19._1
{
    public class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 1) throw new ArgumentException();
            if (File.Exists(args[0]) == false) throw new FileNotFoundException();

            string input;
            List<(string, string)> replacements;
            ParseFile(args, out input, out replacements);

            var molecules = TestMolecules(input, replacements);

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

        public static void ParseFile(string[] args, out string input, out List<(string, string)> replacements)
        {
            input = string.Empty;
            replacements = new List<(string, string)>();
            using (var file = File.OpenText(args[0]))
            {
                while (true)
                {
                    var line = file.ReadLine();
                    if (line == null) break;

                    if (line == string.Empty) continue;

                    var lr = line.Split(" => ");

                    if (lr.Length == 2) replacements.Add((lr[0], lr[1]));
                    else input = lr[0];
                }
            }
        }

        public static HashSet<string> TestMolecules(string input, List<(string, string)> replacements)
        {
            HashSet<string> molecules = new HashSet<string>();
            foreach ((string orig, string replace) in replacements)
            {
                int index = 0;

                while (true)
                {
                    index = input.IndexOf(orig, index);
                    if (index == -1) break;

                    string molecule = input.Substring(0, index) + replace + input.Substring(index + orig.Length);
                    molecules.Add(molecule);

                    index++;
                }
            }
            return molecules;
        }
    }
}