Instructions on how to compile/run your program.
1) method 1: Use terminal 
   * step 1: go to src/
   * step 2: javac *.java
   * step 3: java SystemRunner
   (you can also define your input and output path here)
2) method 2: Run in eclipse
   * step 1: create java project
   * step 2: put files into project
   * step 3: Right click on SystemRunner.java
   * step 4: click Run Configurations
   * step 5: in Arguments put "./in/inputfile1.txt ./in/inputfile2.txt ./out/output.txt"
   * step 6: click run.


Brief description of implementation.
1) About coding.
    * Code environment: JAVA 8.
    * Some of the methods or member are marked as "default" access other than private for the reason of
 unit test and integration test.
 
2) A few assumptions:

    * Suppose the price is integer. (Based on the summary example in the pdf file)

    * When book a passenger to a flight, suppose that the passenger choose a random seat.

    * Suppose one passenger book the same flight at most once.
       For example, in inputfile2.txt "BookPassenger,MikeSmith,LAS,LAX" appears two times, 
       but we only book one ticket for MikeSmith for the flight "A124".
       But for "BookPassenger,KennethHarris,CHI,DFW", since there are two flights, and the first time
       KennethHarris pick "K792" which has the lower price, the second time KennethHarris pick "A792"
       They are two different flights, thus we book two tickets for KennethHarris.

    * Suppose when the following scenario happens:
       - The passenger booked a flight at price 100
       - The flight changed price to 80.
       - The passenger request to book the flight the second time.
       In this case, the passenger's flight price is still 100, unless it cancel the flight and book again.

    * Suppose if a passenger booked two flights with the same origin and destination, if the passenger cancel
      a flight, then we should cancel the most expensive flight for the passenger.

    * Suppose we just consider just one type of passengers here.

    * Suppose we use single thread to handle the transaction.

3) Implementation.
   * Class, data structure and logic: 
      - Flight: information about flight, contains flight basic information and reservation.
               1. Member of the class: 
                        Flight number, 
                        Total number of seats in flight,
                        Price per seat.
                        Origin code
                        Destination code
                        Flight reservation list
                        Seat pool, stored as LinkedList.
               2. HashMap<Passenger, ReservationItem> to store all reservation on this flight
               3. LinkedList<Integer> to store all available seats and generate random seats for reservation.
               4. Each time there is a BookPassenger happens, a ReservationItem will be created and stored into
                  this HashMap<Passenger, ReservationItem>. And at the same time we remove the random generated 
                  seat from LinkedList<Integer> seat pool.
               5. Each time there is a CancelPassenger happens, we remove ReservationItem from Map and restore the 
                  seat to LinkedList<Integer> seat pool.
               6. When changePrice happens, we update the price of the flight.

      - FlightReservationSystem: store all flights information and handle transactions.
               1. HashMap<OriginDestinationPair, TreeSet<Flight>>, to store all the flights have the same origin and 
                  destination. Use a TreeSet to keep the order of the flights, it is sorted by the flight price, thus 
                  it enables us to choose the cheapest flight for the passenger.
               2. HashMap<String, Flight> flightNumberToFlightMap, enable us to fast get a flight by flight number.
               3. When process BookPassenger transaction, we first find all flights by origin and destination. Then 
                  choose a cheapest available flight for passenger.
               4. When process CancelPassenger, we cancel the most expensive flight for passenger.
               5. When changePrice, we change the price of the flight and reorder the flights TreeSet. 

      - SystemRunner: entrance of program, to process transactions and create report.
      - FlightInCSVIndexEnum: Enum of flight's columns index in CSV file. For example, Flight number is at 
        the first column, thus it's index is 0.
      - FlightSummary: contains a flight's total avenue, available seats, etc.
      - OriginDestinationPair: contains Origin and Destination.
      - Passenger: passenger information, contains passenger name.
      - ReservationItem: a reservation on a flight, contains passenger, seat number, price.
      - TransactionTypeEnum: transaction type, contains: book, cancel and change price.