dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Oct 06, 2009 at 10:05:24PM +0200, Johan Hake wrote:
> On Tuesday 06 October 2009 21:58:29 Andy Ray Terrel wrote:
> > On Tue, Oct 6, 2009 at 2:43 PM, Anders Logg <logg@xxxxxxxxx> wrote:
> > > On Tue, Oct 06, 2009 at 02:28:40PM -0500, Andy Ray Terrel wrote:
> > >> Sorry for being dense but computing the SHA1 hash for what essentially
> > >> is a string compare seems a bit overkill.
> > >
> > > I want to avoid a call to strcmp in a piece of code that needs to be
> > > fast. What I do (it seems to work now) is to compute the 20 byte SHA-1
> > > hash from the ~40 byte finite element signature which is typically
> > > something like
> > >
> > >  FiniteElement('Lagrange', 'triangle', 1)
> > >
> > > I then cast the SHA-1 hash to an 8 byte void* and use that for
> > > comparison. This may not be very safe but this is not exactly
> > > cryptography. The number of possible hashes is the same as the address
> > > space so it should be fine.
> > >
> >
> > Yes I guess if you have a large number of comparisons it might be
> > faster.  I forget there are a lot of signature checks for the function
> > space stuff.
> >
> > Computing the hash is O(n) just like strcmp, it really depends on the
> > constants.  I would just do the timing before I said anything.
>
> Would it be an idea to extend ufc::finite_element with a hash_signature()?
> Then you can compute the hash using hashlib in python during code creation.
>
> Johan

Sounds like a good idea. Only it requires changes to DOLFIN, UFC, FFC
and SyFi...

Are there any objections to this?

--
Anders


> > I looked there are a lot of libraries out there for this but OpenSSL
> > is by far the most pervasive.
> >
> > -- Andy
> >
> > >> On Tue, Oct 6, 2009 at 2:18 PM, Anders Logg <logg@xxxxxxxxx> wrote:
> > >> > I'm contemplating adding a dependency to OpenSSL for computing hashes
> > >> > of things like element signatures.
> > >> >
> > >> > This is what breaks Kristian's code. Comparing element pointers does
> > >> > not work since the UFC class creates its own elements which are
> > >> > different from the ones existing in FunctionSpace classes.
> > >> >
> > >> > Would there be any difficulties with depending on OpenSSL? Are there
> > >> > simpler alternatives for a function that takes a string a returns a
> > >> > message digest like the SHA1 function in OpenSSL?
> > >> >
> > >> >
> > >> > -----BEGIN PGP SIGNATURE-----
> > >> > Version: GnuPG v1.4.9 (GNU/Linux)
> > >> >
> > >> > iEYEARECAAYFAkrLmCAACgkQTuwUCDsYZdEk3ACgl7+tHjclHzLesh+g/1vOzIo9
> > >> > tJsAn1fHGu+dzyA/ndNpy88/RFWNTbEB
> > >> > =24AA
> > >> > -----END PGP SIGNATURE-----
> > >> >
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> > >>
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> > >
> > > -----BEGIN PGP SIGNATURE-----
> > > Version: GnuPG v1.4.9 (GNU/Linux)
> > >
> > > iEYEARECAAYFAkrLndAACgkQTuwUCDsYZdFlWACePT0FglJU+NTPnOBXi/h7WcHC
> > > E5EAn2vinxerlg6jTNuA4T+0Kp+CFuhz
> > > =68Dm
> > > -----END PGP SIGNATURE-----
> > >
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