/**
 * A notification link is a relative app path: it starts with "/", not with
 * "//" or "/\", carries no scheme and is at most 300 characters long. Anything
 * else is a programming error, never user input (the code that records the
 * notification builds the link).
 */
const APP_LINK_PATTERN = /^\/(?![/\\])[A-Za-z0-9\-._~/?=&%]{0,299}$/;

export function assertAppLink(link: string): void {
  if (!APP_LINK_PATTERN.test(link)) {
    throw new Error(`Notification link is not a relative app path: ${link}`);
  }
}

/** One key per recipient, so a fan-out repeated for the same event writes nothing new. */
export function notificationDedupeKey(prefix: string, userId: string): string {
  return `${prefix}:${userId}`;
}