On Sat, 12 Oct 2024 17:37:25 GMT, fabioromano1 <d...@openjdk.org> wrote:
>> An optimized algorithm for `BigDecimal.stripTrailingZeros()` that uses
>> repeated squares trick.
>
> fabioromano1 has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Minor change
After the proposed modifications, which aim to clarify the numeric aspects,
I'll wait for a couple of days before approval for you to commit possible last
minute changes.
src/java.base/share/classes/java/math/BigDecimal.java line 5242:
> 5240: }
> 5241:
> 5242: private static final double LOG_5_OF_2 = Math.log(2.0) /
> Math.log(5.0);
Suggestion:
private static final double LOG_5_OF_2 = 0.43067655807339306; // double
closest to log5(2)
to be sure that `LOG_5_OF_2` is the best possible, although it doesn't matter
much.
src/java.base/share/classes/java/math/BigDecimal.java line 5270:
> 5268:
> 5269: intVal = intVal.shiftRight(powsOf2); // remove powers of 2
> 5270: // maxPowsOf5 >= floor(log5(intVal)) >= max{n : (intVal % 5^n)
> == 0}
Suggestion:
// Let k = max{n : (intVal % 5^n) == 0}, m = max{n : 5^n <= intVal}, so
m >= k.
// Let b = intVal.bitLength(). It can be shown that
// | b * LOG_5_OF_2 - b log5(2) | < 2^(-21) (fp viz. real arithmetic),
// which entails m <= maxPowsOf5 <= m + 1, where maxPowsOf5 is as below.
// Hence, maxPowsOf5 >= k and is never off by more than 1 from the
theoretical m.
-------------
PR Review: https://git.openjdk.org/jdk/pull/21323#pullrequestreview-2364917422
PR Review Comment: https://git.openjdk.org/jdk/pull/21323#discussion_r1798328808
PR Review Comment: https://git.openjdk.org/jdk/pull/21323#discussion_r1798329279
