Scalania7.scala

import scala.annotation.tailrec

object S99_P17 {
  def split_1l[T](n: Int, ts: Seq[T]) = ts.splitAt(n)

  def split_tr[T](n: Int, ts: Seq[T]): (Seq[T], Seq[T]) = {
    @tailrec
    def go(n: Int, lists: (Seq[T], Seq[T])): (Seq[T], Seq[T]) = (n, lists) match {
      case (n, (heads, e +: tails)) if (n > 0) => go(n - 1, (e +: heads, tails))
      case (_, (heads, tails)) => (heads.reverse, tails)
    }
    go(n, (Nil, ts))
  }

  def split_f[T](n: Int, ts: Seq[T]): (Seq[T], Seq[T]) = {
    val (left, right, _) = ts.foldLeft((Seq[T](), Seq[T](), 0)) {
      case ((left, right, i), e) =>
        if (i < n) (e +: left, right, i + 1)
        else (left, e +: right, i + 1)
    }
    (left.reverse, right.reverse)
  } 
}

object S99_P18 {
  def slice_tr[T](from: Int, to: Int, ts: Seq[T]): Seq[T] = {
    @tailrec
    def go(from: Int, to: Int, ts: Seq[T], acc: Seq[T]): Seq[T] =
      ts match {
        case head :: tail if from > 0 => go(from - 1, to - 1, tail, acc)
        case head :: tail if to > 0 => go(0, to - 1, tail, head +: acc)
        case _ => acc.reverse
      }
    go(from, to, ts, Nil)
  }
}

object S99_P19 {
  def rotate[T](n: Int, ts: Seq[T]): Seq[T] = {
    @tailrec
    def go(n: Int, ts: Seq[T], acc: Seq[T]): Seq[T] = (n, ts) match {
      case (n, h +: t) if n > 0 => go(n - 1, t, h +: acc)
      case _ => ts ++ acc.reverse
    }
    if (n >= 0) go(n % ts.length, ts, Nil)
    else go(-n % ts.length, ts.reverse, Nil).reverse
  }
}