part1.ts

import * as fs from 'fs';

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

const rows = input.length;
const columns = input[0].length;
const invalidDirections = ['^v', 'v^', '<>', '><'];
const visitedPositions = { '1,0^': new Set(['1,0']) };
const neighbors = { '0,1': [1, 1] };
neighbors[[rows - 1, columns - 2].join(',')] = [rows - 2, columns - 2];

for (let i = 1; i < rows - 1; i++) {
	// skip first row
	for (let j = 1; j < columns - 1; j++) {
		// skip first column
		if (input[i][j] === '#') {
			continue;
		}
		const neighbor = [];
		if (input[i - 1][j] !== '#') neighbor.push([i - 1, j, '^']);
		if (input[i + 1][j] !== '#') neighbor.push([i + 1, j, 'v']);
		if (input[i][j - 1] !== '#') neighbor.push([i, j - 1, '<']);
		if (input[i][j + 1] !== '#') neighbor.push([i, j + 1, '>']);
		neighbors[[i, j].join(',')] = neighbor;
	}
}

let result = 0;

function walk(row: number, col: number, visited: Set<string>, direction: string) {
	if (row === rows - 1 && col === columns - 2) {
		// endpoint
		result = Math.max(result, visited.size);
		return;
	}
	const position = [row, col].join(',');
	const key = position + direction;
	if (
		!Array.isArray(neighbors[position][0]) &&
		key in visitedPositions &&
		[...visited].every((v) => visitedPositions[key].has(v))
	) {
		return;
	}
	if (invalidDirections.includes(direction + input[row][col])) {
		return;
	}
	visited.add(position);
	visitedPositions[key] = new Set(visited);
	for (const [coordRow, coordCol, dir] of neighbors[position]) {
		if (!visited.has([coordRow, coordCol].join(','))) {
			walk(coordRow, coordCol, new Set(visited), dir);
		}
	}
}

walk(1, 1, new Set(['0,1']), 'v');

console.log(result);