/**
 * A tiny app-level cache of the metadata needed to open a previously-visited
 * page while offline (issue #38). It maps a page URL (pond slug + page slug) to
 * the ids the editor and collab layer need. This is persisted by the app in
 * localStorage — deliberately NOT by the service worker — so API responses are
 * never cached, yet a visited page can still be resolved with no network.
 *
 * The store is bounded to the most recently visited pages so it cannot grow
 * without limit; each entry is a few hundred bytes.
 */

const STORAGE_KEY = 'dorfteich:offline-pages';
const MAX_ENTRIES = 50;

export interface CachedPage {
  pondSlug: string;
  pondId: string;
  pageSlug: string;
  pageId: string;
  title: string;
  /** Last write time, for bounded recency-based eviction. */
  savedAt: number;
}

type Store = Record<string, CachedPage>;

function keyOf(pondSlug: string, pageSlug: string): string {
  return `${pondSlug}/${pageSlug}`;
}

function readStore(): Store {
  try {
    const raw = localStorage.getItem(STORAGE_KEY);
    return raw ? (JSON.parse(raw) as Store) : {};
  } catch {
    return {};
  }
}

function writeStore(store: Store): void {
  try {
    localStorage.setItem(STORAGE_KEY, JSON.stringify(store));
  } catch {
    // Best-effort: ignore quota errors or disabled storage.
  }
}

/** Record the metadata of a page loaded online, for later offline resolution. */
export function rememberPage(entry: Omit<CachedPage, 'savedAt'>): void {
  const store = readStore();
  store[keyOf(entry.pondSlug, entry.pageSlug)] = { ...entry, savedAt: Date.now() };

  const keys = Object.keys(store);
  if (keys.length > MAX_ENTRIES) {
    keys
      .sort((a, b) => (store[a]?.savedAt ?? 0) - (store[b]?.savedAt ?? 0))
      .slice(0, keys.length - MAX_ENTRIES)
      .forEach((key) => delete store[key]);
    writeStore(store);
  } else {
    writeStore(store);
  }
}

/** Look up a previously-visited page's metadata by its URL slugs. */
export function recallPage(pondSlug: string, pageSlug: string): CachedPage | null {
  return readStore()[keyOf(pondSlug, pageSlug)] ?? null;
}