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Implement LazyList tailRecM using Iterator.unfold #2964

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Aug 2, 2019
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Implement LazyList tailRecM using Iterator.unfold #2964

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54 changes: 6 additions & 48 deletions core/src/main/scala-2.13+/cats/instances/lazyList.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -69,56 +69,14 @@ trait LazyListInstances extends cats.kernel.instances.StreamInstances {
fa.zipWithIndex

def tailRecM[A, B](a: A)(fn: A => LazyList[Either[A, B]]): LazyList[B] = {
val it: Iterator[B] = new Iterator[B] {
var stack: List[Iterator[Either[A, B]]] = Nil
var state: Either[A, Option[B]] = Left(a)

@tailrec
def advance(): Unit = stack match {
case head :: tail =>
if (head.hasNext) {
head.next match {
case Right(b) =>
state = Right(Some(b))
case Left(a) =>
val nextFront = fn(a).iterator
stack = nextFront :: stack
advance()
}
} else {
stack = tail
advance()
}
case Nil =>
state = Right(None)
}

@tailrec
def hasNext: Boolean = state match {
case Left(a) =>
// this is the first run
stack = fn(a).iterator :: Nil
advance()
hasNext
case Right(o) =>
o.isDefined
}

@tailrec
def next(): B = state match {
case Left(a) =>
// this is the first run
stack = fn(a).iterator :: Nil
advance()
next()
case Right(o) =>
val b = o.get
advance()
b
val kernel = Iterator.unfold[Option[B], Iterator[Either[A, B]]](Iterator(Left(a))) { it =>
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two small notes, I bet this would be faster if we did:

unfold[Option[B], List[Iterator[Either[A, B]]])(Iterator.single(Left(a)) :: Nil) {
  case Nil => None
  case nonempty@(h :: t) =>
    if (!h.hasNext) Some((None, t))
    else h.next match {
      case Left(a) => Some((None, fn(a) :: nonempty))
      case Right(b) => Some((Some(b), nonempty))
    } 
}

if (!it.hasNext) None
else it.next match {
case Left(a) => Some((None, fn(a).iterator ++ it))
case Right(b) => Some((Some(b), it))
}
}

LazyList.from(it)
LazyList.from(kernel.collect { case Some(v) => v })
}

override def exists[A](fa: LazyList[A])(p: A => Boolean): Boolean =
Expand Down