Shingling – it’s not just for roofers!

I was catching up on the Information Architecture Institute mailing list and some feed reader backlog when I came across the concept of shingling. Seeing as how I have never heard of this term in the 8+ years I’ve been working, I decided it was high time that I learned about it.

In essence, shingling seeks to solve the problem of indexing large quantities of data:

  • How can you tell if two pieces of content are the same? You compare them.
  • What happens when you want to compare a lot of pages? You have to make a very large number of comparisons.
  • What happens when you’re google? You can’t solve this problem by bulk comparisons

The number of computations required to compare all of your content rapidly trends toward infinity. Enter shingling.

Shingling is nothing more than taking a single document, splitting it into smaller chunks, and generating a sufficiently sized unique fingerprint from the shingles. This way, when you are indexing content, you can immediately break down the document into the constituent shingles, generate a fingerprint, and then see if that fingerprint already exists. What you do at this point is up to you.

I ran into a similar problem when implementing an n-gram search engine that ultimately proved to be inefficient. Essentially, the computation required to generate the n-grams for a given page of content, check the index for an existing n-gram, and associate the indexed word/content chunk with the given n-gram soon proved terribly inefficient. This n-gram search engine only had to index names, addresses, and email addresses — it wasn’t even attempting to provide an index of user generated content.

I have to wonder how full-text indexing providers, like SQL Server and Ferret, will (or already have) handle the challenge of shingling indexed content. It seems like it would be a concern both on the front of storage consolidation and for the purposes of optimizing CPU cycles — for retrieval, indexing, and comparison.

References
Near-duplicates and shingling
Navigating the network of knowledge: Mining quotations from massive-scale digital libraries of books
N-gram, From Wikipedia, the free encyclopedia

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