Your plane lands with plenty of time to spare. The final leg of your journey is a ferry that goes directly to the tropical island where you can finally start your vacation. As you reach the waiting area to board the ferry, you realize you're so early, nobody else has even arrived yet!

By modeling the process people use to choose (or abandon) their seat in the waiting area, you're pretty sure you can predict the best place to sit. You make a quick map of the seat layout (your puzzle input).

Read the full puzzle.

using System;
using System.Linq;

namespace AdventOfCode.Y2020.Day11;

[ProblemName("Seating System")]
class Solution : Solver {

    public object PartOne(string input) => Solve(input, 4, _ => true);
    public object PartTwo(string input) => Solve(input, 5, place => place != '.');

    int Solve(string input, int occupiedLimit, Func<char, bool> placeToCheck) {
        var (crow, ccol) = (input.Split("\n").Length, input.IndexOf('\n'));

        char PlaceInDirection(char[] st, int idx, int drow, int dcol) {
            var (irow, icol) = (idx / ccol, idx % ccol);
            while (true) {
                (irow, icol) = (irow + drow, icol + dcol);
                var place =
                    irow < 0 || irow >= crow ? 'L' :
                    icol < 0 || icol >= ccol ? 'L' :
                    st[irow * ccol + icol];
                if (placeToCheck(place)) {
                    return place;
                }
            }
        }

        int OccupiedPlacesAround(char[] st, int idx) {
            var directions = new[] { (0, -1), (0, 1), (-1, 0), (1, 0), (-1, -1), (-1, 1), (1, -1), (1, 1) };
            var occupied = 0;
            foreach (var (drow, dcol) in directions) {
                if (PlaceInDirection(st, idx, drow, dcol) == '#') {
                    occupied++;
                }
            }
            return occupied;
        }

        var prevState = new char[0];
        var state = input.Replace("\n", "").Replace("L", "#").ToArray();
        while (!prevState.SequenceEqual(state)) {
            prevState = state;
            state = state.Select((place, i) =>
                place == '#' && OccupiedPlacesAround(state, i) >= occupiedLimit ? 'L' :
                place == 'L' && OccupiedPlacesAround(state, i) == 0             ? '#' :
                place /*otherwise*/
            ).ToArray();
        }
        return state.Count(place => place == '#');
    }
}

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