function init_toc_heading() {
  const headings = document.querySelectorAll("main > article:first-of-type h2");
  const toc_names = document.querySelectorAll("main > nav#toc li");

  const toc_mapped_links = new Map(
    Array.from(toc_names).map((name) => [
      name.children[0].hash.substring(1),
      name,
    ]),
  );

  const position_list = Array.from(headings).map((heading, idx, headings) => [
    heading.offsetTop + heading.offsetHeight,
    headings[idx + 1]?.offsetTop ?? Infinity,
    toc_mapped_links.get(heading.id),
  ]);

  const updateToc = (scrollingElement) => {
    const scrollTop = scrollingElement.scrollTop;
    const vptHeight = scrollingElement.clientHeight;

    const vptBgnOffset = scrollTop;
    const vptEndOffset = scrollTop + vptHeight;

    position_list.forEach(([eltBgnOffset, eltEndOffset, element]) => {
      if (eltBgnOffset < vptEndOffset && eltEndOffset > vptBgnOffset)
        element.classList.add("visible");
      else element.classList.remove("visible");
    });
  };

  document.addEventListener("scroll", () => {
    updateToc(document.scrollingElement);
  });

  window.addEventListener("resize", () => {
    updateToc(document.scrollingElement);
  });

  updateToc(document.scrollingElement);
}

init_toc_heading();