export interface ViewedArticle {
  /** the article read, by its identity — the canonical version's slug. An
   *  entry written before versions had addresses of their own may hold a
   *  document's own slug, which the reader still resolves. */
  slug: string
  title: string
  /** the language it was last read in, so a jump returns to that version.
   *  Absent from entries written when one address stood for the whole article */
  lang?: string
  viewedAt: number
}

const STORAGE_KEY = 'mathew-recently-viewed'
const MAX_ENTRIES = 8

/** where a jump leads: the version the article was last read in, or — for an
 *  entry that recorded no language — the short address, which the reader turns
 *  onto whichever version the slug speaks */
export function viewedHref(viewed: Pick<ViewedArticle, 'slug' | 'lang'>): string {
  return viewed.lang
    ? versionHref(viewed.slug, viewed.lang)
    : `/wiki/${encodeURIComponent(viewed.slug)}`
}

let loaded = false

function readStored(): ViewedArticle[] {
  try {
    const raw = localStorage.getItem(STORAGE_KEY)
    return raw ? (JSON.parse(raw) as ViewedArticle[]) : []
  } catch {
    /* ignore corrupt storage */
    return []
  }
}

/**
 * Tracks which articles the visitor has opened, in `localStorage`, so the
 * sidebar can offer a "recently viewed" list. Shared across pages via
 * `useState`, so it updates live as you browse without extra fetches.
 */
export function useRecentlyViewed() {
  const viewed = useState<ViewedArticle[]>('recently-viewed', () => [])

  /* read the saved history once, after mount, so the server render and the
     client hydration both start from an empty list */
  onMounted(() => {
    if (loaded) return
    loaded = true
    const stored = readStored()
    if (stored.length) viewed.value = stored
  })

  function record(article: {
    slug: string
    title: string
    baseSlug?: string | null
    language?: string
  }) {
    /* filed by the article's identity and turned to the language it was read
       in: an article read in two languages is still one article remembered
       once, at the version it was last opened at */
    const entry = {
      slug: identityOf(article),
      lang: articleLanguage(article),
      title: article.title,
    }
    /* merge with what's already saved so recording before the list has
       loaded (e.g. deep-linking straight to an article) can't clobber it */
    const base = viewed.value.length ? viewed.value : readStored()
    const fresh = { ...entry, viewedAt: Date.now() }
    viewed.value = [fresh, ...base.filter((v) => v.slug !== entry.slug)].slice(0, MAX_ENTRIES)
    loaded = true
    try {
      localStorage.setItem(STORAGE_KEY, JSON.stringify(viewed.value))
    } catch {
      /* private mode etc. — history just won't persist */
    }
  }

  return { viewed, record }
}