mathew/frontend/app/composables/useArticleTabs.ts

export interface ArticleTab {
  /** the article this page is of — its canonical slug, and the page's key */
  identity: string
  /** the language this page is turned to */
  lang: string
  /** the version in view, as the library files it: its stored slug */
  slug: string
  /** and the document behind it — which a retitle keeps, so the article's page
   *  follows it to its new address instead of a second page opening beside it */
  docId?: string
  title: string
}

const MAX_TABS = 10

/** The address a page of the book was read from: the version it is turned to. */
export function tabHref(tab: Pick<ArticleTab, 'identity' | 'lang'>): string {
  return versionHref(tab.identity, tab.lang)
}

/** Which page of the stack an address points at, if any. An address that
 *  names no language points at the article's page however it is turned. */
export function tabAt(tabs: readonly ArticleTab[], path: string): ArticleTab | null {
  const loc = parseReaderPath(path)
  if (!loc) return null
  /* one article is one page of the book, in whichever language that page is
     turned to — so the address names its page by the article alone */
  return tabs.find((t) => t.identity === loc.identity) ?? null
}

/**
 * The session stack of articles opened in the reader — the pages of the
 * book, in the order they were opened, newest at the right. One article is one
 * page however many languages it exists in, so turning an article to another
 * language turns that page over instead of adding another to the row. The page
 * currently in view is simply wherever the route points; the stack lives in
 * shared state and survives every client navigation (reset on reload), so the
 * open pages are still there when you come back to the reading surface.
 */
export function useArticleTabs() {
  const route = useRoute()
  const router = useRouter()
  const tabs = useState<ArticleTab[]>('article-tabs', () => [])
  /* the last page seen outside the reading surface, so closing the last tab
     returns there instead of always going home */
  const origin = useState<string>('article-tabs:origin', () => '')
  /* the article last in view, so the stack can be found again from any page
     that hides the reading surface — the editor, a new article, the graph */
  const lastIdentity = useState<string>('article-tabs:last', () => '')

  /**
   * Lay one version on the book: a fresh page at the right of the row, or —
   * when the article already has a page open — that same page turned to this
   * language. Reading an article in French is not opening a second article,
   * so the page keeps its place in the row and only its text changes.
   */
  function openVersion(article: {
    _id?: string
    slug: string
    title: string
    language?: string
    baseSlug?: string | null
  }) {
    const identity = identityOf(article)
    const lang = articleLanguage(article)
    /* the article's own page, or — failing that — the page holding this very
       document, which is how a retitle is recognised: the article moved to a
       new address and its page follows it rather than doubling up */
    const page = tabs.value.find(
      (t) => t.identity === identity || (!!article._id && t.docId === article._id)
    )
    if (page) {
      /* the page follows the version it holds: the article's identity, the
         document its language asks for, and the title that version carries (a
         save that retitled an article moves it and relabels the page; a
         language turn relabels it again and takes it to another document) */
      page.identity = identity
      page.docId = article._id
      page.slug = article.slug
      page.lang = lang
      page.title = article.title
      return
    }
    const stack = [
      ...tabs.value,
      {
        identity,
        lang,
        slug: article.slug,
        docId: article._id,
        title: article.title,
      },
    ]
    if (stack.length > MAX_TABS) {
      /* don't stack forever — retire the oldest page that isn't this one */
      const drop = stack.findIndex((t) => t.identity !== identity)
      if (drop !== -1) stack.splice(drop, 1)
    }
    tabs.value = stack
  }

  /* Close a page. Closing one that isn't in view just takes it out of the
     stack and the rest settle; closing the page in view turns back to the
     page before it — or returns to the last page outside the book once the
     last page is gone. */
  function closeTab(identity: string) {
    const at = tabs.value.findIndex((t) => t.identity === identity)
    const isActive = tabAt(tabs.value, route.path)?.identity === identity
    tabs.value = tabs.value.filter((t) => t.identity !== identity)
    if (isActive) {
      const back = tabs.value[Math.max(0, Math.min(at - 1, tabs.value.length - 1))]
      void router.push(back ? tabHref(back) : origin.value || '/')
    }
  }

  return { tabs, origin, lastIdentity, openVersion, closeTab }
}

/**
 * Keeps the book's stack of pages in step with the route: which page was last
 * in view, and the last plain page outside the reading surface. The stack
 * itself is left alone — it survives the whole session, so wandering off to
 * the graph or the article list and coming back finds the pages as they were.
 * Call once, high in the app.
 */
export function useArticleTabsSync() {
  const route = useRoute()
  const { tabs, lastIdentity, origin } = useArticleTabs()

  watch(
    () => route.fullPath,
    (to) => {
      /* filed under the article's identity, so a version turned to another
         language is still found as the same page of the book */
      if (isReaderPath(to))
        lastIdentity.value = tabAt(tabs.value, to)?.identity ?? wikiSlugFromPath(to) ?? ''
      else if (!isReadingSurface(to)) origin.value = to
    },
    { immediate: true }
  )
}