OrderBook with helper methods to peek the top order and to decrease quantity of the top outstanding order

OrderBook.scala

class OrderBook(side: Side, orderTypes: (Order => OrderType)) {

  private var limitBook: List[(Double, List[Order])] = Nil
  private val priceOrdering = if (side == Sell) Ordering[Double] else Ordering[Double].reverse

  def top: Option[Order] = limitBook.headOption.map({
    case (_, orders) => orders.head
  })

  def decreaseTopBy(qty: Double) {

    limitBook match {
      case ((level, orders) :: tail) => {
        val (top :: rest) = orders
        limitBook = (qty == top.qty, rest.isEmpty) match {
          case (true, true) => tail
          case (true, false) => (level, rest) :: tail
          case _ => (level, orderTypes(top).decreasedBy(qty) :: rest) :: tail
        }
      }
      case Nil => throw new IllegalStateException("No top order in the empty book")
    }
  }

  // the rest of the implementation is omitted
}