On the outskirts of the North Pole base construction project, many Elves are collecting lumber.

The lumber collection area is 50 acres by 50 acres; each acre can be either open ground (.), trees (|), or a lumberyard (#). You take a scan of the area (your puzzle input).

Read the full puzzle.

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

namespace AdventOfCode.Y2018.Day18;

[ProblemName("Settlers of The North Pole")]
class Solution : Solver {

    public object PartOne(string input) => Iterate(input, 10);
    public object PartTwo(string input) => Iterate(input, 1000000000);

    int Iterate(string input, int lim) {
        var seen = new Dictionary<string, int>();
        var mtx = input.Split("\n");
        
        for (var t = 0; t < lim; t++) {
            var hash = string.Join("", mtx);
            if (seen.ContainsKey(hash)) {
                var loopLength = t - seen[hash];
                var remainingSteps = lim - t - 1;
                var remainingLoops = remainingSteps / loopLength;
                t += remainingLoops * loopLength;
            } else {
                seen[hash] = t;
            }
            mtx = Step(mtx);
        }
        var res = string.Join("", mtx);
        return Regex.Matches(res, @"\#").Count * Regex.Matches(res, @"\|").Count;
    }

    string[] Step(string[] mtx) {
        var res = new List<string>();
        var crow = mtx.Length;
        var ccol = mtx[0].Length;
       
        for (var irow = 0; irow < crow; irow++) {
            var line = "";
            for (var icol = 0; icol < ccol; icol++) {
                var (tree, lumberyard, empty) = (0, 0, 0);
                foreach (var drow in new[] { -1, 0, 1 }) {
                    foreach (var dcol in new[] { -1, 0, 1 }) {
                        if (drow != 0 || dcol != 0) {
                            var (icolT, irowT) = (icol + dcol, irow + drow);
                            if (icolT >= 0 && icolT < ccol && irowT >= 0 && irowT < crow) {
                                switch (mtx[irowT][icolT]) {
                                    case '#': lumberyard++; break;
                                    case '|': tree++; break;
                                    case '.': empty++; break;
                                }
                            }
                        }
                    }
                }

                line += mtx[irow][icol] switch {
                    '#' when lumberyard >= 1 && tree >= 1 => '#',
                    '|' when lumberyard >= 3 => '#',
                    '.' when tree >= 3 => '|',
                    '#' => '.',
                    var c => c
                };
            }
            res.Add(line);
        }
        return res.ToArray();
    }
}

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