"use client";

import { useEffect } from "react";

const SELECTOR = "[data-reveal], [data-reveal-group]";

/**
 * Marks `[data-reveal]` / `[data-reveal-group]` elements with `data-in` as they scroll into view.
 * Render it as the last child of a page: its effect then runs only after the page's markup has
 * hydrated, so touching those elements can't cause a hydration mismatch.
 */
export default function RevealObserver() {
  useEffect(() => {
    const io = new IntersectionObserver(
      (entries) => {
        for (const entry of entries) {
          if (entry.isIntersecting) {
            (entry.target as HTMLElement).dataset.in = "";
            io.unobserve(entry.target);
          }
        }
      },
      { rootMargin: "0px 0px -12% 0px", threshold: 0.12 },
    );
    const watch = (root: ParentNode) =>
      root.querySelectorAll<HTMLElement>(SELECTOR).forEach((el) => {
        if (!("in" in el.dataset)) io.observe(el);
      });

    watch(document);
    const mo = new MutationObserver((records) => {
      for (const r of records) {
        r.addedNodes.forEach((n) => {
          if (n instanceof HTMLElement) {
            if (n.matches(SELECTOR) && !("in" in n.dataset)) io.observe(n);
            watch(n);
          }
        });
      }
    });
    mo.observe(document.body, { childList: true, subtree: true });

    return () => {
      io.disconnect();
      mo.disconnect();
    };
  }, []);

  return null;
}