dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

Johan Hake wrote:
> 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? 

How would I write it by hand? It's common for new methods that the 
UFC-compliant code is hand generated.

Garth

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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> > > > > > > -----BEGIN PGP SIGNATURE-----
> > > > > > > Version: GnuPG v1.4.9 (GNU/Linux)
> > > > > > >
> > > > > > > iEYEARECAAYFAkrLndAACgkQTuwUCDsYZdFlWACePT0FglJU+NTPnOBXi/h7WcHC
> > > > > > > E5EAn2vinxerlg6jTNuA4T+0Kp+CFuhz
> > > > > > > =68Dm
> > > > > > > -----END PGP SIGNATURE-----
> > > > > > >
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> > > > > --
> > > > >
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