Transforming an iterator into an iterator of chunks of duplicates

问题

Suppose I am writing a function foo: Iterator[A] => Iterator[List[A]] to transform a given iterator into an iterator of chunks of duplicates :

def foo[T](it: Iterator[A]): Iterator[List[A]] = ???
foo("abbbcbbe".iterator).toList.map(_.mkString) // List("a", "bbb", "c", "bb", "e")

In order to implement foo I want to reuse function splitDupes: Iterator[A] => (List[A], Iterator[A]) that splits an iterator into a prefix with duplicates and the rest (thanks a lot to Kolmar who suggested it here)

def splitDupes[A](it: Iterator[A]): (List[A], Iterator[A]) = {
  if (it.isEmpty) {
    (Nil, Iterator.empty)
  } else {
    val head = it.next
    val (dupes, rest) = it.span(_ == head)
    (head +: dupes.toList, rest)
  }
}

Now I am writing foo using splitDupes like that:

def foo[A](it: Iterator[A]): Iterator[List[A]] = {
   if (it.isEmpty) {
     Iterator.empty
   } else {
     val (xs, ys) = Iterator.iterate(splitDupes(it))(x => splitDupes(x._2)).span(_._2.nonEmpty)
     (if (ys.hasNext) xs ++ Iterator(ys.next) else xs).map(_._1)
   }
}

This implementation seems working but it's look complicated and clumsy.
How would you improve the foo implementation above ?

回答1:

You can do it like this:

def foo[A](it: Iterator[A]): Iterator[List[A]] = {
  Iterator.iterate(splitDupes(it))(x => splitDupes(x._2))
    .map(_._1)
    .takeWhile(_.nonEmpty)
}

The empty case is already handled in splitDupes. You can safely keep calling splitDupes until it hits this empty case (that is, starts returning Nil in the first tuple element).

This works ok in all cases:

scala> foo("abbbcbbe".iterator).toList.map(_.mkString)
res1: List[String] = List(a, bbb, c, bb, e)

scala> foo("".iterator).toList.map(_.mkString)
res2: List[String] = List()

scala> foo("a".iterator).toList.map(_.mkString)
res3: List[String] = List(a) 

scala> foo("aaa".iterator).toList.map(_.mkString)
res4: List[String] = List(aaa)

scala> foo("abc".iterator).toList.map(_.mkString)
res5: List[String] = List(a, b, c)

来源：https://stackoverflow.com/questions/60025718/transforming-an-iterator-into-an-iterator-of-chunks-of-duplicates

标签

scala

iterator