AdventOfCode 2018 Day 12

void Main()
{
    #region testdata
    var test = new string[] {
    "#..#.#..##......###...###...........",
    "",
    "...## => #",
    "..#.. => #",
    ".#... => #",
    ".#.#. => #",
    ".#.## => #",
    ".##.. => #",
    ".#### => #",
    "#.#.# => #",
    "#.### => #",
    "##.#. => #",
    "##.## => #",
    "###.. => #",
    "###.# => #",
    "####. => #",
    };
    #endregion

    //test
    if(answerA(test, 20) != 325) throw new Exception("failed test a");

    var aoc = new AdventOfCode(2018, 12);
    aoc.SubmitAnswer(answerA(aoc.InputLines, 20), Part.A);
    aoc.SubmitAnswer(answerA(aoc.InputLines, 50000000000), Part.B);
}


public long answerA(string[] input, long cycles)
{
    //get input
    string state = input[0].Replace("initial state: ", "").Trim();

    //create rule dic
    var dicRules = input.Skip(2).Select(x => x.Split(' ')).ToDictionary(x=> x[0], x => x[2]);

    //determine leftsize added
    long appendLeft = 0;

    //each cycle
    for(int i = 0; i < cycles; i++)
    {
        //get next state
        var nextGen = NextGen(state, appendLeft, dicRules);

        //did it change?
        if(nextGen.state == state)
        {
            //nope, what the next state
            var nextGen2 = NextGen(nextGen.state, nextGen.appendLeft, dicRules);

            //determine the delta per cycle
            long delta = nextGen2.appendLeft - nextGen.appendLeft;

            //move append forward in time
            appendLeft += (cycles - i) * delta;
            break; //DONE!
        }
        else
        {
            //switch
            state = nextGen.state;
            appendLeft = nextGen.appendLeft;
        }
    }

    //sum
    long sum = 0;

    //loop through the plants
    for(int i = 0; i < state.Length; i++)
    {
        //add score
        if(state[i] == '#') sum += i - appendLeft;
    }
    return sum;
}

public (string state, long appendLeft) NextGen(string state, long appendLeft, Dictionary<string, string> dicRules)
{
    //append start if needed
    while (state.StartsWith("...") == false)
    {
        state = "." + state;
        appendLeft++;
    }
    //append end if needed
    while (state.EndsWith("...") == false)
    {
        state = state + ".";
    }

    //build the new state
    StringBuilder sb = new StringBuilder();

    //loop through the state
    for (int x = 0; x < state.Length - 4; x++)
    {
        if (dicRules.TryGetValue(state.Substring(x, 5), out string c))
            sb.Append(c);
        else sb.Append(".");
    }

    //create the new state (.. is default as those fall off per rules
    state = sb.ToString() + "..";
    appendLeft -= 2;

    //done
    return(state, appendLeft);
}

// Define other methods and classes here