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Re: Efficient pattern-matching in a large array [message #77224 is a reply to message #77173]

Tue, 16 August 2011 09:41

guillermo.castilla.ca is currently offline

guillermo.castilla.ca
Messages: 27
Registered: September 2008

Junior Member

> Hint: use ULONG64 (8 byte) comparison:...

Lajos, this is an awesome trick, hats off!!
It took me a while to understand it, so assuming there are more people
out there who are intrigued by this (and also for myself, to
understand it at a later date), here's a layman explanation:
Chris has a large 1-D array of Byte type (i.e., numbers ranging from
0-255). He wants to find occurrences of a specific sequence (what he
calls 'pattern') of eight numbers (in his case [234, 203, 138, 216,
21, 52, 117, 39]). It happens that ULONG64 numbers are stored as a
sequence of eight Byte numbers (in his case his 8 byte sequence =
2843236008186268650). Therefore you can convert each sequence of eight
consecutive numbers in the array to a ULONG64 number, and then the
problem is reduced to finding occurrences of that number in the 8
shifted,reduced (by a factor of 1/8) ULONG64 versions of the original
array (where the shift is performed using the offset argument in the
FIX function). Brilliant!

Lajos, I was wondering if your solution can be generalized to patterns
of any length, could it?

Guillermo

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[Message index]

		Efficient pattern-matching in a large array By: Chris O'Dell on Tue, 02 August 2011 11:05
		Re: Efficient pattern-matching in a large array By: Foldy Lajos on Tue, 02 August 2011 13:04
		Re: Efficient pattern-matching in a large array By: ez569x on Fri, 12 August 2011 05:18
		Re: Efficient pattern-matching in a large array By: Brian Daniel on Fri, 12 August 2011 07:38
		Re: Efficient pattern-matching in a large array By: guillermo.castilla.ca on Tue, 16 August 2011 09:41
		Re: Efficient pattern-matching in a large array By: guillermo.castilla.ca on Wed, 17 August 2011 06:41
		Re: Efficient pattern-matching in a large array By: JDS on Tue, 16 August 2011 16:01
		Re: Efficient pattern-matching in a large array By: Bob[4] on Wed, 17 August 2011 14:14
		Re: Efficient pattern-matching in a large array By: JDS on Thu, 18 August 2011 10:00

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