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Actually, the main use for bitvectors in the NetFlix challenge. is that you can do very fast relational math.

So performing the select to find all the people who have rated all the same movies (and the movie we are trying to generate a rating for), that we already have a rating for by the person we are trying to generate a rating for, is just a tight loop over the dataset performing bitstring boolean operations which are very fast (in perl).

Doing the same operations (AND/OR/XOR/etc. of every bit of one bitstring against every bit of another bitstring for every customer in the DB) using Judy arrays, would not benefit at all from any of their special properties (cache line locality), and be extremely slow.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

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In the absence of evidence, opinion is indistinguishable from prejudice.

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In reply to Re^2: Compact and sparse bit vector by BrowserUk
in thread Compact and sparse bit vector by diotalevi

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