Is there a built-in way to do something like `groupBy { }`, but only group on elements where the key...

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[natario1]

Is there a built-in way to do something like groupBy { }, but only group on elements where the key is contiguous?

Say I have a List<Sample> where sample is class Sample(duration: Int).
I need something like a List<List<Sample>>, where samples are grouped by duration, but not reordered. If durations are 100, 100, 100, 200 and 100, that should be three groups, not two.

[pedro]

You can do that with fold. Not the simplest use of it, but doable…

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/fold.html

[natario1]

Thanks. That's kind of what I have right now, but I have the feeling that something better could be done.

samples.fold(mutableListOf<MutableList<Sample>>()) { groups, sample ->
    val duration = groups.lastOrNull()?.lastOrNull()?.duration
    if (sample.duration == duration) {
        groups.last().add(sample)
    } else {
        groups.add(mutableListOf(sample))
    }
    groups
}

[pedro]

yeah, I can’t see a better alternative with this approach… 😬

[diesieben07]

This was considered as part of sliding window functions, but not implemented as too niche

[Kroppeb]

I found this in my util library for advent of code

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