Andy Ray Terrel wrote:
> On Tue, Oct 6, 2009 at 2:43 PM, Anders Logg <l...@simula.no> 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.
>>
>> --
>> Anders
>>
>>
>
> 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.
>
> I looked there are a lot of libraries out there for this but OpenSSL
> is by far the most pervasive.
>
What about boost::hash since we already depend on Boost?
Garth
> -- Andy
>
>
>
>>> On Tue, Oct 6, 2009 at 2:18 PM, Anders Logg <l...@simula.no> 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)
>>
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>> E5EAn2vinxerlg6jTNuA4T+0Kp+CFuhz
>> =68Dm
>> -----END PGP SIGNATURE-----
>>
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