Hi,
Included below is a copy of a message I sent to the py-tutor list. It
didn't garner much in the way of a solution and it was suggested that
this list might be helpful.
Thanks,
-----Forwarded Message----- From: Aaron Straup Cope <as*@vineyard.n et> To: pytutor <tu***@python.o rg> Subject: creating bitmasks (for bloom filters) Date: Fri, 11 Jun 2004 08:49:20 -0400
Hi,
A quick introduction : I am mostly a Perl hacker, but I am teaching myself Python because it seems like the right tool for the job in question. [1] I am not, by any stretch, much of a math weenie.
I am trying to port the Bloom::Filter Perl module [2] for use with my tool. I know that there is a "pybloom" sourceforge project [3] but licensing issues aside it's reliance on Unix math libs makes it problematic for use with a cross-platform GUI tool.
At this point, my not very deep grasp of Python and my poor understanding of some basic compsci/math principles have combined to frustrate my efforts.
I was hoping that the the Perl/Python operator and function set would be similar enough that, with only a few modifications, I could clone the B:F.pm package pretty much "as is". All other bugs aside, I am stuck trying to create a bitmask of a key :
"Given a key, hashes it using the list of salts and returns a bitmask the same length as the Bloom filter. Note that Perl will pad the bitmask out with zeroes so it's a muliple of 8."
# from the docs for the _make_bitmask method
Specifically, it's not clear to me :
* how to map the various Perl pack/unpack formats to their Python equivalents (things like Perl's "b*" which is "a bit string, in ascending bit order inside each byte" where "*" means to use all the bytes from the input field")
* how to account for the fact that unpack returns a list in Perl and Python returns a string
* how to pass a salt to the Python sha module
* how to XOR the pieces of an unpacked sha digest (which may simply be a function of a bad format unpacking stuff)
* A 'vec' [4] function in Python; does one exist?
In short, I don't really have any idea what I'm doing :-(
As much as I relish the challenge purely on the grounds of learning new stuff I am starting to reach the point of irrational frustration and wanting to just get this done so I can move on to other things.
The tool should be written in Python because it's the best choice for the job. The tool needs to be able to read/write Bloom Filters. Python needs a Bloom Filter library, so I will step up and write it. Except it appears to be beyond my skill level right now...
Any thoughts, suggestions or clue-bats would be very much welcomed. Pointers to an already existing implementation that I've missed would be even better but I'll take what I can get. :-)
Thanks,
---
[1] http://aaronland.info/weblog/categor...plication_the/ [2] http://search.cpan.org/src/MCEGLOWS/...0.02/Filter.pm [3] http://cvs.sourceforge.net/viewcvs.py/pybloom/ [4] http://www.perldoc.com/perl5.8.4/pod/func/vec.html 0 1781 This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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