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Optimise ZStream.fromIteratorSucceed #10083

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Merged
merged 3 commits into from
Aug 25, 2025
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Optimise ZStream.fromIteratorSucceed #10083

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Optimise `ZStream.fromIteratorSucceed`
guizmaii Aug 5, 2025
b20e2ed
Review: Materialize `maxChunkSize`
guizmaii Aug 22, 2025
c4dea63
Review: Minimize diff
guizmaii Aug 22, 2025
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37 changes: 20 additions & 17 deletions streams/shared/src/main/scala/zio/stream/ZStream.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -4523,37 +4523,40 @@ object ZStream extends ZStreamPlatformSpecificConstructors {
def fromIteratorSucceed[A](iterator: => Iterator[A], maxChunkSize: => Int = DefaultChunkSize)(implicit
trace: Trace
): ZStream[Any, Nothing, A] = {

def writeOneByOne(iterator: Iterator[A]): ZChannel[Any, Any, Any, Any, Nothing, Chunk[A], Any] =
if (iterator.hasNext)
ZChannel.write(Chunk.single(iterator.next())) *> writeOneByOne(iterator)
else
ZChannel.unit

def writeChunks(iterator: Iterator[A]): ZChannel[Any, Any, Any, Any, Nothing, Chunk[A], Any] =
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AFAICT the main optimization is to use val builder = ChunkBuilder.make[A](maxChunkSize) is that right?

If that's the case maybe keep this method and make the optimization within it just to keep blame / git history with as few changes as possible (which makes it easier to track potential bugs)

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Done! I minimized the changes

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@kyri-petrou , I think there's another small gain here by eliminating intermediate lambdas/thunks. basic idea: have one ZChannel.succeed evaluating chunk size and creating the builder if needed, then calling either writeOneByOne or writeChunks - both taking eager parameters and potentially running the first steps under the top level thunk (line 4553 in the original code)

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@eyalfa I'm not sure I understand your suggestion. Can you provide a code example?

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I think what @eyalfa is saying is that the optimisation is not only val builder = ChunkBuilder.make[A](maxChunkSize) but also to eliminate the ZChannel.succeed which was used in writeChunks but wasn't required as the execution was already suspended with ZChannel.suspend

ZChannel.succeed(ChunkBuilder.make[A]()).flatMap { builder =>
def loop(iterator: Iterator[A]): ZChannel[Any, Any, Any, Any, Nothing, Chunk[A], Any] = {
builder.clear()
var count = 0
while (count < maxChunkSize && iterator.hasNext) {
builder += iterator.next()
count += 1
}
if (count > 0)
ZChannel.write(builder.result()) *> loop(iterator)
else
ZChannel.unit
def writeChunks(iterator: Iterator[A], maxChunkSize0: Int): ZChannel[Any, Any, Any, Any, Nothing, Chunk[A], Any] = {
val builder = ChunkBuilder.make[A](maxChunkSize0)

def loop(iterator: Iterator[A]): ZChannel[Any, Any, Any, Any, Nothing, Chunk[A], Any] = {
builder.clear()
var count = 0
while (count < maxChunkSize0 && iterator.hasNext) {
builder += iterator.next()
count += 1
}

loop(iterator)
if (count > 0)
ZChannel.write(builder.result()) *> loop(iterator)
else
ZChannel.unit
}

loop(iterator)
}

ZStream.fromChannel {
ZChannel.suspend {
if (maxChunkSize == 1)
val maxChunkSize0 = maxChunkSize

if (maxChunkSize0 == 1)
writeOneByOne(iterator)
else
writeChunks(iterator)
writeChunks(iterator, maxChunkSize0)
}
}
}
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