part2.ts

import * as fs from 'fs';
const input = fs.readFileSync('input', 'utf8').split('\n');

const getOptions = (check: string, key: string): string[] => {
	const [x, y] = key.split(',').map((n) => Number(n));
	if (check === 'N') return [x, x - 1, x + 1].map((x) => `${x},${y - 1}`);
	if (check === 'S') return [x, x - 1, x + 1].map((x) => `${x},${y + 1}`);
	if (check === 'W') return [y, y - 1, y + 1].map((y) => `${x - 1},${y}`);
	if (check === 'E') return [y, y - 1, y + 1].map((y) => `${x + 1},${y}`);
	return [];
};

const move = (elves: Set<string>, proposals: Map<string, string[]>): number => {
	let count = 0;
	for (const key of proposals.keys()) {
		const moving = proposals.get(key);
		if (moving?.length === 1) {
			elves.delete(moving[0]);
			elves.add(key);
			count++;
		}
	}
	return count;
};

const round = (elves: Set<string>, checks: string[]): number => {
	const proposals = new Map();
	for (const key of elves.keys()) {
		const all = checks.flatMap((check) => getOptions(check, key));
		if (all.every((option) => !elves.has(option))) {
			// do nothing if no other Elves in surrounding
			continue;
		}
		for (const check of checks) {
			const options = getOptions(check, key);
			if (options.every((option) => !elves.has(option))) {
				const decision = options[0];
				proposals.set(decision, (proposals.get(decision) || []).concat(key));
				break;
			}
		}
	}
	checks.push(checks.shift() as string);
	return move(elves, proposals);
};

const checks = ['N', 'S', 'W', 'E'];
const elves = new Set<string>();
input.forEach((line: string, y: number) =>
	line.split('').forEach((cell: string, x: number) => {
		if (cell === '#') {
			elves.add(`${x},${y}`);
		}
	}),
);

let result = 1;
while (round(elves, checks) > 0) {
	result++;
}

console.log(result);