const { fetchJson, clampPage, stripTags } = require('./http');

const BASE = 'https://openstax.org';
const CATALOG_URL = `${BASE}/apps/cms/api/books`;
const DETAIL_BASE = `${BASE}/apps/cms/api/v2/pages`;
const PAGE_SIZE = 20;
const CATALOG_TTL = 30 * 60 * 1000;

let catalogPromise = null;
let catalogAt = 0;

function getJson(url) {
  return fetchJson(url);
}

async function loadCatalog() {
  const now = Date.now();
  if (catalogPromise && now - catalogAt < CATALOG_TTL) return catalogPromise;
  const promise = getJson(CATALOG_URL).then((j) => {
    if (!j || !Array.isArray(j.books)) throw new Error('OpenStax 返回了无法识别的书目');
    return j.books.filter((b) => b && b.id && b.book_state === 'live');
  });
  catalogPromise = promise;
  catalogAt = now;
  try {
    return await promise;
  } catch (e) {
    if (catalogPromise === promise) {
      catalogPromise = null;
      catalogAt = 0;
    }
    throw e;
  }
}

function validateId(postId) {
  const id = String(postId || '');
  if (!/^\d+$/.test(id)) throw new Error('无效的 OpenStax ID');
  return id;
}

function pageUrl(book) {
  const slug = String(book && book.slug || '').replace(/^\/+/, '');
  return slug ? `${BASE}/details/${slug}` : `${BASE}/subjects`;
}

function toItem(book) {
  return {
    postId: String(book.id),
    title: book.title || '(无标题)',
    cover: book.cover_url || '',
    date: '',
    url: pageUrl(book),
    subtitle: (book.subjects || []).slice(0, 3).join(' · ')
  };
}

function pack(books, page) {
  const start = (page - 1) * PAGE_SIZE;
  return {
    items: books.slice(start, start + PAGE_SIZE).map(toItem),
    maxPage: Math.max(1, Math.ceil(books.length / PAGE_SIZE)),
    page
  };
}

function matches(book, keyword) {
  const tokens = String(keyword || '').trim().toLocaleLowerCase().split(/\s+/).filter(Boolean);
  if (!tokens.length) return true;
  const text = [
    book.title,
    ...(book.subjects || []),
    ...(book.subject_categories || [])
  ].filter(Boolean).join(' ').toLocaleLowerCase();
  return tokens.every((token) => text.includes(token));
}

function authorsOf(book) {
  return (book.authors || []).map((author) => {
    if (typeof author === 'string') return author;
    return author && author.value && author.value.name
      ? author.value.name
      : (author && author.name) || '';
  }).filter(Boolean);
}

function subjectNames(value) {
  const items = Array.isArray(value) ? value : (value ? [value] : []);
  return items.map((item) => (
    typeof item === 'string' ? item : (item && (item.subject_name || item.name)) || ''
  )).filter(Boolean);
}

async function bookDetail(postId) {
  const id = validateId(postId);
  const book = await getJson(`${DETAIL_BASE}/${id}/`);
  if (!book || !book.id) throw new Error('未找到该 OpenStax 教材');
  return book;
}

function detailUrl(book) {
  return (book.meta && book.meta.html_url) || `${BASE}/details/books/${book.meta && book.meta.slug || book.id}`;
}

function licenseLabel(book) {
  return [book.license_name, book.license_version].filter(Boolean).join(' ');
}

function safeName(title) {
  return String(title || 'OpenStax 教材').replace(/[\\/:*?"<>|]/g, '_').slice(0, 80);
}

module.exports = {
  id: 'openstax',
  name: 'OpenStax 开放教材',
  supportsSearch: true,

  async list(page) {
    page = clampPage(page);
    return pack(await loadCatalog(), page);
  },

  async search(keyword, page) {
    page = clampPage(page);
    const books = (await loadCatalog()).filter((book) => matches(book, keyword));
    return pack(books, page);
  },

  async detail(postId) {
    const book = await bookDetail(postId);
    const subjects = [
      ...subjectNames(book.book_subjects),
      ...subjectNames(book.book_categories)
    ];
    return {
      postId: String(book.id),
      title: book.title || '(无标题)',
      cover: book.cover_url || '',
      authors: authorsOf(book),
      date: String(book.publish_date || '').slice(0, 10),
      tags: [
        ...subjects.slice(0, 4).map((subject) => `主题：${subject}`),
        licenseLabel(book) ? `许可：${licenseLabel(book)}` : '',
        book.digital_isbn_13 ? `ISBN：${book.digital_isbn_13}` : ''
      ].filter(Boolean),
      brief: stripTags(book.description || ''),
      url: detailUrl(book),
      links: [
        { name: 'OpenStax 页', url: detailUrl(book) },
        ...(book.webview_link || book.webview_rex_link
          ? [{ name: '在线阅读', url: book.webview_link || book.webview_rex_link }]
          : [])
      ]
    };
  },

  async download(postId) {
    const book = await bookDetail(postId);
    const files = [];
    const seen = new Set();
    for (const [url, label] of [
      [book.pdf_url, 'PDF'],
      [book.high_resolution_pdf_url, '高清 PDF']
    ]) {
      if (!url || seen.has(url)) continue;
      seen.add(url);
      files.push({
        name: `${safeName(book.title)}${label === '高清 PDF' ? '-高清' : ''}.pdf`,
        link: url,
        format: 'PDF'
      });
    }
    const links = [{ name: 'OpenStax 页', url: detailUrl(book) }];
    if (book.webview_link || book.webview_rex_link) {
      links.push({ name: '在线阅读', url: book.webview_link || book.webview_rex_link });
    }
    if (book.license_url) links.push({ name: '许可说明', url: book.license_url });
    return { files, links };
  }
};