[java] Fix BigDecimal ROUND_HALF_EVEN

[Per Bothner]

Tom Tromey wrote:

>>>>>>"Jerry" == Jerry Quinn <jlquinn@optonline.net> writes:
> 
> 
> Jerry> The following program should output 0.21 after scaling, but
> Jerry> outputs 0.20.  The attached patch fixes the bug.
> 
> I don't know this code very well, so I'm reluctant to approve the
> patch.  Per, could you look at it?

I can't say I understand the code very well either.
The test for 'roundDigit > 5' in case ROUND_HALF_DOWN
looks suspicious also.

It might be possible to simplify and optimize the code.
I'd remove the "+ 1" in the setting of 'power', and
then just set unrounded to to parts[0].  I'd drop
roundDigit, whihc which makes me feel very uncomfortable,
and just compare the remainder (parts[1]) with valIntVal:

For simplicity, assume all the numbers are positive.
Then for ROUND_FLOOR or ROUND_DOWN, ignore the remainer.
Then for ROUND_CEILING, ROUND_UP, add one to unrounded
iff the remainder is non-zero.

Otherwise, double the remainder, and compare it with valIntVal.
If the doubled remainder is less than valIntVal, ignore it.
If it is greater, treat as ROUND_UP.  If it is equal,
it depends on the rounding mode in the obvious manner.

Adjust as appropriate if this and/or val are negative.

Does this logic seem correct?  If so, do we have a volunteer
to try implementing (and testing) it?
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/