On Mon, 15 Sep 2025 17:59:28 GMT, fabioromano1 <d...@openjdk.org> wrote:
>> HI @fabioromano1, >> Ah, I haven't worked on that code for exact divide in a while! >> For some historical context, the method was added as part of JSR 13, Decimal >> Arithmetic Enhancement, in JDK 5. A fuller derivation of the old bound is >> discussed in: >> >> Fixed, Floating, and Exact Computation with Java's BigDecimalFixed, >> Floating, and Exact Computation with Java's BigDecimal >> Dr. Dobb's Journal ยท Jun 1, 2004 >> https://web.archive.org/web/20210505132021/http://www.drdobbs.com/jvm/fixed-floating-and-exact-computation-wit/184405721 >> >> At the time I wrote the code, I looked around at the usual sources, Knuth, >> etc. and asked around for a bound of decimal digits of 1/b if 1/b is >> representable, but didn't find anything. >> >> I see you're still revising the implementation. Once this settles down, a >> review comment from me will be "restore some textual discussion of what the >> algorithm is doing." >> >> For testing, I think it would be good to probe at some values where the old >> and new digit estimation techniques differ. >> >> Also, I think both @rgiulietti and myself look at this PR before it goes >> back; I'll adjust the reviewer count accordingly. > > @jddarcy The technique used here is the following: take $a/b$, compute $b' = > b/(2^i 5^j)$, where $i = \max(n \in N \mid b \equiv 0 \mod 2^n)$ and $j = > \max(n \in N \mid b \equiv 0 \mod 5^n)$. If $a \not\equiv 0 \mod b'$, then > $a/b$ is not a finite decimal number. Otherwise: > > - if $i \le j$, then $a/b = (a/b') \cdot 2^{j-i} \cdot 10^{-j}$; > - if $i > j$, then $a/b = (a/b') \cdot 5^{i-j} \cdot 10^{-i}$. @fabioromano1 Please put the [comment above](https://github.com/openjdk/jdk/pull/27271#issuecomment-3293326363) in the appropriate spot in the `divide()` method, around L.1800. More generally, everything not obvious should go in a comment in the code to help future improvements. ------------- PR Comment: https://git.openjdk.org/jdk/pull/27271#issuecomment-3328160026
