function toBase64Url(value: string) {
  if (typeof Buffer !== 'undefined') {
    return Buffer.from(value, 'utf8').toString('base64url');
  }

  return btoa(value)
    .replace(/\+/g, '-')
    .replace(/\//g, '_')
    .replace(/=+$/g, '');
}

export function buildMessengerBotStartUrl(baseUrl: string, email: string) {
  const normalizedEmail = email.trim().toLowerCase();
  if (!baseUrl || !normalizedEmail) {
    return '';
  }

  const payload = encodeURIComponent(toBase64Url(`login:${normalizedEmail}`));
  const separator = baseUrl.includes('?') ? '&' : '?';
  return `${baseUrl}${separator}start=${payload}`;
}