/*
Copyright 2020, 2021 The Matrix.org Foundation C.I.C.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

/*
Account for id attributes that are in the sidebar nav
*/
function populateIds() {
  const navItems = document.querySelectorAll(".td-sidebar-nav li");
  return Array.from(navItems).map(item => item.id).filter(id => id != "");
}

/*
Given an ID and an array of IDs, return s version of the original ID that's
not equal to any of the IDs in the array.
*/
function uniquifyHeadingId(id, uniqueIDs) {
  const baseId = id;
  let counter = 0;
  while (uniqueIDs.includes(id)) {
    counter = counter + 1;
    id = baseId + "-" + counter.toString();
  }
  return id;
}

/*
Given an array of heading nodes, ensure they all have unique IDs.

We have to do this mostly because of client-server modules, which are
rendered separately then glued together with a template.
Because heading IDs are generated in rendering, this means they can and will
end up with duplicate IDs.
*/
function uniquifyHeadingIds(headings) {
  const uniqueIDs = populateIds();
  for (let heading of headings) {
    const uniqueID = uniquifyHeadingId(heading.id, uniqueIDs);
    uniqueIDs.push(uniqueID);
    heading.id = uniqueID;
  }
}

/*
The document contains "normal" headings, and these have corresponding items
in the ToC.

The document might also contain H1 headings that act as titles for blocks of
rendered data, like HTTP APIs or event schemas. Unlike "normal" headings,
these headings don't appear in the ToC. But they do have anchor IDs to enable
links to them. When someone follows a link to one of these "rendered data"
headings we want to scroll the ToC to the item corresponding to the "normal"
heading preceding the "rendered data" heading we have visited.

To support this we need to add `data` attributes to ToC items.
These attributes identify which "rendered data" headings live underneath
the heading corresponding to that ToC item.
*/
function setTocItemChildren(toc, headings) {
  let tocEntryForHeading  = null;
  for (const heading of headings) {
    // H1 headings are rendered-data headings
    if (heading.tagName !== "H1") {
      tocEntryForHeading = document.querySelector(`nav li a[href="#${heading.id}"]`);
    } else {
      // on the ToC entry for the parent heading,
      // set a data-* attribute whose name is the child's fragment ID
      tocEntryForHeading.setAttribute(`data-${heading.id}`, "true");
    }
  }
}

/*
Generate a table of contents based on the headings in the document.
*/
function makeToc() {

  // make the title from the H1
  const h1 = document.body.querySelector("h1");
  const title = document.createElement("a");
  title.id = "toc-title";
  title.setAttribute("href", "#");
  title.textContent = h1.textContent;

  // make the content
  const content = document.body.querySelector(".td-content");
  let headings = [].slice.call(content.querySelectorAll("h2, h3, h4, h5, h6, .rendered-data > details > summary > h1"));

  // exclude headings that don't have IDs.
  headings = headings.filter(heading => heading.id);
  uniquifyHeadingIds(headings);

  // exclude .rendered-data > h1 headings from the ToC
  const tocTargets = headings.filter(heading => heading.tagName !== "H1");

  // we have to adjust heading IDs to ensure that they are unique
  const nav = document.createElement("nav");
  nav.id = "TableOfContents";

  const section = makeTocSection(tocTargets, 0);
  nav.appendChild(section.content);
  // append title and content to the #toc placeholder
  const toc = document.body.querySelector("#toc");
  toc.appendChild(title);
  toc.appendChild(nav);

  // tell ToC items about any rendered-data headings they contain
  setTocItemChildren(section.content, headings);
}

// create a single ToC entry
function makeTocEntry(heading) {
  const li = document.createElement("li");
  const a = document.createElement("a");
  a.setAttribute("href", `#${heading.id}`);
  a.textContent = heading.textContent;
  li.appendChild(a);
  return li;
}

/*
Each ToC section is an `<ol>` element.
ToC entries are `<li>` elements and these contain nested ToC sections,
whenever we go to the next heading level down.
*/
function makeTocSection(headings, index) {
  const ol = document.createElement("ol");
  let previousHeading = null;
  let previousLi = null;
  let i = index;
  const lis = [];
  for (i; i < headings.length; i++) {
    const thisHeading = headings[i];
    if (previousHeading && (thisHeading.tagName > previousHeading.tagName)) {
      // we are going down a heading level, create a new nested section
      const section = makeTocSection(headings, i);
      previousLi.appendChild(section.content);
      i = section.index -1;
    }
    else if (previousHeading && (previousHeading.tagName > thisHeading.tagName)) {
      // we have come back up a level, so a section is finished
      for (let li of lis) {
        ol.appendChild(li);
      }
      return {
        content: ol,
        index: i
      }
    }
    else {
      // we are still processing this section, so add this heading to the current section
      previousLi = makeTocEntry(thisHeading);
      lis.push(previousLi);
      previousHeading = thisHeading;
    }
  }
  for (let li of lis) {
    ol.appendChild(li);
  }
  return  {
    content: ol,
    index: i
  }
}

/*
Set a new ToC entry.
Clear any previously highlighted ToC items, set the new one,
and adjust the ToC scroll position.
*/
function setTocEntry(newEntry) {
  const activeEntries = document.querySelectorAll("#toc a.active");
  for (const activeEntry of activeEntries) {
    activeEntry.classList.remove('active');
  }

  newEntry.classList.add('active');
  // don't scroll the sidebar nav if the main content is not scrolled
  const nav = document.querySelector("#td-section-nav");
  const content = document.querySelector("html");
  if (content.scrollTop !== 0) {
    nav.scrollTop = newEntry.offsetTop - 100;
  } else {
    nav.scrollTop = 0;
  }
}

/*
Test whether a node is in the viewport
*/
function isInViewport(node) {
  const rect = node.getBoundingClientRect();
  return (
    rect.top >= 0 &&
    rect.left >= 0 &&
    rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
    rect.right <= (window.innerWidth || document.documentElement.clientWidth)
  );
}

/*
The callback we pass to the IntersectionObserver constructor.

Called when any of our observed nodes starts or stops intersecting
with the viewport.
*/
function handleIntersectionUpdate(entries) {

  /*
  Special case: If the current URL hash matches a ToC entry, and
  the corresponding heading is visible in the viewport, then that is
  made the current ToC entry, and we don't even look at the intersection
  observer data.
  This means that if the user has clicked on a ToC entry,
  we won't unselect it through the intersection observer.
  */
  const hash = document.location.hash;
  if (hash) {
    let tocEntryForHash = document.querySelector(`nav li a[href="${hash}"]`);
    // if the hash isn't a direct match for a ToC item, check the data attributes
    if (!tocEntryForHash) {
      const fragment = hash.substring(1);
      tocEntryForHash = document.querySelector(`nav li a[data-${fragment}]`);
    }
    if (tocEntryForHash) {
      const headingForHash = document.querySelector(hash);
      if (headingForHash && isInViewport(headingForHash)) {
        setTocEntry(tocEntryForHash);
        return;
      }
    }
  }

  let newEntry = null;

  for (const entry of entries) {
    if (entry.intersectionRatio > 0) {
      const heading = entry.target;
      /*
      This sidebar nav consists of two sections:
      * at the top, a sitenav containing links to other pages
      * under that, the ToC for the current page

      Since the sidebar scrolls to match the document position,
      the sitenav will tend to scroll off the screen.

      If the user has scrolled up to (or near) the top of the page,
      we want to show the sitenav so.

      So: if the H1 (title) for the current page has started
      intersecting, then always scroll the sidebar back to the top.
      */
      if (heading.tagName === "H1" && heading.parentNode.tagName === "DIV") {
        const nav = document.querySelector("#td-section-nav");
        nav.scrollTop = 0;
        return;
      }
      /*
      Otherwise, get the ToC entry for the first entry that
      entered the viewport, if there was one.
      */
      const id = entry.target.getAttribute('id');
      let tocEntry = document.querySelector(`nav li a[href="#${id}"]`);
      // if the id isn't a direct match for a ToC item,
      // check the ToC entry's `data-*` attributes
      if (!tocEntry) {
        tocEntry = document.querySelector(`nav li a[data-${id}]`);
      }
      if (tocEntry && !newEntry) {
        newEntry = tocEntry;
      }
    }
  }

  if (newEntry) {
    setTocEntry(newEntry);
    return;
  }
}

/*
Track when headings enter the viewport, and use this to update the highlight
for the corresponding ToC entry.
*/
window.addEventListener('DOMContentLoaded', () => {

  makeToc();

  const toc = document.querySelector("#toc");
  toc.addEventListener("click", event => {
    if (event.target.tagName === "A") {
      setTocEntry(event.target);
    }
  });

  const observer = new IntersectionObserver(handleIntersectionUpdate);

  document.querySelectorAll("h1, h2, h3, h4, h5, h6").forEach((section) => {
    observer.observe(section);
  });

});
