dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

On Tuesday 06 October 2009 22:18:04 Anders Logg wrote:
> On Tue, Oct 06, 2009 at 09:13:45PM +0100, Garth N. Wells wrote:
> > Anders Logg wrote:
> > > 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?
> >
> > I'm not sure that this is a good idea. It complicates UFC.

Complicates? One function returning a string? But I agree with Anders that it 
requires a lot to change and that it therefore is a good thing to think about 
it.

> It's either that or using boost. I'm looking at both options.
> 
> Calling boost is simpler since it requires the smallest change. If we
> decide to go with that option now, we can keep this in mind for the
> next update of UFC.

Sounds good.

Johan

> --
> Anders
> 
> > Garth
> >
> > >>> 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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> > >>>>>> DOLFIN-dev@xxxxxxxxxx
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> > >>>>>
> > >>>>> _______________________________________________
> > >>>>> DOLFIN-dev mailing list
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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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