Lazily generate all subsequences of an Iterator or Iterable, up to the specified length, returning a new Iterator.

Subseqs.scala

import scala.collection.mutable.ListBuffer

// See also http://stackoverflow.com/questions/22633598/creating-an-iterator-of-subsequences-from-an-iterator-of-items/

/** Lazily generate all subsequences of an Iterator or Iterable, up to the specified length, returning a new Iterator. */
class Subseqs[A](n: Int)(i: Iterator[A]) extends Iterator[List[A]] {
  val buffer = new ListBuffer[A]()
  //buffer ++= i.take(n) // NO - inconsistent behaviour of take() with iterators
  // see http://stackoverflow.com/questions/7619642/consume-items-from-a-scala-iterator
  (1 to n).foreach(_ => buffer += i.next())
  var index = 1
  def hasNext = buffer.length > 1
  def next = {
    def increment() = { val i = index; index += 1; i }
    if (index > buffer.length) {
      index = 1
      buffer.remove(0)
      if (i.hasNext) buffer += i.next()
    }
    buffer.slice(0, increment()).toList
  }
}

/** Pimp my library*/
object Subseqs {
  implicit class SubseqIterable[A](i: Iterable[A]) {
    def subseqs(n: Int) = new Subseqs(n)(i.iterator)
  }

  implicit class SubseqIterator[A](i: Iterator[A]) {
    def subseqs(n: Int) = new Subseqs(n)(i)
  }
}