part2.ts

import * as fs from 'fs';

const input = fs.readFileSync('example', 'utf8').split('\n');

const getNumber = (indices: [number, number]) => {
	const i = indices[0];
	const j = indices[1];
	let number = '' + input[i][j];
	let jMinor = j;
	while (jMinor > 0 && /\d/.test(input[i][--jMinor])) {
		number = input[i][jMinor] + number;
	}
	let jPlus = j;
	while (jPlus < input[0].length - 1 && /\d/.test(input[i][++jPlus])) {
		number = number + input[i][jPlus];
	}
	return +number;
};

let result = 0;

for (let i = 0; i < input.length; i++) {
	const matches = Array.from(input[i].matchAll(/\*/g));
	for (const match of matches) {
		const j = match.index;
		const surrounding = [
			(input[i - 1] ?? '')[j - 1] ?? '.',
			(input[i - 1] ?? '')[j] ?? '.',
			(input[i - 1] ?? '')[j + 1] ?? '.',
			(input[i] ?? '')[j - 1] ?? '.',
			(input[i] ?? '')[j] ?? '.',
			(input[i] ?? '')[j + 1] ?? '.',
			(input[i + 1] ?? '')[j - 1] ?? '.',
			(input[i + 1] ?? '')[j] ?? '.',
			(input[i + 1] ?? '')[j + 1] ?? '.',
		];
		const indices = [
			[i - 1, j - 1],
			[i - 1, j],
			[i - 1, j + 1],
			[i, j - 1],
			[i, j],
			[i, j + 1],
			[i + 1, j - 1],
			[i + 1, j],
			[i + 1, j + 1],
		];
		const localNumberIndices = [];
		for (let k = 0; k < surrounding.length; k++) {
			if (/\d/.test(surrounding[k]) && (!/\d/.test(surrounding[k - 1] ?? '') || k % 3 == 0)) {
				localNumberIndices.push(indices[k]);
			}
		}
		if (localNumberIndices.length === 2) {
			let number1 = getNumber(localNumberIndices[0]);
			let number2 = getNumber(localNumberIndices[1]);
			result += number1 * number2;
		}
	}
}

console.log(result);