The year 518 is significantly more underground than your history books implied. Either that, or you've arrived in a vast cavern network under the North Pole.

After exploring a little, you discover a long tunnel that contains a row of small pots as far as you can see to your left and right. A few of them contain plants - someone is trying to grow things in these geothermally-heated caves.

Read the full puzzle.

using System.Collections.Generic;
using System.Linq;

namespace AdventOfCode.Y2018.Day12;

[ProblemName("Subterranean Sustainability")]
class Solution : Solver {

    public object PartOne(string input) => Iterate(input, 20);

    public object PartTwo(string input) => Iterate(input, 50000000000);

    long Iterate(string input, long iterations) {
        var (state, rules) = Parse(input);

        var dLeftPos = 0L;

        while (iterations > 0) {
            var prevState = state;
            state = Step(state, rules);
            iterations--;
            dLeftPos = state.left - prevState.left;
            if (state.pots == prevState.pots) {
                state = new State { left = state.left + iterations * dLeftPos, pots = state.pots };
                break;
            }
        }

        return Enumerable.Range(0, state.pots.Length).Select(i => state.pots[i] == '#' ? i + state.left : 0).Sum();
    }

    State Step(State state, Dictionary<string, string> rules) {
        var pots = "....." + state.pots + ".....";
        var newPots = "";
        for (var i = 2; i < pots.Length - 2; i++) {
            var x = pots.Substring(i - 2, 5);
            newPots += rules.TryGetValue(x, out var ch) ? ch : ".";
        }

        var firstFlower = newPots.IndexOf("#");
        var newLeft = firstFlower + state.left - 3;

        newPots = newPots.Substring(firstFlower);
        newPots = newPots.Substring(0, newPots.LastIndexOf("#") + 1);
        var res = new State { left = newLeft, pots = newPots };

        return res;
    }

    (State state, Dictionary<string, string> rules) Parse(string input) {
        var lines = input.Split("\n");
        var state = new State { left = 0, pots = lines[0].Substring("initial state: ".Length) };
        var rules = (from line in lines.Skip(2) let parts = line.Split(" => ") select new { key = parts[0], value = parts[1] }).ToDictionary(x => x.key, x => x.value);
        return (state, rules);
    }
}

class State {
    public long left;
    public string pots;
}

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