(function createIndex () {
  function setRealPageNumbers () {
    let pages = document.querySelectorAll('.pagedjs_page'),
        realPageCounter = 0;

    const pageCounterResetPatt = /page (\d+)/;
    // Set an extra data-attribute with the 'real' pagenumber
    // Loop through all the pages.
    for (let i = 0; i < pages.length; i++) {
      const page = pages[i],
            m = window.getComputedStyle(page).getPropertyValue('counter-reset').match(pageCounterResetPatt);
      // Test through getComputedStyles whether there is a counter reset.
      if (m && m.length) {
        // Set js counter to that value
        console.log('Page counter reset to:', m[1], 'on page', page.dataset.pageNumber);
        realPageCounter = parseInt(m[1]); 
      }
      
      // Increment js counter by one
      realPageCounter++;
      // Set it on the page.
      page.dataset.realPageNumber = realPageCounter;
    }
  }

  function triggerDownload (content, filename, mime) {
    const a = document.createElement('a');
    a.setAttribute('href', `data:${mime};charset=utf-8,${encodeURIComponent(content)}`);
    a.setAttribute('download', filename);
    
    if (document.createEvent) {
      const event = document.createEvent('MouseEvents');
      event.initEvent('click', true, true);
      a.dispatchEvent(event);
    }
    else {
        a.click();
    }
  }

  function getPageNum (el) {
    if ('realPageNumber' in el.dataset) {
      return parseInt(el.dataset.realPageNumber);
    }
    else if (el.parentElement) {
      return getPageNum(el.parentElement);
    }
  }

  function createIndex(config){
    let indexElements = document.querySelectorAll(config.indexElementQuery);
    index = {};

    for(let i = 0; i < indexElements.length; ++i){
      let indexElement = indexElements[i];

      // create array with all data-book-index
      let indexKey = indexElement.dataset.bookIndex,
          pageNum = getPageNum(indexElement);
      

      if (pageNum) {
        if (! index[indexKey]) {
          index[indexKey] = [];
        }

        index[indexKey].push(pageNum);
      }
    }

    return index;
  }

  function createJsonIndex (config) {
    let urlParameters = new URLSearchParams(window.location.search),
        pathParts = window.location.pathname.split('/'),
        lastPathPart = pathParts[pathParts.length - 1],
        downloadName = 'index.json';
    
    if (lastPathPart) {
      downloadName = 'index-' + lastPathPart.replace('.html', '.json'); // Ugly, insecure, but here it'll work. MVP for the win.
    }
    
    if (urlParameters.has('makeIndex')) {
      setRealPageNumbers();
      const index = createIndex(config);
      triggerDownload(JSON.stringify(index), downloadName, 'application/json');
    }
  }

  window.createJsonIndex = createJsonIndex;
})();