const ANONYMOUS_ID_STORAGE_KEY = "agent-native.anonymous_id";
const SESSION_ID_STORAGE_KEY = "agent-native.session_id";
const SESSION_LAST_ACTIVITY_STORAGE_KEY = "agent-native.session_last_activity";
// 30-minute idle timeout matches GA4 / Mixpanel defaults: a tab left open
// overnight starts a new session in the morning instead of stretching one visit.
const SESSION_IDLE_TIMEOUT_MS = 30 * 60 * 1000;

function generateVisitorId(): string {
  try {
    if (
      typeof crypto !== "undefined" &&
      typeof crypto.randomUUID === "function"
    ) {
      return crypto.randomUUID();
    }
  } catch {
    // fall through to Math.random
  }
  return (
    Date.now().toString(36) +
    Math.random().toString(36).slice(2) +
    Math.random().toString(36).slice(2)
  );
}

function safeStorageGet(key: string): string | null {
  try {
    return window.localStorage.getItem(key);
  } catch {
    return null;
  }
}

function safeStorageSet(key: string, value: string): void {
  try {
    window.localStorage.setItem(key, value);
  } catch {
    // private browsing / storage disabled -- best-effort
  }
}

export function getOrCreateAnalyticsAnonymousId(): string | undefined {
  if (typeof window === "undefined") return undefined;
  let id = safeStorageGet(ANONYMOUS_ID_STORAGE_KEY);
  if (!id) {
    id = generateVisitorId();
    safeStorageSet(ANONYMOUS_ID_STORAGE_KEY, id);
  }
  return id;
}

export function getOrCreateAnalyticsSessionId(): string | undefined {
  if (typeof window === "undefined") return undefined;
  const now = Date.now();
  const lastActivityRaw = safeStorageGet(SESSION_LAST_ACTIVITY_STORAGE_KEY);
  const lastActivity = lastActivityRaw
    ? Number.parseInt(lastActivityRaw, 10)
    : 0;
  let id = safeStorageGet(SESSION_ID_STORAGE_KEY);
  const expired =
    !lastActivity ||
    Number.isNaN(lastActivity) ||
    now - lastActivity > SESSION_IDLE_TIMEOUT_MS;
  if (!id || expired) {
    id = generateVisitorId();
    safeStorageSet(SESSION_ID_STORAGE_KEY, id);
  }
  safeStorageSet(SESSION_LAST_ACTIVITY_STORAGE_KEY, String(now));
  return id;
}