Comparing large strings in JavaScript with a hash

Question

I have a form with a textarea that can contain large amounts of content (say, articles for a blog) edited using one of a number of third party rich text editors. I'm trying to implement something like an autosave feature, which should submit the content through ajax if it's changed. However, I have to work around the fact that some of the editors I have as options don't support an "isdirty" flag, or an "onchange" event which I can use to see if the content has changed since the last save.

So, as a workaround, what I'd like to do is keep a copy of the content in a variable (let's call it lastSaveContent), as of the last save, and compare it with the current text when the "autosave" function fires (on a timer) to see if it's different. However, I'm worried about how much memory that could take up with very large documents.

Would it be more efficient to store some sort of hash in the lastSaveContent variable, instead of the entire string, and then compare the hash values? If so, can you recommend a good javascript library/jquery plugin that implements an appropriate hash for this requirement?

Answer 1

In short, you're better off just storing and comparing the two strings.

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Computing a proper hash is not cheap. For example, check out the pseudo code or an actual JavaScript implementation for computing the MD5 hash of a string. Furthermore, all proper hash implementations will require enumerating the characters of the string anyway.

Furthermore, in the context of modern computing, a string has to be really, really long before comparing it against another string is slow. What you're doing here is effectively a micro-optimization. Memory won't be an issue, nor will the CPU cycles to compare the two strings.

As with all cases of optimizing: check that this is actually a problem before you solve it. In a quick test I did, computing and comparing 2 MD5 sums took 382ms. Comparing the two strings directly took 0ms. This was using a string that was 10000 words long. See http://jsfiddle.net/DjM8S.

If you really see this as an issue, I would also strongly consider using a poor-mans comparison; and just comparing the length of the 2 strings, to see if they have changed or not, rather than actual string comparisons.

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