Giovanni, hi!
With some piece of stubbornness :) I (guess) have found a way with
Source.last():
def sinkFoldM[U, T](zero: U)(f: (U, T) ⇒ Future[U]): Sink[T, Future[U]] =
Sink.fromGraph {
val last = Sink.last[U]
GraphDSL.create(last) { implicit b => last =>
import GraphDSL.Implicits._
val zip = b.add(ZipWith(f))
val bcast = b.add(Broadcast[U](2))
val merge = b.add(Merge[U](2))
Source.single(zero) ~> merge ~> bcast ~> last
bcast ~> zip.in0
zip.out.mapAsync(1)(identity) ~> merge
SinkShape(zip.in1)
}
}
And in hands I have got a task with needing to fold starting with future
zero. So, I have tried:
def sinkFoldMM[U, T](zeroFu: Future[U])(f: (U, T) ⇒ Future[U]): Sink[T,
Future[U]] = Sink.fromGraph {
val last = Sink.last[U]
GraphDSL.create(last) { implicit b => last =>
import GraphDSL.Implicits._
val zip = b.add(ZipWith(f))
val bcast = b.add(Broadcast[U](2))
val merge = b.add(Merge[U](2))
Source.fromFuture(zeroFu) ~> merge ~> bcast ~> last
bcast ~> zip.in0
zip.out.mapAsync(1)(identity) ~> merge
SinkShape(zip.in1)
}
}
By some reason this test
val upTo = 20000000;
//val upTo = 0;
val result = Source(0 until upTo).runWith(sinkFoldMM(Future(0))((a, x) =>
Future(a + x)))
println(Await.result(result, 10.minutes))
sys.exit(0)
takes two times more CPU time in comparison with sinkFoldM (~50 second vs.
~25 second).
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