def pitDepth(p: Int, q: Int, r: Int) = (p - q) min (r - q)

def updateDepths(pqrd: (Int,Int,Int,Int), x: Int): (Int,Int,Int,Int) = pqrd match {
  case (p,q,r,d) if /*conds*/  => /*upslope reset*/
  case (p,q,r,d) if /*conds*/  => /*update q*/
  case (p,q,r,d) if /*conds*/  => /*update r and d*/
  case (p,q,r,d) if /*conds*/  => /*update d and reset*/
}

def maxDepth(list: List[Int]): Int = list match {
  case Nil => -1
  case head :: tail => tail.foldLeft((head,Int.MaxValue,Int.MinValue,-1)) ((quad,x)=>updateDepths(quad,x))._4
}