/**
 * Glue between the document store (one open document) and where documents
 * live:
 *
 * - **local** documents: in this browser's IndexedDB (src/io/library.ts),
 *   URL `/?doc=<id>`. Without an account, that's all there is.
 * - **server** documents: on the numolus server (src/io/api.ts), URL
 *   `/d/<id>`. With an account, new documents are created there and local
 *   ones can be uploaded; they can be shared. A share link
 *   (`/d/<id>#k=<token>`) also opens a server document without an account --
 *   the token is kept in this browser (localStorage) and sent along.
 *
 * Persistence: the store's debounced save lands in `persist()` and goes to
 * IndexedDB or to the server. Server documents have a single-editor lock
 * (until live collaboration exists): whoever holds it edits, everyone else
 * sees the document read-only and follows its changes by polling.
 *
 * Other tabs: a BroadcastChannel keeps lists current and lets a local
 * document open in two tabs follow the other tab's saves.
 */

import { computed, reactive } from 'vue';
import { useDocument, setPersistHandler, setReadOnlyNotifier, contentFingerprint } from './document';
import type { DocumentState } from './types';
import { DocumentLibrary, newDocId, type DocMeta } from '../io/library';
import { api, ApiError, type Role, type ServerDoc, type ServerDocMeta } from '../io/api';
import { validateDocument } from '../io/json';
import { createBlankDocument, createSeedDocument } from '../data/seed';
import { useToasts } from '../composables/useToasts';
import { useSession } from './session';

/** The single-document localStorage keys used before the library existed --
 *  read once and moved into the library as its first document. */
const LEGACY_KEYS = ['numolus:doc:v2', 'numbers-study:doc:v2'];
/** share-link tokens this browser holds without an account */
const LINKS_KEY = 'numolus:links';
const CHANNEL = 'numolus:library';
const DEFAULT_TITLE = 'Unbenannt';
const APP_TITLE = 'numolus';
const UPLOAD_DELAY = 800; // ms after the last change
const HEARTBEAT = 30_000; // refresh the edit lock (server TTL is 90 s)
const POLL = 10_000; // read-only: look for someone else's changes

export type DocSource = 'local' | 'server';
export interface DocRef {
  source: DocSource;
  id: string;
}
export interface LinkDoc {
  id: string;
  token: string;
  title: string;
  role: Role;
}

type Msg =
  | { type: 'list' }
  | { type: 'saved'; id: string }
  | { type: 'deleted'; id: string }
  | { type: 'hello'; id: string }
  | { type: 'here'; id: string };

function metaOf(d: ServerDoc): ServerDocMeta {
  const { id, title, version, role, owner_name, updated_at, updated_by_name } = d;
  return { id, title, version, role, owner_name, updated_at, updated_by_name };
}

function readLinks(): LinkDoc[] {
  try {
    return JSON.parse(localStorage.getItem(LINKS_KEY) || '[]') as LinkDoc[];
  } catch {
    return [];
  }
}
function writeLinks(links: LinkDoc[]) {
  try {
    localStorage.setItem(LINKS_KEY, JSON.stringify(links));
  } catch {
    /* private mode -- the link itself still works */
  }
}

function createLibrary() {
  const store = useDocument();
  const toasts = useToasts();
  const session = useSession();

  const state = reactive({
    ready: false,
    /** false = IndexedDB unavailable (private mode in some browsers, ...) */
    available: true,
    /** local documents (IndexedDB) */
    docs: [] as DocMeta[],
    /** server documents of the signed-in user: own + shared with them */
    serverDocs: [] as ServerDocMeta[],
    /** documents opened via share link without an account */
    linkDocs: readLinks(),
    current: null as DocRef | null,
    /** the open server document's metadata (title, role, version, ...) */
    server: null as ServerDocMeta | null,
    /** someone else holds the edit lock on the open server document */
    lockedBy: null as string | null,
    /** server saves waiting or failing */
    syncState: 'idle' as 'idle' | 'saving' | 'offline',
  });

  const currentTitle = computed(() => {
    const c = state.current;
    if (!c) return DEFAULT_TITLE;
    if (c.source === 'server') return state.server?.title ?? DEFAULT_TITLE;
    return state.docs.find((d) => d.id === c.id)?.title ?? DEFAULT_TITLE;
  });
  /** role in the open document; local documents are always the user's own */
  const currentRole = computed<Role>(() =>
    state.current?.source === 'server' ? (state.server?.role ?? 'viewer') : 'owner',
  );
  const canEdit = computed(() => currentRole.value !== 'viewer' && !state.lockedBy);

  let lib: DocumentLibrary | null = null;
  let channel: BroadcastChannel | null = null;
  /** content fingerprint of what's saved for the open document */
  let savedContent = '';
  /** `updatedAt` of the open local document as this tab last wrote or read it */
  let knownUpdatedAt = 0;
  /** local writes run one after another, in order */
  let writeQueue: Promise<void> = Promise.resolve();
  const warnedShared = new Set<string>();

  // server document state
  let uploadTimer: ReturnType<typeof setTimeout> | undefined;
  let uploading: Promise<void> | null = null;
  let pendingJson: string | null = null;
  let pendingFp = '';
  let holdsLock = false;
  let heartbeatTimer: ReturnType<typeof setInterval> | undefined;
  let pollTimer: ReturnType<typeof setInterval> | undefined;

  const post = (m: Msg) => channel?.postMessage(m);
  const enqueue = (job: () => Promise<void>) => {
    writeQueue = writeQueue.then(job).catch((err) => {
      console.error(err);
      toasts.error(`Speichern fehlgeschlagen: ${(err as Error).message}`);
    });
    return writeQueue;
  };
  const shareToken = (id: string) => state.linkDocs.find((l) => l.id === id)?.token ?? null;
  const isCurrent = (ref: DocRef) => state.current?.source === ref.source && state.current.id === ref.id;

  async function refreshList() {
    if (lib) state.docs = await lib.list();
  }
  async function refreshServerList() {
    if (!session.signedIn.value) return;
    try {
      state.serverDocs = (await api<{ documents: ServerDocMeta[] }>('GET', '/documents')).documents;
    } catch (err) {
      console.error(err);
    }
  }

  function uniqueTitle(base: string, taken: string[]): string {
    const set = new Set(taken.map((t) => t.toLowerCase()));
    if (!set.has(base.toLowerCase())) return base;
    for (let n = 2; ; n++) {
      const t = `${base} ${n}`;
      if (!set.has(t.toLowerCase())) return t;
    }
  }

  function setUrl(ref: DocRef, push: boolean) {
    const url = new URL(location.href);
    url.hash = '';
    if (ref.source === 'server') {
      url.pathname = `/d/${ref.id}`;
      url.searchParams.delete('doc');
    } else {
      url.pathname = '/';
      url.searchParams.set('doc', ref.id);
    }
    if (url.href === location.href) return;
    if (push) history.pushState(null, '', url);
    else history.replaceState(null, '', url);
  }

  function updateWindowTitle() {
    document.title = `${currentTitle.value} – ${APP_TITLE}`;
  }

  function applyReadOnly() {
    store.ui.readOnly =
      state.current?.source !== 'server'
        ? null
        : currentRole.value === 'viewer'
          ? 'Nur Lesen – du kannst dieses Dokument ansehen, aber nicht bearbeiten.'
          : state.lockedBy
            ? `${state.lockedBy} bearbeitet gerade – nur Lesen, bis die Bearbeitung beendet ist.`
            : null;
  }

  /** Put `next` into the store as the open document. */
  function show(ref: DocRef, next: DocumentState, keepView = false) {
    if (keepView && next.sheets.some((s) => s.id === store.doc.activeSheetId)) {
      // someone else's save: take the content, keep this tab's cursor/sheet
      next.activeSheetId = store.doc.activeSheetId;
      next.activeObjectId = store.doc.activeObjectId;
      next.selection = store.doc.selection;
    }
    state.current = ref;
    store.replaceDocument(next);
    // it came from where it's saved -- writing it straight back is pointless
    store.cancelSave();
    savedContent = contentFingerprint(store.doc);
    updateWindowTitle();
  }

  // ---- persistence -----------------------------------------------------
  function persist(doc: DocumentState) {
    const c = state.current;
    if (!c) return;
    if (c.source === 'server') return persistServer(doc);
    if (!lib) return;
    const id = c.id;
    const json = JSON.stringify(doc);
    const fp = contentFingerprint(doc);
    const changed = fp !== savedContent;
    savedContent = fp;
    void enqueue(async () => {
      const meta = (await lib!.getMeta(id)) ?? {
        id,
        title: currentTitle.value,
        createdAt: Date.now(),
        updatedAt: Date.now(),
      };
      if (!changed) {
        // only the cursor/active sheet moved: saved so reopening restores
        // it, but never over newer content another tab has written meanwhile
        if (meta.updatedAt !== knownUpdatedAt) return;
        await lib!.put(meta, json);
        return;
      }
      const updatedAt = Math.max(Date.now(), meta.updatedAt + 1);
      await lib!.put({ ...meta, updatedAt }, json);
      if (isCurrent({ source: 'local', id })) knownUpdatedAt = updatedAt;
      await refreshList();
      post({ type: 'saved', id });
    });
  }

  function persistServer(doc: DocumentState) {
    if (!canEdit.value) return;
    const fp = contentFingerprint(doc);
    if (fp === savedContent && pendingJson === null) return; // cursor move only
    pendingJson = JSON.stringify(doc);
    pendingFp = fp;
    clearTimeout(uploadTimer);
    uploadTimer = setTimeout(() => void upload(), UPLOAD_DELAY);
  }

  /** Send the pending change of the open server document (one at a time). */
  async function upload(): Promise<void> {
    clearTimeout(uploadTimer);
    uploadTimer = undefined;
    if (uploading) {
      await uploading;
      if (pendingJson !== null) return upload();
      return;
    }
    const c = state.current;
    const meta = state.server;
    if (!c || c.source !== 'server' || !meta || pendingJson === null) return;
    const json = pendingJson;
    const fp = pendingFp;
    pendingJson = null;
    state.syncState = 'saving';
    uploading = (async () => {
      try {
        const r = await api<{ document: ServerDocMeta }>('PATCH', `/documents/${c.id}`, {
          body: { content: json, base_version: meta.version },
          shareToken: shareToken(c.id),
        });
        if (isCurrent(c)) {
          state.server = { ...meta, ...r.document, role: meta.role };
          savedContent = fp;
        }
        state.syncState = 'idle';
      } catch (err) {
        await uploadFailed(c, err, json, fp);
      } finally {
        uploading = null;
      }
    })();
    await uploading;
    if (pendingJson !== null && isCurrent(c)) return upload();
  }

  async function uploadFailed(c: DocRef, err: unknown, json: string, fp: string) {
    if (!(err instanceof ApiError) || err.status === 0 || err.status >= 500) {
      // offline / server trouble: keep it and try again
      state.syncState = 'offline';
      if (pendingJson === null) {
        pendingJson = json;
        pendingFp = fp;
      }
      clearTimeout(uploadTimer);
      uploadTimer = setTimeout(() => void upload(), 5000);
      return;
    }
    state.syncState = 'idle';
    // the server's version wins; edits made meanwhile would be based on the
    // rejected state, so they go too
    pendingJson = null;
    clearTimeout(uploadTimer);
    if (err.status === 423) {
      holdsLock = false;
      state.lockedBy = (err.body.holder_name as string) || 'Jemand anderes';
      toasts.error(`${state.lockedBy} bearbeitet dieses Dokument inzwischen – deine letzte Änderung wurde nicht gespeichert.`);
    } else if (err.status === 409) {
      toasts.error('Das Dokument wurde inzwischen anderswo geändert – deine letzte Änderung wurde nicht gespeichert.');
    } else {
      toasts.error(`Speichern fehlgeschlagen: ${err.message}`);
    }
    if (isCurrent(c)) await reloadServer(c.id, true);
    startServerTimers();
  }

  /** Before leaving the open server document: send what's pending. */
  async function flushServer() {
    store.flushSave();
    if (pendingJson !== null || uploading) await upload();
  }

  // ---- edit lock + polling ---------------------------------------------
  function stopServerTimers() {
    clearInterval(heartbeatTimer);
    clearInterval(pollTimer);
    heartbeatTimer = pollTimer = undefined;
  }

  function startServerTimers() {
    stopServerTimers();
    if (state.current?.source !== 'server') return;
    if (holdsLock) heartbeatTimer = setInterval(() => void heartbeat(), HEARTBEAT);
    else pollTimer = setInterval(() => void poll(), POLL);
  }

  /** Try to take the edit lock of the open server document. */
  async function takeLock(id: string): Promise<boolean> {
    if (currentRole.value === 'viewer') return false;
    try {
      const r = await api<{ lock: { mine: boolean; holder_name: string | null } }>('POST', `/documents/${id}/lock`, {
        shareToken: shareToken(id),
      });
      holdsLock = r.lock.mine;
      state.lockedBy = r.lock.mine ? null : r.lock.holder_name || 'Jemand anderes';
    } catch {
      holdsLock = false;
    }
    applyReadOnly();
    return holdsLock;
  }

  async function heartbeat() {
    const c = state.current;
    if (c?.source !== 'server') return;
    const had = holdsLock;
    await takeLock(c.id);
    if (had && !holdsLock) {
      toasts.error(`${state.lockedBy ?? 'Jemand anderes'} bearbeitet dieses Dokument jetzt.`);
      startServerTimers();
    }
  }

  async function poll() {
    const c = state.current;
    if (c?.source !== 'server' || document.visibilityState === 'hidden') return;
    if (currentRole.value !== 'viewer' && (await takeLock(c.id))) {
      await reloadServer(c.id, true);
      toasts.info('Du kannst das Dokument jetzt bearbeiten.');
      startServerTimers();
      return;
    }
    await reloadServer(c.id, true);
  }

  function releaseLock(id: string) {
    if (!holdsLock) return;
    holdsLock = false;
    void api('DELETE', `/documents/${id}/lock`, { shareToken: shareToken(id), keepalive: true }).catch(() => {});
  }

  /** Fetch the server document again; with `keepView` only when it changed. */
  async function reloadServer(id: string, keepView: boolean) {
    const known = keepView ? state.server?.version : undefined;
    try {
      const q = known ? `?known_version=${known}` : '';
      const r = await api<{ document: ServerDoc; unchanged?: boolean }>('GET', `/documents/${id}${q}`, {
        shareToken: shareToken(id),
      });
      if (!isCurrent({ source: 'server', id })) return;
      const { content } = r.document;
      state.server = metaOf(r.document);
      if (!r.unchanged && content !== undefined) {
        show({ source: 'server', id }, validateDocument(JSON.parse(content)), keepView);
      }
      updateWindowTitle();
      applyReadOnly();
    } catch (err) {
      if (err instanceof ApiError && err.status === 404) {
        toasts.error('Kein Zugriff mehr auf dieses Dokument.');
        await openFallback();
      }
    }
  }

  // ---- leaving / opening -----------------------------------------------
  async function leaveCurrent() {
    const c = state.current;
    if (!c) return;
    if (c.source === 'server') {
      await flushServer();
      stopServerTimers();
      releaseLock(c.id);
      state.lockedBy = null;
      state.server = null;
    } else {
      store.flushSave();
      await writeQueue;
    }
  }

  async function openLocal(id: string, opts: { push?: boolean } = {}): Promise<boolean> {
    if (!lib) return false;
    if (isCurrent({ source: 'local', id })) return true;
    const [meta, json] = await Promise.all([lib.getMeta(id), lib.getContent(id)]);
    if (!meta || json === undefined) return false;
    let next: DocumentState;
    try {
      next = validateDocument(JSON.parse(json));
    } catch (err) {
      toasts.error(`Dokument konnte nicht geladen werden: ${(err as Error).message}`);
      return false;
    }
    await leaveCurrent();
    show({ source: 'local', id }, next);
    knownUpdatedAt = meta.updatedAt;
    applyReadOnly();
    setUrl({ source: 'local', id }, opts.push ?? true);
    post({ type: 'hello', id });
    return true;
  }

  async function openServer(id: string, opts: { push?: boolean } = {}): Promise<boolean> {
    if (isCurrent({ source: 'server', id })) return true;
    let r: { document: ServerDoc };
    try {
      r = await api<{ document: ServerDoc }>('GET', `/documents/${id}`, { shareToken: shareToken(id) });
    } catch (err) {
      toasts.error(
        err instanceof ApiError && err.status === 404
          ? 'Dieses Dokument gibt es nicht, oder du hast keinen Zugriff darauf.'
          : `Dokument konnte nicht geladen werden: ${(err as Error).message}`,
      );
      return false;
    }
    const { content } = r.document;
    const meta = metaOf(r.document);
    let next: DocumentState;
    try {
      next = validateDocument(JSON.parse(content ?? ''));
    } catch (err) {
      toasts.error(`Dokument konnte nicht geladen werden: ${(err as Error).message}`);
      return false;
    }
    await leaveCurrent();
    state.server = meta;
    state.lockedBy = null;
    holdsLock = false;
    show({ source: 'server', id }, next);
    // remember the title for link-only documents
    const link = state.linkDocs.find((l) => l.id === id);
    if (link && (link.title !== meta.title || link.role !== meta.role)) {
      Object.assign(link, { title: meta.title, role: meta.role });
      writeLinks(state.linkDocs);
    }
    applyReadOnly();
    if (meta.role !== 'viewer') await takeLock(id);
    startServerTimers();
    setUrl({ source: 'server', id }, opts.push ?? true);
    return true;
  }

  function open(ref: DocRef, opts: { push?: boolean } = {}) {
    return ref.source === 'server' ? openServer(ref.id, opts) : openLocal(ref.id, opts);
  }

  // ---- creating --------------------------------------------------------
  /** Add a document and open it: on the server when signed in, else locally. */
  async function create(doc: DocumentState, title: string): Promise<DocRef | null> {
    const t = title.trim() || DEFAULT_TITLE;
    if (session.signedIn.value) {
      try {
        const r = await api<{ document: ServerDocMeta }>('POST', '/documents', {
          body: { title: uniqueTitle(t, state.serverDocs.map((d) => d.title)), content: JSON.stringify(doc) },
        });
        await refreshServerList();
        post({ type: 'list' });
        await openServer(r.document.id);
        return { source: 'server', id: r.document.id };
      } catch (err) {
        toasts.error(`Anlegen auf dem Server fehlgeschlagen: ${(err as Error).message}`);
        return null;
      }
    }
    if (!lib) {
      store.replaceDocument(doc); // no storage at all: at least show it
      return null;
    }
    const id = newDocId();
    const now = Date.now();
    await enqueue(() =>
      lib!.put({ id, title: uniqueTitle(t, state.docs.map((d) => d.title)), createdAt: now, updatedAt: now }, JSON.stringify(doc)),
    );
    await refreshList();
    post({ type: 'list' });
    await openLocal(id);
    return { source: 'local', id };
  }

  const createBlank = () => create(createBlankDocument(), DEFAULT_TITLE);
  const createSample = () => create(createSeedDocument(), 'Muster · Umsatzanalyse');

  // ---- managing ----------------------------------------------------------
  async function rename(ref: DocRef, title: string) {
    const t = title.trim();
    if (!t) return;
    if (ref.source === 'server') {
      try {
        const r = await api<{ document: ServerDocMeta }>('PATCH', `/documents/${ref.id}`, {
          body: { title: t },
          shareToken: shareToken(ref.id),
        });
        if (isCurrent(ref) && state.server) state.server = { ...state.server, title: r.document.title };
        await refreshServerList();
      } catch (err) {
        toasts.error(`Umbenennen fehlgeschlagen: ${(err as Error).message}`);
      }
    } else {
      if (!lib) return;
      await enqueue(() => lib!.patchMeta(ref.id, { title: t }));
      await refreshList();
    }
    if (isCurrent(ref)) updateWindowTitle();
    post({ type: 'list' });
  }

  async function contentOf(ref: DocRef): Promise<{ title: string; json: string } | null> {
    if (isCurrent(ref)) {
      if (ref.source === 'server') await flushServer();
      else {
        store.flushSave();
        await writeQueue;
      }
    }
    if (ref.source === 'server') {
      const r = await api<{ document: ServerDoc }>('GET', `/documents/${ref.id}`, { shareToken: shareToken(ref.id) });
      return r.document.content === undefined ? null : { title: r.document.title, json: r.document.content };
    }
    if (!lib) return null;
    const [meta, json] = await Promise.all([lib.getMeta(ref.id), lib.getContent(ref.id)]);
    return meta && json !== undefined ? { title: meta.title, json } : null;
  }

  /** A copy next to the original (server copies go to the own account). */
  async function duplicate(ref: DocRef) {
    try {
      const src = await contentOf(ref);
      if (!src) return;
      const title = `${src.title} (Kopie)`;
      if (ref.source === 'server') {
        await api('POST', '/documents', {
          body: { title: uniqueTitle(title, state.serverDocs.map((d) => d.title)), content: src.json },
        });
        await refreshServerList();
      } else if (lib) {
        const now = Date.now();
        await enqueue(() =>
          lib!.put(
            { id: newDocId(), title: uniqueTitle(title, state.docs.map((d) => d.title)), createdAt: now, updatedAt: now },
            src.json,
          ),
        );
        await refreshList();
      }
      post({ type: 'list' });
    } catch (err) {
      toasts.error(`Duplizieren fehlgeschlagen: ${(err as Error).message}`);
    }
  }

  /** Delete (own / local) or leave (shared with me / link). */
  async function remove(ref: DocRef) {
    if (ref.source === 'server') {
      const wasCurrent = isCurrent(ref);
      if (wasCurrent) await leaveCurrent();
      if (session.signedIn.value) {
        try {
          await api('DELETE', `/documents/${ref.id}`);
        } catch (err) {
          if (!(err instanceof ApiError && err.status === 404)) {
            toasts.error(`Löschen fehlgeschlagen: ${(err as Error).message}`);
            return;
          }
        }
      }
      state.linkDocs = state.linkDocs.filter((l) => l.id !== ref.id);
      writeLinks(state.linkDocs);
      await refreshServerList();
      post({ type: 'list' });
      if (wasCurrent) {
        state.current = null;
        await openFallback();
      }
      return;
    }
    if (!lib) return;
    await enqueue(() => lib!.remove(ref.id));
    await refreshList();
    post({ type: 'deleted', id: ref.id });
    if (isCurrent(ref)) {
      state.current = null;
      await openFallback();
    }
  }

  /**
   * Move local documents into the account (server). Each one is removed from
   * this browser once it's safely on the server; an open one is reopened
   * from there. Returns the server id of each uploaded document.
   */
  async function upload_(localIds: string[]): Promise<Map<string, string>> {
    const done = new Map<string, string>();
    if (!lib || !session.signedIn.value) return done;
    for (const id of localIds) {
      try {
        const src = await contentOf({ source: 'local', id });
        if (!src) continue;
        const r = await api<{ document: ServerDocMeta }>('POST', '/documents', {
          body: { title: src.title, content: src.json },
        });
        done.set(id, r.document.id);
        const wasCurrent = isCurrent({ source: 'local', id });
        if (wasCurrent) await openServer(r.document.id, { push: false });
        await enqueue(() => lib!.remove(id));
        post({ type: 'deleted', id });
      } catch (err) {
        toasts.error(`Hochladen fehlgeschlagen: ${(err as Error).message}`);
      }
    }
    await Promise.all([refreshList(), refreshServerList()]);
    post({ type: 'list' });
    return done;
  }

  /** Most recently changed document, or a new blank one. */
  async function openFallback() {
    state.current = null;
    const server = state.serverDocs[0];
    const local = state.docs[0];
    const serverNewer = server && (!local || Date.parse(server.updated_at) > local.updatedAt);
    if (serverNewer && (await openServer(server.id, { push: false }))) return;
    if (local && (await openLocal(local.id, { push: false }))) return;
    await createBlank();
  }

  // ---- share links -------------------------------------------------------
  /** Redeem `#k=<token>` for document `id` (from the URL). */
  async function joinByLink(id: string, token: string) {
    try {
      const r = await api<{ document: ServerDocMeta }>('POST', `/documents/${id}/join`, { body: { token } });
      if (session.signedIn.value) {
        await refreshServerList(); // it's in the account now
      } else {
        state.linkDocs = [
          { id, token, title: r.document.title, role: r.document.role },
          ...state.linkDocs.filter((l) => l.id !== id),
        ];
        writeLinks(state.linkDocs);
      }
    } catch {
      toasts.error('Dieser Link ist nicht (mehr) gültig.');
    }
  }

  /** After signing in: documents opened via link become part of the account. */
  async function adoptLinkDocs() {
    if (!session.signedIn.value || !state.linkDocs.length) return;
    for (const l of state.linkDocs) {
      await api('POST', `/documents/${l.id}/join`, { body: { token: l.token } }).catch(() => {});
    }
    state.linkDocs = [];
    writeLinks([]);
  }

  // ---- other tabs --------------------------------------------------------
  async function onMessage(m: Msg) {
    if (m.type === 'list') {
      await Promise.all([refreshList(), refreshServerList()]);
      state.linkDocs = readLinks();
      updateWindowTitle();
      return;
    }
    const cur = state.current?.source === 'local' ? state.current.id : null;
    if (m.type === 'saved') {
      await refreshList();
      // our own unsaved edit wins (it's written right after); otherwise
      // follow -- a pending save that's only a cursor move doesn't count
      const localEdit = store.hasPendingSave() && contentFingerprint(store.doc) !== savedContent;
      if (m.id === cur && !localEdit && lib) {
        const [meta, json] = await Promise.all([lib.getMeta(m.id), lib.getContent(m.id)]);
        if (meta && json !== undefined && isCurrent({ source: 'local', id: m.id })) {
          show({ source: 'local', id: m.id }, validateDocument(JSON.parse(json)), true);
          knownUpdatedAt = meta.updatedAt;
        }
      }
    } else if (m.type === 'deleted') {
      await refreshList();
      if (m.id === cur) {
        toasts.info('Dieses Dokument ist nicht mehr in diesem Browser (gelöscht oder hochgeladen).');
        state.current = null;
        await refreshServerList();
        await openFallback();
      }
    } else if (m.id === cur) {
      // hello / here: the same local document is open in another tab
      if (m.type === 'hello') post({ type: 'here', id: m.id });
      if (!warnedShared.has(m.id)) {
        warnedShared.add(m.id);
        toasts.info(
          'Dieses Dokument ist auch in einem anderen Tab geöffnet. Gleichzeitige Änderungen können sich überschreiben.',
        );
      }
    }
  }

  // ---- start -------------------------------------------------------------
  async function migrateLegacy() {
    for (const key of LEGACY_KEYS) {
      let raw: string | null = null;
      try {
        raw = localStorage.getItem(key);
      } catch {
        return;
      }
      if (!raw) continue;
      try {
        const doc = validateDocument(JSON.parse(raw));
        const now = Date.now();
        await lib!.put({ id: newDocId(), title: 'Mein Dokument', createdAt: now, updatedAt: now }, JSON.stringify(doc));
      } catch {
        continue; // corrupt -- leave it where it is
      }
      for (const k of LEGACY_KEYS) localStorage.removeItem(k);
      return;
    }
  }

  /** What the URL asks for: `/d/<id>[#k=<token>]` or `/?doc=<id>`. */
  function refFromUrl(): { ref: DocRef | null; token: string | null } {
    const m = location.pathname.match(/^\/d\/([0-9a-f-]{36})\/?$/i);
    if (m) {
      const token = new URLSearchParams(location.hash.slice(1)).get('k');
      return { ref: { source: 'server', id: m[1] }, token };
    }
    const id = new URL(location.href).searchParams.get('doc');
    return { ref: id ? { source: 'local', id } : null, token: null };
  }

  async function init() {
    setReadOnlyNotifier((why) => toasts.info(why));
    try {
      lib = await DocumentLibrary.open();
      await migrateLegacy();
      await refreshList();
    } catch (err) {
      console.error(err);
      lib = null;
      state.available = false;
      if (!session.signedIn.value) {
        toasts.error('Dokumente können in diesem Browser nicht gespeichert werden (IndexedDB nicht verfügbar).');
      }
    }
    await adoptLinkDocs();
    await refreshServerList();

    if (typeof BroadcastChannel !== 'undefined') {
      channel = new BroadcastChannel(CHANNEL);
      channel.onmessage = (e: MessageEvent<Msg>) => void onMessage(e.data);
    }
    setPersistHandler(persist);

    const { ref, token } = refFromUrl();
    if (ref?.source === 'server' && token) await joinByLink(ref.id, token);
    if (ref && !(await open(ref, { push: false })) && ref.source === 'local') {
      toasts.error('Das angeforderte Dokument gibt es in diesem Browser nicht.');
    }
    if (!state.current) await openFallback();

    window.addEventListener('popstate', () => {
      const { ref: r } = refFromUrl();
      if (r && !isCurrent(r)) void open(r, { push: false });
    });
    // don't lose the last moments of typing when the tab goes away
    window.addEventListener('pagehide', () => {
      store.flushSave();
      if (state.current?.source === 'server') {
        if (pendingJson !== null) void upload();
        releaseLock(state.current.id);
      }
    });
    window.addEventListener('beforeunload', (e) => {
      if (pendingJson !== null || uploading) e.preventDefault(); // unsaved server changes
    });
    document.addEventListener('visibilitychange', () => {
      if (document.visibilityState === 'hidden') store.flushSave();
    });
    state.ready = true;
  }

  return {
    state,
    currentTitle,
    currentRole,
    canEdit,
    init,
    open,
    create,
    createBlank,
    createSample,
    rename,
    duplicate,
    remove,
    upload: upload_,
    refreshServerList,
  };
}

let instance: ReturnType<typeof createLibrary> | null = null;

export function useLibrary() {
  if (!instance) instance = createLibrary();
  return instance;
}