'use strict';

const crypto = require('crypto');

function b64url(input) {
  return Buffer.from(input).toString('base64url');
}

function signHS256(payload, secret, expiresInSec = 3600) {
  const now = Math.floor(Date.now() / 1000);
  const body = { ...payload, iat: now, exp: now + expiresInSec };
  const header = b64url(JSON.stringify({ alg: 'HS256', typ: 'JWT' }));
  const claims = b64url(JSON.stringify(body));
  const data = `${header}.${claims}`;
  const sig = crypto.createHmac('sha256', secret).update(data).digest('base64url');
  return `${data}.${sig}`;
}

function verifyHS256(token, secret) {
  if (!token || !secret) return null;
  const parts = String(token).split('.');
  if (parts.length !== 3) return null;
  const [header, claims, sig] = parts;
  const expected = crypto.createHmac('sha256', secret).update(`${header}.${claims}`).digest('base64url');
  const a = Buffer.from(sig);
  const b = Buffer.from(expected);
  if (a.length !== b.length || !crypto.timingSafeEqual(a, b)) return null;
  let payload;
  try {
    payload = JSON.parse(Buffer.from(claims, 'base64url').toString('utf8'));
  } catch {
    return null;
  }
  if (payload.exp && payload.exp < Math.floor(Date.now() / 1000)) return null;
  return payload;
}

module.exports = { signHS256, verifyHS256 };