import cache from '../../core/base/cache';
import { querySelectorAll } from '../../core/utils';

// test for hrefs that start with # or /# (for angular)
const isInternalLinkRegex = /^\/?#[^/!]/;

/**
 * Determines if element is a skip link
 * @method isSkipLink
 * @memberof axe.commons.dom
 * @instance
 * @param  {Element} element
 * @return {Boolean}
 */
function isSkipLink(element) {
  if (!isInternalLinkRegex.test(element.getAttribute('href'))) {
    return false;
  }

  let firstPageLink;
  if (typeof cache.get('firstPageLink') !== 'undefined') {
    firstPageLink = cache.get('firstPageLink');
  } else {
    // define a skip link as any anchor element whose href starts with `#...`
    // and which precedes the first anchor element whose href doesn't start
    // with  `#...` (that is, a link to a page)
    firstPageLink = querySelectorAll(
      // TODO: es-module-_tree
      axe._tree,
      'a:not([href^="#"]):not([href^="/#"]):not([href^="javascript"])'
    )[0];

    // null will signify no first page link
    cache.set('firstPageLink', firstPageLink || null);
  }

  // if there are no page links then all all links will need to be
  // considered as skip links
  if (!firstPageLink) {
    return true;
  }

  return (
    element.compareDocumentPosition(firstPageLink.actualNode) ===
    element.DOCUMENT_POSITION_FOLLOWING
  );
}

export default isSkipLink;