exchange.scala

object Bank {
    def placeFillOrKillBuyOrder(price: Double, vol: Int, stock: StockID, buyer: UserID): Unit {

        val (askPrice,askVol) = OrderDepth.getLowestAsk(stock)
        if (price >= askPrice && vol <= askVol) {
            /* The Bank updates the user's account to reflect
               the trade which it was able to fill at once. */
            Account.decreaseBalance(UserID, price * vol)
            Account.increaseStock(UserID, stock, vol)
            Account.notify("Stock bought successfully")

            /* The Bank buys the amount of stock that it
               owes the user  */
            OrderBook.placeBuyOrder(price, vol, stock, Bank.userid)
        }
    }
}

object OrderBook {
    def placeBuyOrder(price: Double, vol: Int, stock: StockID, buyer: UserID): Unit {
        ...
        ...
        matchOrder(stock)
    }
    def matchOrder(stock: StockID) {
        
    }
}