Change SkConsistentChecksum to use SuperFastHash

Change SkConsistentChecksum to use SuperFastHash

This eliminates known hash collisions and makes use of a proven algorithm,
also allowing for incremental hashing (as will be useful for image checksums).
BUG=981

[epoger, 2012-11-22 06:29:42]

https://codereview.appspot.com/6847087/diff/2001/src/utils/SkConsistentChecks...
File src/utils/SkConsistentChecksum.h (right):

https://codereview.appspot.com/6847087/diff/2001/src/utils/SkConsistentChecks...
src/utils/SkConsistentChecksum.h:5: *  
http://www.azillionmonkeys.com/qed/weblicense.html
From reading those license details and looking at
http://code.google.com/p/smhasher/source/browse/trunk/SuperFastHash.cpp , I
*think* we're in the clear...

[reed1, 2012-11-26 13:50:53]

lets get a nod from cdibona or someone else in open-source at google to bring in
this code.

[epoger, 2012-11-27 16:13:06]

Joe- any thoughts on the licensing issue, or who might take a look at it for us?

[jcgregorio_google, 2012-11-27 16:41:38]

Yeah, you should get someone on the OSS team to look at that. I will
send the internal link
for that in a separate email.

Also, you might consider looking for other fast hash libraries we have already
cleared for Google third-party usage. For example,
http://code.google.com/p/cityhash/,
probably isn't equivalent to what's going on here, but it was written
and open sourced by Google.

  -joe

On Tue, Nov 27, 2012 at 11:13 AM,  <epoger@google.com> wrote:
> Joe- any thoughts on the licensing issue, or who might take a look at it
> for us?
>
> https://codereview.appspot.com/6847087/

[epoger, 2012-11-27 18:19:09]

On 2012/11/27 16:41:38, jcgregorio_google wrote:
> Also, you might consider looking for other fast hash libraries we have already
> cleared for Google third-party usage. For example,
> http://code.google.com/p/cityhash/,
> probably isn't equivalent to what's going on here, but it was written
> and open sourced by Google.

Yeah, I looked at cityhash, and it would be fine EXCEPT it does not provide an
API for "incremental" hashing (one line of pixels at a time, as opposed to
assembling all the lines into a contiguous bytearray).  See
http://code.google.com/p/cityhash/source/browse/trunk/src/city.h .

For the same reason, I can't make use of the SuperFastHash algorithm by SVN deps
on http://code.google.com/p/smhasher/source/browse/trunk/SuperFastHash.cpp (also
maintained by Googlers).

As far as I know, these are my options:

1. Find a lawyer (or lawyerish person) at Google to approve this code import.
2. Find another open-sourced hash algorithm that provides an incremental API,
which I can use via SVN deps.  (I looked for one without success.)
3. Approach the maintainers of CityHash about adding an incremental API, and
then use it via SVN deps.
4. Import CityHash as-is via SVN deps.  To use it, I will either need to make
successive calls to CityHash64WithSeed() for each pixel row, or copy all the
pixel rows into a contiguous bytearray.

Mike/Joe/anyone: is there an option I'm not thinking of that might be better?

In the absence of other options, I think my best bet is to go with #4 (worse
performance but no messy licensing issues) for now, and then see if I can get #1
or #3 to work over time.  What do you think?

[epoger, 2012-11-27 19:52:59]

Daniel Berlin (of OSPO) has advised me to NOT use SuperFashHash, and to use
CityHash instead.  So I will suspend work on this CL (probably closing it before
too long) and create another that uses CityHash via SVN deps.

[epoger, 2012-12-04 20:56:00]

On 2012/11/27 19:52:59, epoger wrote:
> Daniel Berlin (of OSPO) has advised me to NOT use SuperFashHash, and to use
> CityHash instead.  So I will suspend work on this CL (probably closing it
before
> too long) and create another that uses CityHash via SVN deps.

I just posted https://codereview.appspot.com/6867060/ ('Replace
SkConsistentChecksum with SkCityHash (now including CityHash via DEPS)') for
review... that should replace this CL.

[epoger, 2012-12-07 15:14:28]

Closed in favor of https://codereview.appspot.com/6867060/ ('Replace
SkConsistentChecksum with SkCityHash (now including CityHash via DEPS)')