EnumeratedStream

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On Sat, Apr 20, 2024 at 8:29 PM ІП-24 Олександр Ротань <
rotan.olexandr at gmail.com> wrote:

> Gatherers could be effectively terminal, but I don't think Gatherer API
> designers intended it to be. In related JEP, gatherers are described as a
> way to declare custom intermediate operations, and introducing "terminal"
> gatherers would be misleading.
>
I will show you an API that expands on collectors too.

>
> Talking about performance, not even considering gather() method itself,
> creating an instance of Indexed object for each value in stream is costy
> and might turn into a nightmare for infinite streams.
>
We will have value objects soon (Project Valhalla), by then the object
creation will be much more lightweight as it's effectively a tuple. This is
not really a thing of concern.

>
> As for internals of Stream API right now, I am not aware about its current
> state, so not much to say here except that adding a new type of streams
> that just slightly extends existing functionality might not be that
> harmful, I guess.
>
> Regarding API exposure, I don't think that moving it from stream directly
> to Gatherers factory would be much of a deal since to see index-aware
> methods user must explicitly convert stream to enumerated.
>
Having another set of EnumeratedStream APIs and implementations is going to
be a nightmare to maintain. Also it has bad interpolatability to convert
itself back into regular streams for other usages, while Stream<Indexed<T>>
are much better in that aspect.

>
> I also think the I didn't express my point about index consistency clear
> enough. Consider following:
>
Notice that Stream.filter retains instead of drops the matching elements!
Since otherwise your results don't make sense. I will assume you mean
"filter" like "dropIf"

>
> List.of(1,2,3).stream().enumerated().filter((idx, i) -> i*idx <
> 2).map((idx, val) -> idx * val)
>
> Result : (4, 9)
>
Does idx start from 0 or 1? All Java indices start from 0, having it start
from 1 in enumerated is really weird. I will assume you mean [2, 6]=[1*2,
2*3]
Your use case is even simpler, this case can be covered in a simple
gatherer like this:
Gatherer.ofSequential(() -> new int[] {0},
            Gatherer.Integrator.ofGreedy((counter, v, sink) ->
                    sink.push(new Indexed<>(counter[0]++, v))))
Better than writing thousands lines of new code. The functionalities are
already implemented in my sample project in
https://github.com/liachmodded/IndexedStream/tree/main/src/main/java/com/github/liachmodded/indexedstream

>
> With your approach
>
> List.of(1,2,3).stream().gather(Gatherers.filterIndexed((idx, val) ->
> idx*val < 2)).gather(Gatherers.mapIndexed((idx, val) -> idx * val))
>
> Result : (2, 6)
>
I will assume you mean [0, 3]=[0*2, 1*3]

>
> Not only second option is much more verbose, but also indexes of stream
> are inconsistent between operations.
>
Then what does EnumeratedStream serve? We would assume the enumeration
means the enumeration at the current stage so we want the number
immediately. If you want the enumeration at a particular stage, it's better
to zip your stream items with an index like my gatherer above at an
explicit step.

Also I have already made a showcase git repository with your two test cases
at
https://github.com/liachmodded/IndexedStream/blob/main/src/test/java/com/github/liachmodded/indexedstream/IndexedGatherersTest.java
Feel free to rant and tell what you really want that aren't served by these
few simple classes.

>
> PS: regarding findIndex, I did some benchmarking today, you might like to
> take a look. On average list-based version outperform collector and
> gatherer-based in more then 10 times. And also lists in stdlib doesn't have
> any hashcode-based implementations. I wrote 19 implementations in list
> subclasses and none of them had anything but simple traversing logic inside
> indexOf. But that's topic of another thread
>
Thanks for the prompt, I will look at it and reply in the other thread.
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