// @ts-check
/**
 * Notebooks kept in the browser, for Kokkoja opened from a web site with
 * no local program on the computer.
 *
 * The notebooks follow the same rules as the directories the local program
 * keeps (`notebook.js`): a notebook holds `Current.svg`, the page being
 * written, and dated kept pages such as `2026_10_02_01.svg`.  They live in
 * the browser's IndexedDB for this site, which survives closing the
 * browser but belongs to this browser on this computer alone; the way to
 * take a page out is to download it.  The identity is kept in the
 * browser's localStorage in the same way.
 */

const DATABASE = 'kokkoja';
const PAGES = 'pages';
const IDENTITY_KEY = 'kokkoja_identity';
const CURRENT_PAGE = 'Current.svg';
const KEPT_PAGE = /^(\d{4}_\d{2}_\d{2})_(\d{2,})\.svg$/;

/**
 * @typedef {object} StoredPage
 * @property {string} notebook
 * @property {string} name
 * @property {string} text
 * @property {number} changed  When the page was last written, in milliseconds.
 */

/** @type {Promise<IDBDatabase> | null} */
let opening = null;

/** @returns {Promise<IDBDatabase>} */
function database() {
  opening ??= new Promise((resolve, reject) => {
    const request = indexedDB.open(DATABASE, 1);
    request.onupgradeneeded = () => {
      const store = request.result.createObjectStore(PAGES, { keyPath: ['notebook', 'name'] });
      store.createIndex('notebook', 'notebook');
    };
    request.onsuccess = () => resolve(request.result);
    request.onerror = () => reject(request.error ?? new Error('The browser’s storage cannot be opened.'));
  });
  return opening;
}

/**
 * @template T
 * @param {IDBRequest<T>} request
 * @returns {Promise<T>}
 */
function result(request) {
  return new Promise((resolve, reject) => {
    request.onsuccess = () => resolve(request.result);
    request.onerror = () => reject(request.error ?? new Error('The browser’s storage failed.'));
  });
}

/**
 * Run work in one transaction on the pages.
 * @template T
 * @param {IDBTransactionMode} mode
 * @param {(pages: IDBObjectStore) => Promise<T>} work
 * @returns {Promise<T>}
 */
async function transaction(mode, work) {
  const db = await database();
  const tx = db.transaction(PAGES, mode);
  const done = new Promise((resolve, reject) => {
    tx.oncomplete = () => resolve(undefined);
    tx.onerror = () => reject(tx.error ?? new Error('The browser’s storage failed.'));
    tx.onabort = () => reject(tx.error ?? new Error('The browser’s storage failed.'));
  });
  const value = await work(tx.objectStore(PAGES));
  await done;
  return value;
}

/**
 * @param {IDBObjectStore} pages
 * @param {string} notebook
 * @returns {Promise<StoredPage[]>}  The notebook's pages, by name.
 */
async function pagesOf(pages, notebook) {
  const found = /** @type {StoredPage[]} */ (await result(pages.index('notebook').getAll(notebook)));
  return found.sort((a, b) => (a.name < b.name ? -1 : a.name > b.name ? 1 : 0));
}

/**
 * @param {number} time  Milliseconds.
 * @returns {string}  The day, as 2026_10_02.
 */
function dayOf(time) {
  const date = new Date(time);
  return [date.getFullYear(), date.getMonth() + 1, date.getDate()]
    .map((part) => String(part).padStart(2, '0')).join('_');
}

/** @type {import('./notebook.js').Backend} */
export const backend = {
  async readIdentity() {
    return localStorage.getItem(IDENTITY_KEY);
  },

  async keepIdentity(text) {
    if (localStorage.getItem(IDENTITY_KEY) !== null) return false;
    localStorage.setItem(IDENTITY_KEY, text);
    return true;
  },

  async listNotebooks() {
    const names = await transaction('readonly', async (pages) => {
      const keys = /** @type {[string, string][]} */ (await result(pages.getAllKeys()));
      return [...new Set(keys.map(([notebook]) => notebook))].sort();
    });
    return { directory: '', notebooks: names };
  },

  async listPages(notebook) {
    return transaction('readonly', async (pages) => (await pagesOf(pages, notebook)).map((page) => page.name));
  },

  async readPage(notebook, pageName) {
    const page = /** @type {StoredPage | undefined} */ (await transaction(
      'readonly', (pages) => result(pages.get([notebook, pageName]))));
    return page ? page.text : null;
  },

  async writePage(notebook, pageName, text) {
    await transaction('readwrite', async (pages) => {
      await result(pages.put({ notebook, name: pageName, text, changed: Date.now() }));
    });
  },

  // The same rules as keep_page() in kokkoja.py: nothing is kept of a
  // blank page or of one already kept; the name is the day the page was
  // last changed and a serial number.
  async keepPage(notebook) {
    return transaction('readwrite', async (pages) => {
      const current = /** @type {StoredPage | undefined} */ (await result(pages.get([notebook, CURRENT_PAGE])));
      const blank = !['data-stroke=', 'data-text=', '<image'].some((mark) => current?.text.includes(mark));
      if (!current || blank) return { kept: null, new: false };
      const kept = (await pagesOf(pages, notebook)).filter((page) => KEPT_PAGE.test(page.name));
      const same = kept.find((page) => page.text === current.text);
      if (same) return { kept: same.name, new: false };
      const day = dayOf(current.changed);
      const serials = kept.map((page) => /** @type {RegExpExecArray} */ (KEPT_PAGE.exec(page.name)))
        .filter((match) => match[1] === day).map((match) => Number(match[2]));
      const name = `${day}_${String(Math.max(0, ...serials) + 1).padStart(2, '0')}.svg`;
      await result(pages.put({ notebook, name, text: current.text, changed: current.changed }));
      return { kept: name, new: true };
    });
  },
};