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Hi!<br>
<br>
I've been thinking a bit about it [the problem] and arrived somewhere along the lines of it not necessarily being worth adding a new type to represent an index and an element (or trying to repurpose something existing), as I think the ergonomics of providing
a mapper BiFunction is better, somewhere along the lines of:<br>
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public static <T, R> Gatherer<T, ?, R> mapIndexed(BiFunction<Long, ? super T, ? extends R> mapper) {</div>
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class Index { long at = 0; }</div>
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return Gatherer.ofSequential(</div>
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Index::new,</div>
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(idx, e, d) -> d.push(mapper.apply(idx.at++, e))</div>
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);</div>
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}</div>
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<br>
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(Using longs for indexing seems sensible for something which could have unbounded length)<br>
<br>
Which would mean that if you have your own Pair class, or want to represent it as a Map.Entry, it's pretty straight-forward to do:<br>
<br>
stream.gather(mapIndexed(Pair::of))...</div>
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<br>
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It is unfortunate that parallelization takes a hit in this use-case, but knowing what indicies a sub-segment of the Stream has depends on the known size of the stream¡Xand I wouldn't be surprised that out-of-order processing of indices can be surprising to people,
so perhaps an ofSequential(¡K) isn't all that bad.<br>
<br>
With that being said, including new Gatherers in the stdlib is important to be done only after thorough evaluation of need.</div>
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Cheers,<br>
¡Ô</div>
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<br>
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<b><br>
</b></div>
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<b>Viktor Klang</b></div>
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Software Architect, Java Platform Group<br>
Oracle</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> core-libs-dev <core-libs-dev-retn@openjdk.org> on behalf of Olexandr Rotan <rotanolexandr842@gmail.com><br>
<b>Sent:</b> Thursday, 5 December 2024 18:20<br>
<b>To:</b> Henrik Wall <xehpuk.dev@gmail.com><br>
<b>Cc:</b> core-libs-dev <core-libs-dev@openjdk.org><br>
<b>Subject:</b> Re: JEP 473: Proposal for new built-in gatherer `indexed`</font>
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<p dir="ltr">Hi. There has been a proposal from me (that Chen mentioned), approximately half a year ago. At the time I have insisted on creating stream sub interface, and even got a working prototype for sequential streams, but there have been such a huge complexity
blowup in parallisation that I have just decided to drop it. Gatherers can be used pretty easily for this task, but using ofSequential, sacrificing parallelism. So basically, parallelism is a pain point here (or performance). I am not saying that it is impossible
to console enumeration and prallelisation, but it will require huge efforts and invasive changes in current *Pipeline implementations, or enormous amounts of code duplication</p>
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<div dir="ltr" class="x_gmail_attr">On Thu, Dec 5, 2024, 18:48 Henrik Wall <<a href="mailto:xehpuk.dev@gmail.com" target="_blank" rel="noreferrer">xehpuk.dev@gmail.com</a>> wrote:<br>
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Hey folks,<br>
<br>
Not having access to the index of an element of a stream is often a<br>
reason to fall back to a traditional loop, at least for me. I'd love<br>
to have `Gatherers.indexed()` that looks something like this:<br>
<br>
public static <TR> Gatherer<TR, ?, Map.Entry<Integer, TR>> indexed() {<br>
return Gatherer.ofSequential(<br>
() -> new int[1],<br>
Gatherer.Integrator.ofGreedy((state, element, downstream) -><br>
downstream.push(Map.entry(state[0]++, element)))<br>
);<br>
}<br>
<br>
(Potentially with a custom pair class to avoid auto-boxing.)<br>
<br>
In other popular languages like Python or Rust, this is also called `enumerate`.<br>
<br>
Any chance to get that in a future release?<br>
<br>
Regards,<br>
Henrik<br>
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