LIFTI and Porter stemming

Back in the original post where I first introduced LIFTI I wrote:

LIFTI will not...:

  • Handle word derivatives, e.g. if "Cats" is searched for, objects containing just the word "Cat" would not be returned.

This was always something that I wanted to handle, but felt that it was better to have a simple system that actually worked than an incomplete complex system. I've been a bit slack in updating the LIFTI code, but this has always been at the top of my list of things to implement.

As it turns out, once I actually got down to it, it has been pretty easy adding this feature on top of the existing code.

Introducing Porter stemming

A full 30 years ago a chap called Martin Porter had a piece of work entitled "An algorithm for suffix stripping" published in Program magazine. (Full history here) This article described a simple process by which common word suffixes could be removed from words, effectively normalizing them to their base form. For example connection, connects and connecting all become connect.

Assuming that the words being indexed and the words being searched upon are all normalized using the same process, searching for connecting in the index will begin to return items indexed against all forms of the word.

Over the years, Martin has refined this process and I've used an implementation of the Porter2 algorithm to stem words in LIFTI. You'll find loads of information about this algorithm on Martin's site.

It's a word splitter again

Much like the previous LIFTI article, Porter stemming can be introduced into LIFTI by creating a new word splitter implementation - each word the splitter returns having been passed through the stemming process.

You'll find the implementation of the new word splitter in the updated LIFTI codebase in the StemmingWordSplitter class - it's a pretty lightweight in its own right, as it relies heavily on the supporting Porter stemming implementation that you'll find lurking in the Lifti.PorterStemmer namespace.

By default the default word splitter is unchanged, so in order to make use of the stemming word splitter you'll need to construct your full text indexer like this:

var indexer = new FullTextIndexer<MyThing>(t => t.Description)
{
    WordSplitter = new StemmingWordSplitter() 
};

What's the catch?

Introducing a stemming algorithm into LIFTI has a few benefits, including:

  • The overall index will be smaller because all the indexed words will be stemmed prior to being stored.
  • You have to be less careful about search words; searching for cats would now return objects indexed against cat.

However this power is not without cost - stemming words introduces additional overhead during indexing and searching. To give you some feel for how much extra cost is involved I've updated the original comparison code to run an additional set of performance tests for an index built with a stemming word splitter.

Note that these results have been produced on my slightly less powerful laptop, so the figures may be slightly worse than in the original article, which were obtained on a workstation.

Number of results (simple) Time in ms (simple) Number of results (stemmer) Time in ms (stemmer)
Initialize index N/A 6.75ms N/A 22.47ms
Search: Jack tells 5 0.0067ms 8 0.0172ms
Search: boars 1 0.0026ms 3 0.0089ms
Search: jack 25 0.0023ms 25 0.009ms
Search: marshal 4 0.0027ms 4 0.0094ms

From these figures it becomes apparent that using a stemming word splitter is about 1/3 as fast as the simple word splitter, though hopefully you'll agree that it's more than 3 times as useful for some scenarios!

But what does it all mean?

What this means is that LIFTI is starting to become useful in a broader set of circumstances and it almost becomes useable for indexing more meaningful text, such as documents

I suspect however there's still quite some way to go before this becomes practical; features like index persistence and the ability to update and remove indexed items will probably be essential. I'm already on the case with the thinking process for these features, but if there's anything else that you think would be good to add, feel free to leave me a comment here or on the discussions board on the LIFTI CodePlex site.

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