Initial request for review of JDK-8006572 DoubleStream.sum()/average() implementations that reduce numerical errors

[Joe Darcy]

Hi Mike,

On 11/20/2013 10:31 AM, Mike Duigou wrote:
> - Collectors averagingDouble could use the same @implNote as in DoublePipeline.
>
> - DoublePipeline implementation could document the usage of the double array indices.
>
> - @summary in test.

I'll reflect these in the next iteration.

>
> - I agree with Paul that refactoring as a testNG test would be nice.

While learning about testNG would be useful, I don't want to take that 
on right at the moment ;-)

>
> - I wondered at the mechanism for combining compensation values. Do you have a reference which explains the correctness? I thought perhaps directly adding the compensations together before doing compensated addition of the two sums might be better.

One of the inquiries I have out to my numerical reviewer is how to best 
combine two compensations.

In the code as written, from the perspective of one compensation, the 
two doubles in the other other compensation are just two more double 
values. So I think this is correct, if not ideal.

The compensation portion of one running sum could be of vastly different 
magnitude than the compensation portion (or the high-order sum bits) of 
the other sum. Therefore, I believe a direct combination of either (sum1 
+ sum2) or (comp1 + comp2) would lose the numerical properties of 
interest here.

Thanks for the feedback,

-Joe

>
> Mike
>
>
> On Nov 20 2013, at 00:21 , Joe Darcy <joe.darcy at oracle.com> wrote:
>
>> Hi Paul,
>>
>> On 11/18/2013 07:38 AM, Paul Sandoz wrote:
>>> Hi Joe,
>>>
>>> You can use the three arg form of collect for DoublePipeline.sum/average impls, which is already used for average:
>>>
>>>      public final OptionalDouble average() {
>>>          double[] avg = collect(() -> new double[2],
>>>                                 (ll, i) -> {
>>>                                     ll[0]++;
>>>                                     ll[1] += i;
>>>                                 },
>>>                                 (ll, rr) -> {
>>>                                     ll[0] += rr[0];
>>>                                     ll[1] += rr[1];
>>>                                 });
>>>          return avg[0] > 0
>>>                 ? OptionalDouble.of(avg[1] / avg[0])
>>>                 : OptionalDouble.empty();
>>>      }
>>>
>>> That would be the most expedient way.
>> Thanks for the tip. For the moment, I'm feeling a bit expedient and used the array-based approach in an iteration of the change:
>>
>>     http://cr.openjdk.java.net/~darcy/8006572.3/
>