const express = require('express');
const router = express.Router();

function factorial(n) {
    if (n === 0 || n === 1) return 1;
    let result = 1;
    for (let i = 2; i <= n; i++) result *= i;
    return result;
}

router.post('/', (req, res) => {
    const n = Math.floor(Number(req.body.input));
    if (n < 0 || n > 170) return res.status(400).json({ error: 'n must be between 0 and 170' });
    res.json({ ret: factorial(n) });
});

router.get('/:n', (req, res) => {
    const n = Math.floor(Number(req.params.n));
    if (n < 0 || n > 170) return res.status(400).json({ error: 'n must be between 0 and 170' });
    res.json({ ret: factorial(n) });
});

module.exports = router;