/***********************************************************************************************************************************************************************
 * File Name: otp.util.js
 * Type of Program: Utility
 * Description: One-Time Password (OTP) generation and validation helpers.
 *              - generateOTP()   → 6-digit numeric string
 *              - getOTPExpiry()  → Date object N minutes from now
 *              - isOTPExpired()  → boolean check on the stored expiry
 * Author: rgrgogu
 * Date Created: Oct. 6, 2025
 ***********************************************************************************************************************************************************************
 * HOW TO USE:
 *   const { generateOTP, getOTPExpiry, isOTPExpired } = require('../utils/otp.util');
 *   user.otp_code      = generateOTP();
 *   user.otp_expires_at = getOTPExpiry();
 ***********************************************************************************************************************************************************************/
const crypto = require('crypto');

/**
 * Generates a cryptographically secure 6-digit OTP.
 * @returns {string} e.g. "048291"
 */
const generateOTP = () => {
  const bytes = crypto.randomBytes(3);           // 3 bytes = 0–16777215
  const num   = bytes.readUIntBE(0, 3) % 1_000_000; // force to 0–999999
  return num.toString().padStart(6, '0');
};

/**
 * Returns a Date object N minutes in the future.
 * @param {number} [minutes=10]
 * @returns {Date}
 */
const getOTPExpiry = (minutes = Number(process.env.OTP_EXPIRY_MINUTES) || 10) => {
  return new Date(Date.now() + minutes * 60 * 1000);
};

/**
 * Checks whether the stored OTP has expired.
 * @param {Date|string} expiresAt
 * @returns {boolean}
 */
const isOTPExpired = (expiresAt) => !expiresAt || new Date() > new Date(expiresAt);

module.exports = { generateOTP, getOTPExpiry, isOTPExpired };