I've spent some time looking at other options and have now come up with the
following
SELECT mod (
(
(((results.myValue - ((results.myValue >> 1) &
3952873730080618203) - ((results.myValue >> 2) & 1317624576693539401)) +
((results.myValue - ((results.myValue >> 1) &
3952873730080618203) - ((results.myValue >> 2) & 1317624576693539401) >>
3)) & 8198552921648689607)
+
((((results.myValue -((results.myValue >> 1) &
3952873730080618203) - ((results.myValue >> 2) & 1317624576693539401)) +
((results.myValue - ((results.myValue >> 1) &
3952873730080618203) - ((results.myValue >> 2) & 1317624576693539401) >>
3)) & 8198552921648689607)
) >> 6
)
) & 8088187786164957247
)
,4095) AS Result
FROM
(
SELECT 1256789 AS myValue
) AS results
With some testing this seems to correctly count bits for 64bit long ints on
at least MYSQL (currently my only test target).
Its a version of a 1970's solution known as HAKMEM 169, which was orginally
coded for the PDP10. Full background can be found on the web and there is
some nice detail here
http://blogs.msdn.com/b/jeuge/archive/2005/06/08/hakmem-bit-count.aspx
A short background is that it works by suming up the number of bits in each
block of 3 bits, it then combines the results of each set of 2 blocks and
then finally uses MOD to create a sum of all the results. All of the long
'magic' numbers make far more sense if viewed as octal values as they are
used as bitwise masks. The reason why the code is so small is because each
operation is working on a total of 22 blocks (21 x 3 bit and 1 x 1 bit).
Its not used nowadays as the MOD function is costly compaired to other
coding options.
For general ease of use I would say that the solution already included in
jOOQ is good enough and far easier for people to understand if they look at
the queries. Compaired to my original post, this solution should be better
across platforms as it only reduces the 64 bit value so there is no
problems with maths overflows.
Roger
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