Question

Suppose you have a Django view that has two functions:

The first function renders some XML using a XSLT stylesheet and produces a div with 1000 subelements like this:

<div id="myText">
    <p id="p1"><a class="note-p1" href="#" style="display:none" target="bot">✽</a></strong>Lorem ipsum</p>
    <p id="p2"><a class="note-p2" href="#" style="display:none" target="bot">✽</a></strong>Foo bar</p>
    <p id="p3"><a class="note-p3" href="#" style="display:none" target="bot">✽</a></strong>Chocolate peanut butter</p>
     (etc for 1000 lines)
    <p id="p1000"><a class="note-p1000" href="#" style="display:none" target="bot">✽</a></strong>Go Yankees!</p>
</div>

The second function renders another XML document using another stylesheet to produce a div like this:

<div id="myNotes">
    <p id="n1"><cite class="note-p1"><sup>1</sup><span>Trololo</span></cite></p>
    <p id="n2"><cite class="note-p1"><sup>2</sup><span>Trololo</span></cite></p>
    <p id="n3"><cite class="note-p2"><sup>3</sup><span>lololo</span></cite></p>
     (etc for n lines)
    <p id="n"><cite class="note-p885"><sup>n</sup><span>lololo</span></cite></p>
</div>

I need to see which elements in #myText have classes that match elements in #myNotes, and display them. I can do this using the following jQuery:

$('#myText').find('a').each(function() {
    var $anchor = $(this);
    $('#myNotes').find('cite').each(function() {
        if($(this).attr('class') == $anchor.attr('class')) {
            $anchor.show();
    });
});

However this is incredibly slow and inefficient for a large number of comparisons.

What is the fastest/most efficient way to do this – is there a jQuery/js method that is reasonable for a large number of items? Or do I need to reengineer the Django code to do the work before passing it to the template?

Answer 1

For best-possible performance, make an index once and then re-use it:

function revealCite() {
  var cites_index = $("#myText").data("cites_index");

  // if no cached index exists, prepare one (one-time hit code section)
  if (!cites_index) {
    var cites = $("#myNotes cite");
    var cites_count = cites.length();
    var cites_index = {};

    for (var i=0; i<cites_count; i++) {
      var cite = cites[i], group = cites_index[cite.className];
      if (!group) cites_index[cite.className] = [];
      group.push(cite);
    }
    $("#myText").data("cites_index", cites_index);
  }

  // use the index to work with related elements ("this" must be an <a> element)
  $(cites_index[this.className]).show();
}

Now trigger the above function any way you like:

$("#myText a").each(revealCite);

PS: You could also do this, in place of the for loop:

cites.each( function() {
  var group = cites_index[this.className];
  if (!group) cites_index[this.className] = [];
  group.push(this);
});

But it’s the same number lof lines of code, and probably is a bit slower.