Finding the number of "monotonicity flips" in a Seq[Int] in Scala

gistfile1.scala

// Finds the monotonicity breaks in a numerical sequence.
def findMonotonicityBreaks(items: Seq[Int]): Int = {
  def comp(op: Symbol, a: Int, b: Int) = if (op == '<=) a <= b else a >= b
  def flip(op: Symbol) = if (op == '<=) '>= else '<=
  
  @tailrec
  def detect(pairs: Seq[(Int, Int)], op: Symbol, breaks: Int): Int = {
    pairs match {
      case Seq() => breaks
      case Seq((a, b), rest @ _*) if comp(op, a, b) => detect(rest, op, breaks)
      case _ => detect(pairs, flip(op), breaks + 1)
    }
  }
  
  if (items.isEmpty) 0
  else {
    val pairs = items.zip(items.tail)
    pairs.collectFirst {
      case (a, b) if a < b => '<=
      case (a, b) if a > b => '>=
    }.map { op => detect(pairs, op, 0)
    }.getOrElse(0)
  }
}

// Usage:

scala> findMonotonicityBreaks(Seq.empty[Int])
res21: Int = 0

scala> findMonotonicityBreaks(Seq(0,1,2,3,4,4,4,4,5,6))
res22: Int = 0

scala> findMonotonicityBreaks(Seq(0,1,2,3,3,3,2,1))
res23: Int = 1

scala> findMonotonicityBreaks(Seq(1,2,3,4,5,6,5,4,5,6,7))
res24: Int = 2

scala> findMonotonicityBreaks(Seq(5,4,3,2,1,0))
res25: Int = 0

scala> findMonotonicityBreaks(Seq(7,6,5,4,5,6,5,4,3,2,1))
res26: Int = 2

scala> findMonotonicityBreaks(Seq(1,2,3,3,3,2,1,0))
res27: Int = 1

scala> findMonotonicityBreaks(Seq(1,2,3,4,5,6,5,4,5,6,7,6,5,4,3))
res28: Int = 3

scala> findMonotonicityBreaks(scala.util.Random.shuffle(Vector.range(1, 100000)))
res29: Int = 66498

scala> findMonotonicityBreaks(Seq(5,5,5,4,3,2,1,0))
res30: Int = 0

scala> findMonotonicityBreaks(Seq(5,5,5,4,3,2,1,0,2,3,4,5))
res31: Int = 1

scala> findMonotonicityBreaks(Seq(1,1,1,1,2))
res32: Int = 0

scala> findMonotonicityBreaks(Seq(1,1,1,1,2,1))
res33: Int = 1

Great solutions, guys! Aren't Scala collections fun?