// Gallery shortcode: opens the full image in a lightbox (a modal <dialog>) with its alt text
// and credit. Arrow buttons, arrow keys and swiping move between the images of the gallery;
// Escape, the close button or a click beside the image close it.
{
  const button = (className, label, symbol) => {
    const element = document.createElement('button');
    element.type = 'button';
    element.className = `lightbox__button ${className}`;
    element.setAttribute('aria-label', label);
    element.innerHTML = `<span aria-hidden="true">${symbol}</span>`;
    return element;
  };

  const lightbox = (gallery) => {
    const links = [...gallery.querySelectorAll('.gallery__link')];
    const labels = gallery.dataset;
    let index = 0;

    const dialog = document.createElement('dialog');
    dialog.className = 'lightbox';
    dialog.setAttribute('aria-label', labels.labelDialog);
    dialog.innerHTML = `
      <figure class="lightbox__figure">
        <img class="lightbox__image" alt="">
        <figcaption class="lightbox__caption" aria-live="polite">
          <span class="lightbox__alt"></span>
          <span class="lightbox__credit"></span>
          <span class="lightbox__counter"></span>
        </figcaption>
      </figure>`;
    const close = button('lightbox__close', labels.labelClose, '×');
    const previous = button('lightbox__previous', labels.labelPrevious, '‹');
    const next = button('lightbox__next', labels.labelNext, '›');
    dialog.append(close);
    if (links.length > 1) dialog.append(previous, next);
    document.body.append(dialog);

    const image = dialog.querySelector('.lightbox__image');
    const preload = (link) => {
      new Image().src = link.href;
    };

    const show = (newIndex) => {
      index = (newIndex + links.length) % links.length;
      const link = links[index];
      image.src = link.href;
      image.width = link.dataset.width;
      image.height = link.dataset.height;
      image.alt = link.dataset.alt;
      // The alt text is shown as the caption, so screen readers get it once, from the image.
      dialog.querySelector('.lightbox__alt').textContent = link.dataset.alt;
      dialog.querySelector('.lightbox__alt').setAttribute('aria-hidden', 'true');
      dialog.querySelector('.lightbox__credit').textContent = link.dataset.credit ?? '';
      dialog.querySelector('.lightbox__counter').textContent =
        links.length > 1 ? `${index + 1} / ${links.length}` : '';
      if (links.length > 1) {
        preload(links[(index + 1) % links.length]);
        preload(links[(index - 1 + links.length) % links.length]);
      }
    };

    links.forEach((link, i) => {
      link.addEventListener('click', (event) => {
        // Let modified clicks open the image in a new tab or window as usual.
        if (event.button !== 0 || event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) return;
        event.preventDefault();
        show(i);
        dialog.showModal();
        close.focus();
      });
    });

    // Closing goes through here, so that focus goes back to the thumbnail of the image that was
    // shown (the browser would restore it to where it was before the lightbox opened).
    const hide = () => {
      dialog.close();
      links[index].focus();
    };

    close.addEventListener('click', hide);
    // Escape, or the back gesture on Android.
    dialog.addEventListener('cancel', (event) => {
      event.preventDefault();
      hide();
    });
    previous.addEventListener('click', () => show(index - 1));
    next.addEventListener('click', () => show(index + 1));

    // A click on the dark area around the image closes the lightbox.
    dialog.addEventListener('click', (event) => {
      if (event.target === dialog || event.target.classList.contains('lightbox__figure')) hide();
    });

    dialog.addEventListener('keydown', (event) => {
      if (links.length < 2) return;
      if (event.key === 'ArrowLeft') show(index - 1);
      else if (event.key === 'ArrowRight') show(index + 1);
      else return;
      event.preventDefault();
    });

    // Swiping left or right on touch screens.
    let startX = null;
    dialog.addEventListener('pointerdown', (event) => {
      if (event.pointerType !== 'mouse') startX = event.clientX;
    });
    dialog.addEventListener('pointerup', (event) => {
      if (startX === null || links.length < 2) return;
      const distance = event.clientX - startX;
      startX = null;
      if (Math.abs(distance) > 50) show(index + (distance < 0 ? 1 : -1));
    });
  };

  document.querySelectorAll('[data-gallery]').forEach(lightbox);
}