Adv2015/D18.2/Program.cs

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

namespace D18._2
{
    class Program
    {
        static void Main(string[] args)
        {
            const int len = 100;

            string input = ParseFile(args[0]);

            for (int i = 0; i < 100; ++i)
                input = Step(len, input);

            int litOn = input.Count(c => c == '#');

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

        private static string ParseFile(string arg)
        {
            var sb = new StringBuilder();
            using (var file = File.OpenText(arg))
            {
                while (true)
                {
                    var line = file.ReadLine();
                    if (line == null) break;

                    sb.Append(line);
                }
            }

            string input = sb.ToString();
            return input;
        }

        private static string Step(int len, string input)
        {
            int edgeDetector = 0;
            int totalSize = len * len;
            int lastLineDetector = len * (len - 1);
            int lastColumn = len - 1;

            var stepb = new StringBuilder();
            for (int i = 0; i < totalSize; ++i)
            {
                char state = input[i];
                char nstate = state;

                bool onFirstLine = i < len;
                bool onLastLine = i >= lastLineDetector;

                if ((onFirstLine && edgeDetector == 0) || (onFirstLine && edgeDetector == lastColumn) || (onLastLine && edgeDetector == 0) || (onLastLine && edgeDetector == lastColumn))
                {
                    edgeDetector = AppendState(edgeDetector, lastColumn, stepb, '#');
                    continue;
                }

                int onNeighbors = DetectNeoghbors(len, input, edgeDetector, lastColumn, i, onFirstLine, onLastLine);

                if (state == '#')
                {
                    if (onNeighbors != 2 && onNeighbors != 3) nstate = '.';
                }
                else
                {
                    if (onNeighbors == 3) nstate = '#';
                }

                edgeDetector = AppendState(edgeDetector, lastColumn, stepb, nstate);
            }

            return stepb.ToString();
        }

        private static int AppendState(int edgeDetector, int lastColumn, StringBuilder stepb, char nstate)
        {
            stepb.Append(nstate);

            edgeDetector = edgeDetector == lastColumn ? 0 : edgeDetector + 1;
            return edgeDetector;
        }

        private static int DetectNeoghbors(int len, string input, int edgeDetector, int lastColumn, int i, bool onFirstLine, bool onLastLine)
        {
            int onNeighbors = 0;

            if (edgeDetector > 0) onNeighbors += input[i - 1] == '#' ? 1 : 0;
            if (edgeDetector < lastColumn) onNeighbors += input[i + 1] == '#' ? 1 : 0;
            if (onFirstLine == false) onNeighbors += input[i - len] == '#' ? 1 : 0;
            if (onLastLine == false) onNeighbors += input[i + len] == '#' ? 1 : 0;
            if (onFirstLine == false && edgeDetector > 0) onNeighbors += input[i - len - 1] == '#' ? 1 : 0;
            if (onFirstLine == false && edgeDetector < lastColumn) onNeighbors += input[i - len + 1] == '#' ? 1 : 0;
            if (onLastLine == false && edgeDetector > 0) onNeighbors += input[i + len - 1] == '#' ? 1 : 0;
            if (onLastLine == false && edgeDetector < lastColumn) onNeighbors += input[i + len + 1] == '#' ? 1 : 0;
            return onNeighbors;
        }
    }
}