Javascript – Anyone have a diff algorithm for rendered HTML?


I'm interested in seeing a good diff algorithm, possibly in Javascript, for rendering a side-by-side diff of two HTML pages. The idea would be that the diff would show the differences of the rendered HTML.

To clarify, I want to be able to see the side-by-side diffs as rendered output. So if I delete a paragraph, the side by side view would know to space things correctly.

@Josh exactly. Though maybe it would show the deleted text in red or something. The idea is that if I use a WYSIWYG editor for my HTML content, I don't want to have to switch to HTML to do diffs. I want to do it with two WYSIWYG editors side by side maybe. Or at least display diffs side-by-side in an end-user friendly matter.

Best Solution

There's another nice trick you can use to significantly improve the look of a rendered HTML diff. Although this doesn't fully solve the initial problem, it will make a significant difference in the appearance of your rendered HTML diffs.

Side-by-side rendered HTML will make it very difficult for your diff to line up vertically. Vertical alignment is crucial for comparing side-by-side diffs. In order to improve the vertical alignment of a side-by-side diff, you can insert invisible HTML elements in each version of the diff at "checkpoints" where the diff should be vertically aligned. Then you can use a bit of client-side JavaScript to add vertical spacing around checkpoint until the sides line up vertically.

Explained in a little more detail:

If you want to use this technique, run your diff algorithm and insert a bunch of visibility:hidden <span>s or tiny <div>s wherever your side-by-side versions should match up, according to the diff. Then run JavaScript that finds each checkpoint (and its side-by-side neighbor) and adds vertical spacing to the checkpoint that is higher-up (shallower) on the page. Now your rendered HTML diff will be vertically aligned up to that checkpoint, and you can continue repairing vertical alignment down the rest of your side-by-side page.