"use client";

import { useCallback, useMemo, useSyncExternalStore } from "react";
import { parseReaderPlace, readerPlaceUrl, READER_VIEWS, type ReaderPlace, type ReaderView } from "../../../lib/report/slides";

// THE READER'S PLACE, FROM THE URL — which view (`?rv=`) and where in it (the
// hash), as lib/report/slides.ts parses it; the export site's reader and
// umtool's both read and write it here. A switch pushes an entry (Back undoes
// it); a step within a view replaces it. Back and a hash change re-read it.

const listeners = new Set<() => void>();
function subscribe(cb: () => void) {
  listeners.add(cb);
  window.addEventListener("popstate", cb);
  window.addEventListener("hashchange", cb);
  return () => {
    listeners.delete(cb);
    window.removeEventListener("popstate", cb);
    window.removeEventListener("hashchange", cb);
  };
}
const snapshot = () => `${window.location.search}${window.location.hash}`;
const serverSnapshot = () => "";

export type PlaceLike = { view: ReaderView; slide?: number | null; anchor?: string | null };

export function readerUrl(place: PlaceLike): string {
  return readerPlaceUrl(window.location.pathname, window.location.search, place);
}

export function goToPlace(place: PlaceLike, push: boolean): void {
  const url = readerUrl(place);
  const here = `${window.location.pathname}${window.location.search}${window.location.hash}`;
  if (url === here) return;
  window.history[push ? "pushState" : "replaceState"](window.history.state, "", url);
  for (const l of listeners) l();
}

// The place in force; a view not offered (`views`) reads as the article.
export function useReaderPlace(views: readonly ReaderView[] = READER_VIEWS): {
  place: ReaderPlace;
  go: (place: PlaceLike, push: boolean) => void;
} {
  const url = useSyncExternalStore(subscribe, snapshot, serverSnapshot);
  const place = useMemo<ReaderPlace>(() => {
    const at = url.indexOf("#");
    const p = parseReaderPlace(at < 0 ? url : url.slice(0, at), at < 0 ? "" : url.slice(at));
    return views.includes(p.view) ? p : { view: "article", slide: null, anchor: p.anchor };
  }, [url, views]);
  const go = useCallback((p: PlaceLike, push: boolean) => goToPlace(p, push), []);
  return { place, go };
}