OrderType registry with pegged order partial function added

OrderType.scala

object OrderType {

  def all(buy: => OrderBook, sell: => OrderBook): PartialFunction[Order, OrderType] = {

    // omitted [unmodified] order type definitions for limit and market orders
    // ...
    
    case self@PegOrder(_, side, _) => new OrderType {
      private lazy val book = side match {
        case Buy => buy
        case Sell => sell
      }

      def bookDisplay: String = s"Peg($bestLimit)"

      def price: PriceLevel = PegPrice

      def crossesAt(price: Double): Boolean = side match {
        case Buy => price <= bestLimit
        case Sell => price >= bestLimit
      }

      def decreasedBy(qty: Double): Order = self.copy(qty = self.qty - qty)

      private def bestLimit =
        book.bestLimit.getOrElse(throw new IllegalStateException("Pegged orders are illegal in a book with no best limit defined"))
    }
  }
}