part2.ts

import * as fs from 'fs';

const input = fs.readFileSync('input', 'utf8').split('\n').map((line => line.split('')));

const startY = input.findIndex(line => line.includes('^'));
const startX = input[startY].indexOf('^');
const start = { x: startX, y: startY, direction: 'up' };

const STEP = {
	up: { x: 0, y: -1, turnDirection: 'right' },
	down: { x: 0, y: 1, turnDirection: 'left' },
	left: { x: -1, y: 0, turnDirection: 'up' },
	right: { x: 1, y: 0, turnDirection: 'down' },
};

input[start.y][start.x] = '.';
let current = { ...start };
const visited = new Set<string>();

while (input[current.y]?.[current.x]) {
	const next = { ...current };
	while (input[next.y]?.[next.x] === '.') {
		visited.add(`${next.x},${next.y}`);
		next.x += STEP[current.direction].x;
		next.y += STEP[current.direction].y;
	}
	if (input[next.y]?.[next.x] === '#') {
		next.x -= STEP[current.direction].x;
		next.y -= STEP[current.direction].y;
		next.direction = STEP[current.direction].turnDirection;
	}
	current = next;
}

let result = 0;
visited.forEach(pos => {
	const [x, y] = pos.split(",").map(Number);
	const map = input.map(row => [...row]);
	map[y][x] = "#";
	current = { ...start };
	const turns = new Set<string>();

	while (map[current.y]?.[current.x]) {
		const next = { ...current };
		while (map[next.y]?.[next.x] === ".") {
			next.x += STEP[current.direction].x;
			next.y += STEP[current.direction].y;
		}
		if (map[next.y]?.[next.x] === "#") {
			next.x -= STEP[current.direction].x;
			next.y -= STEP[current.direction].y;
			next.direction = STEP[current.direction].turnDirection;
			if (turns.has(`${next.x},${next.y},${next.direction}`)) {
				result++;
				break;
			}
			turns.add(`${next.x},${next.y},${next.direction}`);
		}
		current = next;
	}
});

console.log(result);