part2.ts

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

let result = 0;

const rows = input.length;
const cols = input[0].length;

const startingRow = input.findIndex((row) => row.includes('S'));
const startingCol = input[startingRow].indexOf('S');

const canGoUp = ['|', '7', 'F'].includes(input[startingRow - 1][startingCol]);
const canGoDown = ['|', 'L', 'J'].includes(input[startingRow + 1][startingCol]);
const canGoLeft = ['-', 'L', 'F'].includes(input[startingRow][startingCol - 1]);
const canGoRight = ['-', '7', 'J'].includes(input[startingRow][startingCol + 1]);

let direction = 0; // up, right, down, left

if (canGoUp && canGoDown) {
	input[startingRow][startingCol] = '|';
	direction = 0;
} else if (canGoUp && canGoRight) {
	input[startingRow][startingCol] = 'L';
	direction = 0;
} else if (canGoUp && canGoLeft) {
	input[startingRow][startingCol] = 'J';
	direction = 0;
} else if (canGoDown && canGoRight) {
	input[startingRow][startingCol] = 'F';
	direction = 2;
} else if (canGoDown && canGoLeft) {
	input[startingRow][startingCol] = '7';
	direction = 2;
} else if (canGoLeft && canGoRight) {
	input[startingRow][startingCol] = '-';
	direction = 1;
}

const rowDirections = [-1, 0, 1, 0];
const colDirections = [0, 1, 0, -1];

let row = startingRow;
let col = startingCol;
let loopBoard = Array(rows)
	.fill(0)
	.map(() => Array(cols).fill('.'));

while (true) {
	loopBoard[row][col] = input[row][col];

	row += rowDirections[direction];
	col += colDirections[direction];
	switch (input[row][col]) {
		case 'L':
		case '7':
			direction ^= 3;
			break;
		case 'J':
		case 'F':
			direction ^= 1;
			break;
	}

	if (row == startingRow && col == startingCol) {
		break;
	}
}

// Drawing a line between a point to any edge, if it cross the loop odd-number times, the point is within the loop
// Loop thought all points and get the cross section count to left edge, for up direction (|, L, or F) only
for (let i = 0; i < rows - 1; i++) {
	let crossSectionCount = 0;
	for (let j = 0; j < cols - 1; j++) {
		if (loopBoard[i][j] === '.') {
			if (crossSectionCount % 2 === 1) {
				loopBoard[i][j] = '1';
				result++;
			}
		} else if (['|', 'L', 'J'].includes(loopBoard[i][j])) {
			crossSectionCount++;
		}
	}
}

console.log(result);