In case there is a need for extra motivation here:
import java.util.stream.DoubleStream;
public class Test8214761 {
public static void main(String[] args) {
double a = Double.valueOf(args[0]);
if (Math.ulp(a) <= Math.ulp(Math.nextAfter(a, 0))) {
System.out.println("Stable addition");
} else {
double b = Math.signum(a) * Math.ulp(a) / 2;
double sum = a + b;
double streamSum = DoubleStream.of(a, b).sum();
System.out.println(a + " + " + b + "\n => " + sum);
System.out.println("DoubleStream.of(" + a + ", " + b + ").sum()\n => " +
streamSum);
}
}
}
$ java -showversion Test8214761 1
openjdk version "16-internal" 2021-03-16
OpenJDK Runtime Environment (build 16-internal+0-cdennis.0178d0f136e9)
OpenJDK 64-Bit Server VM (build 16-internal+0-cdennis.0178d0f136e9, mixed mode,
sharing)
1.0 + 1.1102230246251565E-16
=> 1.0
DoubleStream.of(1.0, 1.1102230246251565E-16).sum()
=> 0.9999999999999999
That's the sum of two positive doubles returning a result smaller than one of
the two.
Apologies for the zeal,
Chris
On 8/27/20, 10:52 AM, "Chris Dennis" <chris.w.den...@gmail.com> wrote:
Bump...
I've run in to this while running tests that check computation results
against the expected bounds of a Kahan summation. Any chance that this gets
picked up in the near future?
Thanks,
Chris
On 12/13/18, 6:16 PM, "core-libs-dev on behalf of Ivan Gerasimov"
<core-libs-dev-boun...@openjdk.java.net on behalf of ivan.gerasi...@oracle.com>
wrote:
Gentle ping.
On 12/9/18 7:37 PM, Ivan Gerasimov wrote:
> Hello!
>
> DoubleSummaryStatistics takes advantage of Kahan summation algorithm
> to reduce the error of the total sum.
>
> Internally it maintains a field double sumCompensation, which keeps
> lower bits (which were rounded off) of the last addition.
>
> Note, that the compensation has to be subtracted from the result to
> add those bits back:
>
> 166 private void sumWithCompensation(double value) {
> 167 double tmp = value - sumCompensation;
> 168 double velvel = sum + tmp; // Little wolf of rounding
error
> 169 sumCompensation = (velvel - sum) - tmp;
> 170 sum = velvel;
> 171 }
>
> At the line 169, tmp normally has more lower bits than (velvel - sum).
>
> However, when two DoubleSummaryStatistics objects are combined, this
> compensation part is *added* to the total, which may result in a less
> accurate result.
>
> The same bug is replicated in DoubleStreams.
>
> Would you please help review the fix?
>
> BUGURL: https://bugs.openjdk.java.net/browse/JDK-8214761
> WEBREV: http://cr.openjdk.java.net/~igerasim/8214761/00/webrev/
>
--
With kind regards,
Ivan Gerasimov
