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   Re: [xml-dev] [OT] Looking for a text algorithm

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Miles Sabin writes:

 > Judging from your examples it looks like you're after a closeness 
 > criterion derived from longest common subsequences. But I don't see how 
 > you could use that to usefully construct a single characteristic number 
 > for _any_ string of _any_ length: with only 32 or 64 bits to play with, 
 > many many completely unrelated (on any criterion) strings will collide 
 > on the same code.

Quite right.  However, if the alternative is a linear search (say,
using an edit-distance algorithm), then reducing the number of
candidates by a few orders of magnitude would not necessarily be a bad

The problem I was considering (in the shower) was detecting spam
messages with minor variations, such as the insertion of the
recipient's e-mail address in the body or the substitution of Zimbabwe
for Nigeria.  Assume that I have a database containing many millions
of known spam messages, and that I want to check an incoming e-mail
message against it.  If I can narrow the field down to, say, 50
candidates after a very inexpensive operation, then my system will be
much more efficient; I can then use edit-distance against the closest
matches to see if the message really is likely spam.

That said, based on private e-mail from another list member, I suspect
that there may be nothing original about the algorithm I came up with;
nevertheless, here it is for anyone who would care to take a peek:


All the best,


David Megginson, david@megginson.com, http://www.megginson.com/


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