Implementation of liftFirst for FS2

process1.scala

  def liftFirst[F[_],A,B,C](f: Process1[A,B]): Process1[(A,C),(B,C)] = {
    def go(cs: Chunk[C], s: Stepper[A,B]): Handle[Pure,(A,C)] => Pull[Pure,(B,C),Unit] = h =>
      s.step match {
        case Stepper.Done => Pull.done
        case Stepper.Fail(err) => Pull.fail(err)
        case Stepper.Emits(chunk, next) =>
          if (cs.isEmpty) go(cs, next)(h)
          else {
            val last = cs(cs.size - 1)
            val extended =
              if (cs.size < chunk.size) cs.toVector ++ Vector.fill(chunk.size - cs.size)(last)
              else cs.toVector
            val zipped = Chunk.indexedSeq { chunk.toVector zip extended }
            Pull.output(zipped) >> go(Chunk.singleton(last), next)(h)
          }
        case Stepper.Await(recv) => Pull.suspend {
          Pull.awaitOption(h).flatMap {
            case None => go(Chunk.empty, recv(None))(Handle.empty)
            case Some(s) => go(s.head.map(_._2), recv(Some(s.head.map(_._1))))(s.tail)
          }
        }
      }
    _ pull { h => go(Chunk.empty, process1.stepper(f))(h) }
  }